Thinking of changing to an all-electric home: would you really want to?
Dr Alan Jones, PhD, CEng, FIET.
Independent Researcher
6 March 2024
Summary
Achieving Net Zero places the insatiable demand for energy under the spotlight, especially energy derived from hydro-carbon sources. Imagine a world where hydro-carbon based fuels no longer exist and are replaced instead by electricity, and particularly low-no carbon electricity from nuclear and renewables, especially wind and solar.
This paper takes a look at what that might mean for the typical UK household and what, if any, the scale of energy savings might be as well as the cost to the consumer.
The results are revealing – there is cause for hope for the climate, but the benefit does not necessarily translate into cost savings for the majority of end users due to structural issues with the domestic pricing of electricity in relation to gas and petrol.
1 Introduction
This paper examines what it means for the average UK household, the type of household the Energy Regulator (Ofgem) cites when presenting typical figures for domestic gas and electricity consumption, as they transition from fossil-fuels to an all-electric future and how this may affect total annual energy consumption and running cost.
In this scenario the homeowner replaces their internal combustion engine (ICE) petrol car with an equivalent electric vehicle (EV) while an air source heat pump (ASHP) replaces the present gas boiler to provide central heating and hot water.
The results from this investigation are revealing. Simply changing from a petrol car to an EV saves almost 30% of transport related energy consumption for the same equivalent mileage. Furthermore, when the effect of the transition to heat pump technology is included our typical household total energy consumption falls by over 62%.
The reason for this magnitude of energy reduction is down to the superior efficiency of this all-electric technology, with the ICE at around 25% or less while the EV typically achieves over 90%, depending on a number of factors. Similarly, the ASHP can achieve efficiencies of 300% or more compared to a gas boiler, which depending on age may achieve between 80% to 90% or more.
This scale of energy reduction if applied universally could help the UK towards meeting
Net Zero emissions goals. This reduction, however, comes at a cost to our typical homeowner in so far as this reduction in energy consumption does not reduce annual energy bills due to structural problems with the unit pricing of domestic electricity compared to the equivalent fossil-fuels – gas and petrol.
On the other hand, opportunities exist whereby the homeowner can reduce their energy bill, perhaps significantly so, in certain circumstances and these are detailed in the paper.
2 Typical household energy demand
The Regulator, Ofgem, provides typical energy consumption figures for different size properties. For example, Ofgem cites the average 2-3 bedroom household in Great Britain (GB) uses 2700kWh of electricity and 11,500kWh of gas each year, but this can increase to 4100kWh and 17,000kWh respectively for a 4+ bedroom house accommodating 4-5 people, or it can be as little as 1800kWh and 7500kWh for a 1 bedroom flat housing 1-2 people [1].
In this paper let’s consider that typical 2-3 bedroom household with a total energy load of 14,200kWh (2700kWh + 11,500kWh) for lighting, heating and hot water along with a host of other electrical appliances that all require energy to operate. But we shouldn’t forget the car, most likely petrol or diesel. We’ll take a look at the car first.
2.1 A quick look at the energy demand for domestic road transport
With 33.58 million cars licenced in the UK in September 2023 [2], which if spread amongst the 28.2 million households in 2022 [3] represents, on average, 1.19 cars per household. For this discussion, and to make the sums easier, we’ll assume this typical home only has one car. So how much energy will this car, and here let’s assume it’s petrol, consume in a year?
A car leasing company [4], publishes figures for the average UK annual car mileage at 7400 miles, although the breakdown depends on a number of factors such as whether the motorist lives in an urban or rural environment, whether they are young or old as well as their lifestyle and occupational factors. Another organisation surveys the fuel sold over 30 million fill-up transactions by 80,000 UK vehicle owners to calculate the 2023 average consumption rate, which they found to be 36 miles per gallon (mpg) for petrol and 43 mpg for diesel cars [5].
These figures suggest our single petrol car household that does 7400 miles on average each year, uses around 195 gallons or 886 litres of fuel. As the gross calorific value of a litre of petrol, when burned, releases 9.7kWh of energy [6] then our typical household requires a further 8594kWh for personal transport.
If we now sum up our typical household multi-fuel energy consumption, ignoring other things like water consumption, bus, train or travelling by air, all of which consume energy, we have 2700kWh for electricity, 11,500kWh for gas and 8594kWh for petrol. That’s now a total of 22,794kWh of which, at present, electricity only represents 11.8% of the total.
2.2 Summarising the position
From these figures, simple maths would suggest that if our typical average household were to go all-electric, by replacing gas with an air source heat pump, for example, and changing the petrol car for an electric vehicle (EV) then the present annual electrical energy consumption would increase by over eightfold - from 2700kWh to 22,794kWh. But if this is correct what are the consequences for the homeowner. In another paper we’ll examine the implications, if everyone follows our typical homeowner’s example, on the future UK renewable generation capability to meet this 800% plus increase in electrical demand?
Let’s examine each element in turn beginning with a switch to an EV.
3 Switching from a petrol engine vehicle to an EV.
What we know from the source above is that our average petrol engine car achieves 36 mpg, which converted to litres equates to 7.92 miles per litre (mpl). We also know that the calorific value, or energy density, of petrol is 9.7kWh/l so from these two values we can arrive at a figure of miles per kWh (m/kWh) in order to compare both petrol and EV performance.
For our petrol car the result is typically 0.82m/kWh, which is simply calculated from 7.92mpl/9.7kWh/l. The result for an EV is perhaps surprisingly different, and better. Several sources [7, 8] suggest real world figures of between 3.0-3.5m/kWh for modern EVs although, in the same way the petrol engine performance varies, a lot depends on factors such as the terrain, tyre maintenance and driving style. In the case of the EV, however, seasonal factors, such as winter vs. summer driving can have a big impact on efficiency with figures such as 4.0m/kWh in summer falling to around 3.0m/kWh in winter.
Now that we have two performance figures – 0.82m/kWh for our petrol car and 3.0-3.5m/kWh for our EV the obvious question is, why is the petrol engine vehicle performance so poor compared to the EV? The answer is simply down to the fact that the petrol (and diesel too) powered internal combustion engine (ICE) has a poor efficiency. While the EV has a high efficiency, converting around 90% or more of the energy stored in the battery into driving force at the wheels the petrol engine efficiency is around 25%, or less, with the majority of the energy released by combustion turned into heat. This is one of the areas where an EV out-performs the ICE.
With this in mind let’s now return to our typical home that has replaced their petrol ICE vehicle with an EV, and let’s be generous by assuming the EV achieves an average of 3.5m/kWh. We can now ask ourselves, assuming the household annual mileage remains the same at 7400 miles, how much electrical energy will be required each year to cover this distance? The answer is 2114kWh (given by 7400/3.5). Had we been less generous and assumed the EV efficiency as 3.0m/kWh then the result would have been 2467kWh each year, but we’ll assume the lesser annual kWh figure applies here based on a 3.5m/kWh efficiency.
Just to recap therefore, our typical home that once consumed 22,794kWh of energy each year now only consumes 16,314kWh, a 28% reduction, as shown in Table 1, simply because an EV is more efficient at energy conversion than an ICE.
Table 1. Typical 2-3 Bedroom Household Energy Demand
Typical 2-3 bed household | Lighting etc (kWh) | Heating – gas (kWh) | Transport (kWh) | Total (kWh) |
Petrol | 2700 | 11,500 | 8594 | 22,794 |
EV | 2700 | 11,500 | 2114 | 16,314 |
Percentage Change in Annual Energy Demand from Adopting an Electric Vehicle | -28% | |||
And what if our typical home also converts from gas central heating to an air source heat pump (ASHP)? Will this deliver further energy saving benefits by switching to electricity?
4 Switching from gas to an ASHP
Perhaps a simple explanation of what an ASHP is, how it works, the role it can play in helping eliminate fossil-fuel consumption along with the progress being made with implementation is required before we dig deeper and examine what it means for our typical household to switch from gas to heat pump technology.
4.1 A quick look at an ASHP
Put simply, an ASHP extracts the energy contained in the air outside a home so that it can be transferred or converted into useful energy to heat the inside. It is often described as a refrigerator operating in reverse. In this case, external air, or water in the case of a ground source heat pump (GSHP), is blown (or pumped for a GSHP) over the surface of a heat exchanger causing the refrigerant liquid inside the heat exchanger to evaporate and turn into a gaseous state. The gas is then compressed thereby increasing both the pressure and temperature with the heated gas passing over another heat exchanger the output of which can be blown as warm air through a house, or transferred into a wet central heating/hot water system. While air-air transfer can be used for heating, but not for domestic hot water, air-water transfer can be used for both heating and hot water, and is consequently more versatile. To complete this cycle, as the heat is transferred the gas falls in temperature until it returns back to a liquid state and this reverse refrigeration cycle repeats until the home reaches the required temperature [9].
Because heat pump technology operates on the basis of a transfer of energy rather than the generation of heat by combustion of either gas or oil, as in conventional heating boilers, it is rather surprisingly more efficient than conventional heating technologies widely used in the UK today. And this efficiency is what makes ASHPs along with other heat pump alternatives so interesting as a ‘greener’ alternative for the household seeking to go all-electric.
4.2 Why it makes sense to adopt heat pump technology
The latest domestic gas boilers can have efficiencies in the high 90% range, meaning that only a little of the energy contained in the gas in its natural state is lost due to combustion. However, and by contrast, heat pumps can have efficiencies way in excess of 100% and this is what makes them so attractive as a cleaner, greener option for heat and hot water in domestic properties.
To put this into context heat pump performance is measured by a term called the Coefficient of Performance (CoP), which is used to describe the performance, or efficiency, of heat pumps and is expressed as:
CoP = Heat Energy Output (in kWh)/Electrical Energy Input (in kWh)
What this means is that, for example, if a heat pump vendor says their technology has a CoP of 4.0 then for every single unit (kWh) of electrical energy consumed by the heat pump to operate fans and compressors etc. there will be 4 units (kWh’s) of useful heat energy produced that can be used to heat a home and heat water. From this simple example we gather that this one unit of electrical energy is able to transfer three units of energy from the external air outside a property, in the case of an ASHP, or from the ground in the case of a ground source heat pump.
This may sound inconceivable – energy for nothing almost, and the makers of heat pumps recognise this, the too good to be true aspect - and it is true, to an extent.
In our typical household that adopts an ASHP we will examine this claim in greater detail.
4.3 UK rate of progress with implementation
Unfortunately the UK is presently lagging behind in this race, having only about 0.1% of the global heat pump installed capacity, with a modest total figure of over 200,000 units installed since 2008 and currently running at just over 50,000 installations in 2023 - way behind the Government target of 600,000 annually by 2028 to satisfy Net Zero ambitions [10, 11, 12].
Sadly, at this low rate of heat pump installation the UK will not be in a position to meet the anticipated demand from the 2025 ban on installing fossil-fuel boilers in new built homes, which is currently achieving a rate of around 160,000 each year according to the National House Building Council [13]. And this number is trivial compared to the roughly 1.7 million new gas boilers installed annually, largely to replace old or unrepairable heating systems in existing homes [14].
Beyond this, and at a global level, the UK is hardly a player in the heat pump market. Europe had over 21 million heat pumps installed by 2020 and globally there were 177 million installations. Even in some of the coldest-winter European nations - Norway, Sweden, Finland and Estonia for example, the percentage of households with heat pumps at this time was running at 60%, 43%, 41% and 34% respectively [10]. By stark contrast, the UK, with its 28.2 million households in 2022 [15], appears to remain wedded to gas heating with only around 0.7% of households making the switch to heat pumps – one of the lowest in Europe.
This paper does not intend to examine the reasons for this lack of uptake, which itself could form the basis of a further paper, but suffice to say here that chief among many of the reasons offered by commentators, is cost [16, 17, 18, 19, 20, 21]. Not just the extra investment cost over simply replacing one gas boiler with another more efficient model, but also the rising cost of the electricity required to operate heat pumps against the much lower unit cost of gas - a point that is examined further in this paper.
So, having given this basic introduction to heat pumps let’s now turn our attention to the typical property considering switching from gas to an ASHP.
4.4 Before taking the plunge
Before our typical 2-3 bed household that consumes 11,500kWh of gas each year for heating and hot water makes the switch to an ASHP there are a few points worth mentioning if a heat pump installation is to be successful.
The first question to ask is: is the building fabric suitable for an ASHP? In other words, does the energy efficiency of the building meet current standards or does the building leak energy to the point where the low-grade heat from a heat pump would be insufficient to maintain a comfortable living temperature - often cited as 21oC?
This is a material question to our investigation because, according to the House Builders Federation (HBF), around 73% of all homes in England (just under 18 million) were built before 1980 at a time when building standards gave little consideration to the energy performance of domestic properties [22] and is the reason why some 14% of the UK’s annual greenhouse gas (GHG) emissions is attributable to domestic properties [18].
With the thermal transmittance, or U-value, for solid masonry walls of 2.4 against a much better and lower value of 0.5 or less for modern houses (around 5 times better); floors constructed from stone, concrete or timber compared to modern insulated equivalents at least twice as good [23] it becomes obvious why the HBF cites older properties consuming 264kWh/m2/year of energy each year against only 100kWh/m2/year for the average new build property [22].
Put more simply – the average new built home consumes only around 40% of the energy of the average and almost universal UK home built pre-1980. For this reason, and before contemplating replacing gas with ASHP technology, improving the energy efficiency of the building fabric of the average home is an important and necessary first step.
Once this is sorted, even if it is practical to do so, it is highly likely the existing wet central heating system may need attention. For example, to achieve the best outcome underfloor heating is highly recommended as this form of heating provides faster warmth and a more uniform heat distribution, and is better suited to the optimum efficiency low circulating water temperature of 30-35oC from ASHPs[24].
Beyond this, if conventional panel radiators are the only option for an air-water heat pump installation then larger radiators will be required, typically up to twice the surface area of the existing ones [25, 26, 27], simply because radiators are tested and sized according to how much heat they give off based on the difference (Delta T) in temperature between the water circulating in the central heating system and that of the ambient or room temperature, and the return flow. With a condensing gas combi boiler the circulating hot water temperature may be set between 70C-75oC which, based on a 20oC ambient , provides a Delta T of 50oC or higher. However, because the alternative ASHP technology caters most efficiently for a Delta T of 30-35oC, radiators fed from heat pumps need to be sized significantly larger to achieve the same kW rating.
One further point not to be overlooked is the need to accommodate the physical component parts of an ASHP within the household. Pictures of an ASHP located outside a property abound, but little is mentioned about the kit needed inside a property to make it work. To give an example: let’s imaging a property has a gas combi-boiler that heats hot water on demand. Changing to an ASHP requires a domestic hot water cylinder (DHW), which needs locating somewhere convenient to existing pipework. And even if a DHW cylinder already exists it may need to be replaced by one suited to ASHP’s – with a larger heating coil so that it can heat from a lower temperature source [28].
An indoor heat exchange will also be needed in addition to the one located outside to transfer the energy contained in the pressurised hot gas to heat the return water flow to the required flow temperature, which may be boosted up to 55oC or higher with the aid of electrical supplementary heating. This heat exchanger is equivalent to the size of a modern gas boiler although a number of heat pump manufacturers now also offer pre-plumbed monoblock heat pump systems that combines the DWH cylinder with the heat exchanger along with pumps, valves and the associated control system.
These systems, while convenient, are sophisticated and are usually the size of a fridge/freezer, depending on the water capacity of the DHW tank [29]. A 200l capacity monoblock heat pump system from Samsung, for example, will require a volume of around 1500mm (H) x 950mm (W) x 750mm (D) to accommodate the system and allow suitable clearances for installation and maintenance activity [30].
Having now attended to these issues and being satisfied that our typical home satisfies the requirements it is time to focus on the implementation: choosing the right size heat pump and understanding how well it will operate in practice.
4.5 Choosing the right system
Choosing a heat pump that has too large a rating for our household’s heating demand will switch off and on frequently rather than operate on an almost continuous basis, which may lead to premature breakdowns and inefficient operation. On the other hand a heat pump with too low a rating might not be able to heat a home to a comfortable temperature.
Employing a knowledgeable Microgeneration Certification Scheme (MCS) accredited heat pump installer along with an Energy Performance Certificate (EPC) for the property should help resolve this problem and make the sizing task easier [31].
For our hypothetical typical 2-3 bedroom household though, that we assume forms part of the 73% of housing stock in England built before 1980 [22], OVO Energy [32] estimates a 6.0-7.5kW rated ASHP will be needed if the building fabric cannot meet the equivalent new build standard, otherwise a 5kW ASHP may suffice [33]. We will also assume our typical household of 3-4 occupants requires a 200 litre DHW tank [22]. Let’s not forget either that in a non-hypothetical case the actual heat pump size chosen will be based on a number of factors, one of which will be the EPC report and the location of the property (to choose the correct outdoor design temperature for the particular region of the UK) as well as the suitability for retrofitting heat pump technology.
4.6 So what does it mean for our typical household to switch from gas to an ASHP
The questions here, for our average 2-3 bedroom household, are – will adopting heat pump technology save energy and, if so, how much will it save?
The question we need to address is, how much energy will our typical property that is no longer on gas consume in a year? The answer lies in how efficient our ASHP performs over that period and here we should appreciate that the efficiency, or CoP, will vary as either the inlet temperature and/or the heat pump outlet temperature varies throughout the year. This annual performance or averaged performance is known as the Seasonal Coefficient of Performance (SCoP).
To illustrate the effect these two variable, inlet temperature and outlet flow temperature, have on the instantaneous value of CoP, and the longer term SCoP, it is helpful to consider the illustrative Figures, 1 and 2, provided by Kensa Heat Pumps [34]. From Figure 1 it can be seen that the typical heat pump efficiency falls as the external temperature falls because the electrically operated compressor requires more energy at a colder inlet air temperature. This figure also demonstrates that different sizes of heat pump can have different performance efficiencies.
Figure 1 Showing how CoP varies with source temperature and ASHP kW rating. Source: Kensa Heat Pumps
Figure 2, again illustrative, demonstrates that the higher the outlet temperature of the heat pump the more work the compressor has to do to achieve this temperature and therefore the greater energy the electrically powered compressor requires – hence the lower CoP value.
What this graph shows is that the choice of outlet temperature from the heat pump can have a significant effect on the instantaneous CoP and hence the longer-term SCoP, and consequently on the economics of operation through greater than optimum electricity consumption. This is why heat pumps are ideally suited for underfloor heating embedded in a cement screed which requires a low heat pump outlet flow temperature of around 30-35oC. It also demonstrates how retaining the smaller sized radiators used for gas central heating will likely result in a lower SCoP due to the need for supplementary electrical heating to raise the circulating water to a higher flow temperature than can be achieved by a heat pump alone.
Figure 2 Showing how CoP varies with compressor off temperature, which is slightly higher than the flow temperature leaving the heat pump. Source: Kensa Heat Pumps
Interpreting these graphs, especially Figure 1, may lead to the ‘apparently’ logical conclusion that an ASHP achieves a higher SCoP during the summer months than during the winter because of the warmer ambient air temperature and the compressor having to do less work. This is not the case, however, and various sources provide operational experience to confirm this [35, 36].
Energy-Stats UK [35], for example, provides an in-depth review of the steps towards the adoption of renewable energy technology for a typical domestic property - in this case the replacement of a 32kW gas boiler by a 5kW Vaillant ASHP in a 1930’s, semi-detached, 3-bed, 90m2 property in Sheffield that has had larger heat pump compatible radiators and pipework retrofitted while the house itself has adequate insultation in the loft and part insulated cavity walls.
What the results show here is that over the first year of operation the electrical input to the heat pump was 3086kWh while the heat output was 11,200kWh – thus returning a SCoP of 3.6 (based on 11,200/3086). However, during the summer months, at an average external ambient of 16oC the SCoP averages a value of 3.0, typically consuming each month around 75kWh of electricity to yield 215kWh of heat. On the other hand, during the winter when the average external temperature falls to around 4.5oC the SCoP rises to between 3.7-3.8 where, typically, the monthly performance was 550kWh of electricity yielded around 2000kWh of heat.
The reason for the wintertime efficiency improvement is answered by Figure 2 and is due to the fact that summertime use of the ASHP is almost exclusively to heat domestic hot water (DHW), which requires a hotter water temperature than that for central heating – usually heated to around 50oC with a weekly excursion to 60oC as a precaution against Legionella bacteria. Consequently the efficiency falls because more supplementary electrical energy is required to achieve the higher temperature.
By making use of the data from this example we can estimate that if DHW continued to be supplied by gas while employing the ASHP for central heating alone then the annual SCoP performance would rise from 3.6 to almost 4.0, because around 23% of the total heat output is presently used to heat the DHW while consuming around 29% of the annual electricity consumption.
The other observation arising from this example is that the ASHP, especially during the winter months, runs for around 60% of the time. In other words, around 14 hours each day on average although, as the author of this site describes, during the night the thermostat is set back from 20.8C to 19C, otherwise it would run for longer. This is quite unlike the strategy for gas central heating and demonstrates the much more gentle and slower heating capability of heat pump technology – hence the preference for underfloor heating.
Heatpumpmonitor.org [36] provides information on a range of ASHP installations of various sizes and locations for different property and building fabric configurations in the UK. The records here help support the above observations where SCoP’s vary between 3.6 to 4.1 depending on the conditions under which the ASHP is operated and the level of DHW demand ranging from 19% to 25% of the total annual ASHP heat output.
The key question for this investigation, for our typical 2-3 bedroom house, is what value of ScoP to use in our calculation? To answer this question we need to remember that our house is a typical pre-1980’s house. It probably has sufficient loft insulation but it is unlikely to have insulation in the floors or external walls. It is also unlikely to have the larger radiators fitted to accommodate an ASHP and it is even more unlikely to have underfloor heating although it is expected to have at least some double glazing. It also has around 100m2 of liveable space that is typical of a 2-3 bedroom terrace house [37].
For our typical home, therefore, Wocester-Bosch, a ASHP manufacturer, advise that a typical 3-bedroom semi-detached house will achieve a SCoP of 3.0 while Grant Boilers, another ASHP manufacturer, also provides a similar figure for a 6.9kW Aerona R32 model when run at a flow temperature of 55C. A further source [38], based on 2021 survey of 23,000 properties in Scotland with heat pumps, shows an accumulative heat output of 300GWh against 100GWh of electricity input – further suggesting that a SCoP of 3.0 is a reasonable estimate for our property.
One additional point before working out the effect of the heat pump in our typical 2-3 bedroom household is that Ofgem [1] provides the average annual consumption in their data for energy consumption, of 2700kWh for electricity and 11,500kWh of gas for a 2-3 bedroomed house. However, this data may be skewed because, for instance, of the presence of unequal numbers of different property types in this category - from detached, semi-detached, end terrace and mid terrace, each of which may have different energy demands. Equally, the age of the property and hence the condition of the building fabric will impact this average value too. Although it is not the intention to change Ofgem’s figures for this exercise it is worth appreciating that in circumstances like this the median value would be more helpful and descriptive.
In this regard a University College London [39] large-scale longitudinal survey of domestic energy consumption finds the 2021 median values for our 100m2 typical home are 2900kWh of electricity consumption and 12,700kWh of gas. This suggests, for future reference, that the Ofgem figures are reporting under consumption and are therefore, in this particular case, not representative of the true population.
4.7 Energy demand following our transition to an all-electric household
After this rather long discussion our typical 2-3 bed household can now switch from gas to an ASHP with the expectation that the energy required for heating will fall from 11,500kWh each year to 3833kWh (11,500kWh/SCoP of 3.0) where these 3833 units of energy are provided by electricity. Figure 2 below shows the effect of this transition.
Table 2 The effect of switching from gas to an ASHP
Typical 2-3 bed household | Lighting etc (kWh) | Heating – gas (kWh) | Transport (kWh) | Total (kWh) |
Petrol + Gas | 2700 | 11,500 | 8594 | 22,794 |
EV + Gas | 2700 | 11,500 | 2114 | 16,314 |
EV + ASHP | 6533 (2700+3833) | 0 | 2114 | 8647 |
Percentage Change in Annual Energy Demand from adopting an Electric Vehicle and switching to an ASHP | -62.1% | |||
We saw from Chapter 3 and Table 1 that our average 2-3 bedroom household could make an annual energy demand reduction of around 28% by ditching their ICE vehicle in favour of an EV with the reduction coming from the high level of efficiency the EV has in converting stored chemical energy, in the battery, into kinetic and potential energy. Compared with the ICE vehicle, at around 25% efficiency, at best, the EV wins by a wide margin.
Table 2 shows the energy reduction from taking the next stage of the transition, dumping gas as a source of heating and moving to heat pump technology. Here is can be seen that the cumulative annual reduction in energy demand for our household is around 62%, again due to the grossly superior efficiency of this technology.
Achieving this scale of reduction from our typical home, of over 14,000kWh of energy each year that is presently obtained from fossil-fuels, would be impressive especially if it could be replicated throughout the UK. The reduction in carbon emissions would be equally impressive, but not fully realisable until the electricity grid is powered solely from renewable and nuclear energy sources, if that is possible.
The downside of course is that the majority of renewable electricity generation is intermittent in nature and in the case of wind energy and solar, difficult to predict precisely. This could lead to a supply-demand imbalance – something the National Grid Electricity System Operator (NGESO) would not allow to happen.
5 Impact on our all-electric household
Our typical 2-3 bedroom household has now gone from being 11.8% electric to 100% electric and in doing so has reduced their total energy consumption from 22,794kWh each year to 8647kWh – impressive to say the least.
Reduced environmental pollution from carbon dioxide emissions will naturally follow this move to electricity for our energy needs, but at what cost to our typical household? Will making this transition make living in an all-electric household cheaper or more expensive? This is what we intend to explore next.
5.1 Motoring cost for an EV
In this analysis we will ignore the fact that EV’s are currently more expensive than their ICE counterpart. We will only consider the variable cost difference in commuting 7400 miles each year using either form of transport. And because this cost difference will change over time as electricity or petrol prices change let’s take one period, January 2024, as our point of reference.
The Automobile Association (AA) publish fuel prices each month with the UK petrol price returning an average for January 2024 of 139.8p/l, although regional variations exist [40]. If we use this average figure along with the consumption of 886l of petrol from section 2.1 it is possible to calculate that it costs our 2-3 bedroom household £1238.63 each year in fuel.
Having now swapped our petrol ICE vehicle for an EV let’s try and compare the equivalent motoring cost based on the 2114kWh required each year as calculated in Chapter 3. Before we begin though we should acknowledge that this is the difficult part of the exercise because EV charging costs vary depending on what form of charging is used, where it is used and when it is used.
For instance, let’s take the case where our typical 2-3 bedroom home has a driveway - possibly even a garage. In this instance it is possible to install a 7kW smart charger and charge using a low-cost over-night tariff [41, 42, 43, 44]. However, as a recent survey [45] found, 69% of EV owners live in urban or suburban areas so we will assume our new EV owner does too. In these areas many homes do not have the luxury of off-street parking, with Parking Review [46] citing 60% of UK homes in towns and cities fall into this category while a more in-depth review of 7142 properties for sale in 20 city areas by Nationwide Vehicle Contracts [47] finds on average only 47.9% have a driveway. This figure is even lower if London Boroughs are included where, for example, Havering is the highest at 20% while in Wandsworth, Lambeth and Newham homes with driveways represents only 1% of house sales.
We can conclude, therefore, that the majority of EV owners in UK towns and city areas require on-street parking and as such need access to the public charging network, and here we will assume, on the basis of probability, that our typical 2-3 bedroom household falls into this category simply because they are less likely to have a driveway.
For the basis of this exercise we will also consider that our EV driver uses a fast charge facility (rated at less than 50kW) for 80% of their annual charge of 2114kWh and a rapid charge facility (50kW and above) for the remainder. Zapmap [48] calculates a weighted average for EV charging costs based on one million charging sessions each month. The costs for January 2024 were: £0.56/kWh for fast charge and £0.80 for rapid charge.
Computing these values suggests our typical EV homeowner spends in the region of £1265 each year on electricity ((£0.56 x 0.8 x 2114) + (£0.8 x 0.2 x 2114)), which is similar to our ICE driver who spends around £1239 on petrol. The advantage only swings in favour of the EV driver when charging at home becomes an option, allowing our homeowner to make use of low overnight tariffs that can vary from 7-9p/kWh. In such a case our EV homeowner’s electricity cost, using the 80% : 20% scenario above (by this we mean 80% low overnight tariff at say 8p/kWh and 20% using a rapid charger at 80p/kWh) would fall to £473 each year – roughly one third of the cost of petrol or the use of public charging points.
5.2 Comparative running cost for an ASHP
Looking now at the comparative cost for switching from gas to heat pump technology is a little more straight forward as Ofgem [49] publish standard variable rate tariffs. We can see here that on average, during the price cap period in January-March 2024, for consumers paying by direct debit, the price for a unit (kWh) of electricity is £0.2832 whereas the same unit of gas costs £0.0742. Electricity is therefore almost 400% more expensive than gas per unit.
With these rates we can easily compute the cost of our previous gas consumption of 11,500kWh against that of the electricity needed to power our replacement ASHP, of 3833kWh. The answer becomes: gas costs £853.30 a year whereas the equivalent electricity cost is £1085.50. We can see from this that it costs over £200 more each year for our typical 2-3 bedroom household to provide heating and domestic hot water with an ASHP than it did from gas.
5.3 The overall impact for our all-electric household
Having now done the maths, albeit with a few necessary assumptions, it is possible to examine the overall impact on our typical 2-3 bedroom household.
Table 3 shows the results of this analysis as it impacts on the overall cost of annual energy for lighting, heating, domestic hot water and motoring before and after making the switch to an all-electric household.
What becomes obvious from Table 3, based on our roughly 100m2, pre 1980’s home that has limited insulation and a wet heating system designed for a gas boiler along with no driveway to facilitate a smart EV charge point, is that the transition to all-electric living is likely to lead to an increased annual energy cost in the order of 10% based on January-March 2024 energy prices. There is scope, however, to reduce this difference if our homeowner becomes disconnected from the gas main, which would eliminate the standing charge of £108.41 a year [49].
There is also the advantage arising from EV ownership from no annual road tax, or Vehicle Excise Duty (VED), although this advantage will be lost from 1 April 2025 when VED on EV’s is introduced [50]. But for our typical homeowner contemplating moving to an EV one annual revenue cost not to be forgotten is car insurance, which is said to be 25.5% more expensive compared with the ICE equivalent [51, 52].
One further revenue cost disadvantage for our homeowner moving to an all-electric household is the annual maintenance of an ASHP compared to the gas boiler it replaces. Checkatrade [53, 54] data shows the average annual maintenance cost for an ASHP is double that for a gas boiler, and even greater for an oil boiler. Furthermore, repair costs for an ASHP can be as high as £2000 if a replacement compressor is required, or £500 if a fan fails.
Table 3 Comparison of annual energy cost in becoming an all-electric household
Cost of electricity (£) | Cost of gas (£) | Cost of motoring (£) | Total annual cost (£) | |
Before | 764.64 | 853.30 | 1238.63 | 2856.57 |
After | 1850.80 | -------- | 1265.00 | 3115.80 |
6 Discussion
The way we choose to live our lives – from the use of gas to heat our homes and the majority who choose ICE vehicles to satisfy their transport needs, contributes greatly to UK emissions with 17% of all CO2emissions arising from the residential sector, mainly from gas heating, and 34% from transport, partly from the motoring public. It is not surprising, therefore, that Government has moved to encourage the phasing out of gas boilers for new homes and enforced the transition to EV’s in place of new ICE’s in the near future.
This paper attempts to evaluate what this all-electric future – with the deployment of heat pumps and EV’s, might look like for the typical homeowner, and what the relative cost might be to operate such technology on an annual basis. Any discussion of the capital costs involved are excluded although suffice to say the main reason given for those homeowners reluctant to adopting such a move primarily comes down to one of cost.
What this examination highlights, using current energy prices and based on a typical pre-1980’s home of around 100m2 floor area with 2-3 bedrooms and no facility for off-street parking, is:
1 The cost of motoring remains largely similar, if not marginally higher, by moving to an EV. This arises from the higher unit cost from the use of public charge points along with inflated insurance premiums. The introduction of VED for EV’s in 2015 will further increase this margin.
Leaving aside any concern potential EV drivers may have for range anxiety and the availability of public charge points, key to the adoption of EV’s from an economic viewpoint is the ability to charge at home using a smart charging facility to access off-peak rates. Doing so would save our typical homeowner around 2/3 of the cost of off-street charging.
2 Heat pump technology has a clear advantage over gas due to efficiency, from around 90% to 300%, or higher. However, air-water heat pumps require considerable space for technology inside a property, they are most efficient with well insulated building fabric and where the wet heating system is optimised for operation from a low-temperature heating source. Consequently the risk is that inappropriate technology selection and deployment may fail to adequately heat a home or alternatively lead to unwanted breakdowns – which are more costly to repair vs. their gas boiler equivalent.
However, when correctly chosen and applied to suitable building fabrics evidence suggests they work well. The downside, if there is a downside to effectively obtaining energy for free, is that the ratio of the domestic cost of electricity to gas per kWh in Q1 2024 is around 4:1 whereas the typical operational efficiency ratio (SCoP) of a heat pump for heating and domestic hot water is closer to 3:1 averaged over a 12-month period. This ratio becomes the ‘Achilles Heel’ for ASHP’s in terms of retrofitting gas boilers on a UK wide scale.
3 The argument for the all-electric home comes down to one of economics. Leaving aside the issue of capital cost and return on investment (ROI) – an important issue in its own right, the driving force behind the voluntary adoption of both EV’s and heat pump technology for many households comes down to a question of running cost – will it save money each year or will it cost more?.
If a unit of electricity were to half or quarter in price with retail gas price fixed, or alternatively if a unit of gas were to double, or even quadruple in price and electricity prices remaining fixed that would provide the signalling and motivation for households to contemplate the switch from fossil-fuels to more environmentally friendly renewable energy technology – the all-electric home.
What we have instead, and this has emerged from our analysis, is that in our typical 2-3 bedroom household electricity comprises almost 12% of total annual energy consumption. By eliminating petrol and moving to an EV total energy consumption falls by 28% while electricity rises in proportion to 29.5%. When our household takes the final step and becomes all-electric total energy falls by 62% in which electricity now provides the remaining 38%. The observation here, which our typical householder is bound to notice in time, is that for a larger total annual expenditure on energy – electricity, they are only consuming 38% of the energy they did previously. Put another way, the cost of our typical household energy per unit has suddenly become over 260% more expensive!
7 Conclusions
By making use of Ofgem’s average energy consumption for a typical 2-3 bedroom property along with the state of GB’s current housing stock, and several assumptions, this paper has shown how technology, in the form of EV’s for transport and heat pumps for heating and domestic hot water, has the ability to reduce total energy consumption for our household by 62% due to superior operational efficiency over conventional and long-established fossil-fuels based technology.
Exploiting the superior capability of this technology on a wider scale would make a worthy contribution to help reduce UK territorial CO2 emissions and would align with the pathway several European countries, such as Norway, Sweden, Finland and Estonia, have already taken and in which they are leading the way as exemplars.
Achieving this transition, however, presents a dilemma for decision makers: how and whether, as rational beings – in the main, can the population, in sufficient numbers, be persuaded to adopt this technology when a) the cost of energy/unit forsaken (gas and petroleum) is much cheaper than the replacement energy/unit (electricity) even though the benefit (warmth of homes, sufficient hot water and miles driven) along with the annual cost of consumption remain more or less the same, and b) the cost of implementing this new technology comes at a premium over purchasing a replacement gas boiler and an ICE vehicle? Is this question the ‘elephant in the room’ that politicians and those pushing the renewable agenda would rather not address?
For some individuals, however, this dilemma may not arise. The emotional-being will see it as an opportunity to help save the planet regardless of the economic argument. For some, whose homes are already more suited to adopting heat pump technology – where SCoP’s rise to 4.0 and above, and who are able to charge an EV using low-cost, overnight tariff’s then the overall annual cost of energy will fall and the economic, rational argument may prevail.
For many, perhaps even the majority, who live in inadequately insulated homes or without access to off-street parking these technologies may prove inappropriate, resulting, if forced to adopt such measures, in colder homes, increased fuel poverty and higher annual bills.
Retrofitting these existing homes with heat pump technology will remain a challenge. Poor standards of building fabric along with low-temperature heating do not make for happy bedfellows.
The real incentive – the opportunity, for the rational-being to adopt this technology and for Government to signal the importance of this happening arises from the prospect new-build properties present, and the conclusion here is that building standards for future housebuilding do not go far enough.
For ASHP technology to work effectively not only should insulation standards be increased to match Scandinavian equivalents, houses should be designed with underfloor heating as the de facto standard, and in an attempt to maximise the SCoP of heat pump technology - to values of 5.0, 6.0, 7.0 or greater, these new homes require to be equipped with solar panels to reduce the effective electricity input (this behind-the-meter generation is not recorded as consumption) and solar thermal for domestic hot water to further maximise the efficiency by avoiding the need for supplementary electrical heating. To complement this package these new homes need battery energy storage to allow excess PV generation to be used throughout the day to further reduce metered electricity consumption.
Achieving this step change may be difficult - with likely resistance from housebuilding developers on several fronts – from construction delays due to the lack of suitably trained, qualified and competent heat pump installers in sufficient numbers to meet the demand from these new homes and the ability of supply chains to meet the demand for heat pump technology, to the additional cost of these new built homes. On the other side of the coin, the prize - the marketing proposition is – homes with the lowest carbon footprint and where energy comes free, or close to free.
Finally, and as a rider to these conclusions it would be remiss not to at least lay down a future marker against the ‘elephant in the room’ question, which is: can we realistically expect an all-electric future when historically the pre-tax UK domestic electricity price per unit has been four times higher than gas since 1979 [55], when in two of the heat pump exemplar countries, Sweden and Estonia electricity and gas are closer to parity, and where in Norway and Finland there is no competition from gas [56]? A similar disparity could also be levelled against UK electricity and petrol unit energy prices where, since 2013 onwards, the price ratio has gradually increased to around 2:1 by 2022 [57].
Foundational to this question lies a further, deeper, question. Why, if the wholesale price of electricity (which helps set the retail price of electricity, including much of the lowest marginal cost electricity generated from wind and solar) is so dependent on gas power plants with high marginal costs is the domestic standard variable rate of gas, a commodity the world is keen to eliminate, only a small fraction of the standard variable rate of domestic electricity?
One further point, perhaps the subject for a future paper, is how our typical household feels about depending on a single source of energy, electricity, when compared to the present diverse supply that includes gas and petroleum in which the loss of one source does not mean the loss of all sources, and where the UK is able to store sufficient gas to supply the population for several weeks compared to electricity alone where energy storage capacity is minimal and at best a limited supply may last for a few hours. Fortunately such events are rare, but in an all-electric future powered solely by part nuclear and mainly by intermittent wind and solar generation the consequences of such a rare event would be difficult to comprehend.
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