A major change in SAP 10 is the introduction of a lower carbon factor for grid electricity that will lower the regulatory bar to installing electrical heat sources, relative to other fuels. This includes both heat pumps and direct-electric heaters.

Is this a good idea? We take a closer look at the implications of this significant policy shift.

SAP is a building energy and carbon model used to demonstrate compliance with Part L (conservation of fuel and power) of the Building Regulations in the UK. To pass Part L, the carbon emissions calculated for a proposed design must be lower than the emissions from a ‘notional’ building with the same geometry but a standardised specification. The fuels (including electricity) used to deliver heat to the house are weighted using a ‘carbon emissions factor’ which represents the contribution to climate change that using one unit of each different type of fuel makes. Heating fuel selection is therefore of great importance in terms of both ensuring compliance, and to achieving low-carbon homes that minimise our contribution to climate change.

In the last ten years the way the UK generates electricity has shifted significantly. Although less on-shore wind is being built, off-shore is ramping up and solar PV has come to prominence via a combination of small roof top systems and large scale ground mounted arrays. Our aging coal-fired power stations are being phased out, mostly replaced with lower-carbon gas plant. This graph by our friends at Etude tells a compelling story, and of course, something that should be celebrated and continued.

This shows that in both cases, the annual average grid emission factor is slightly higher than the reported values, particularly if we project forward a few years (SAP10 is the first update since SAP2012, which came into force in 2014, under Part L1a 2013. Yes, I know…).

In contrast, the laws of physics and chemistry suggest that if you burnt a litre of Natural Gas or Oil in 1970, it would release the same amount of CO2 if you burnt it today. However, the gas CO2 factors has also been reduced slightly in SAP 10. So it seems reasonable to update SAP, which is in effect the government’s main tool for controlling the carbon emissions of new dwellings, to reflect this happy trend.

A blunt instrument

This rosy picture changes however, if we dig a little deeper. For example, it may be important to consider how the grid carbon factor varies across the year, and across each day. This graph from Ofgem shows that domestic electricity consumption is about a third higher in the winter than the summer – this will be partly down to lighting during the shorter days for example, but it seems reasonable to suggest that much of this uplift will be due to existing electric heating.

Compare that graph with the following one, which shows that the fuel mix has a significant seasonal variation. It is coal, and to a lesser extent gas, that help meet the extra demand during the winter.

This suggests that when it comes to space heating, it is not accurate to use a simple annual average of the grid factor to represent the CO2 impact of space heating, as the factor is higher exactly when most heating is required. The shape of the domestic electricity consumption curve closely resembles the shape of the coal section of the fuel source graph. This suggests that much of the extra electricity use in our homes over the winter is in fact coal-fired – one of the worst carbon emitters when it comes to electricity generation. Its probably a bit strong to say that when you switch on your electric heater, you’re burning coal, but it’s certainly more carbon intensive than say running your washing machine in the middle of a sunny summer day.

We’ve also run a quick simple analysis of the daily pattern. We took the following data from carbonintensity.org, for the period 24/02/2018 to 04/03/2018, otherwise know as The Beast From The East.

Plotting the data does suggest there is a daily cycle underlying some other trends, but it is only when we separated out the peak and off-peak* data that the extent of this can be seen. During this period, electricity was 54% more carbon intensive at peak times than off-peak.

 *Off-peak assumed to be 13:30-16:00 and 23:00-07:00

So perhaps this number is not as accurate as it could be. There is an argument that it doesn’t even need to be accurate. A total of five different forward projections are presented on the graph above, showing a variety of levels that we may, or may not achieve across the next thirty years. As they say, prediction is hard, especially about the future. But perhaps all of this misses a broader point: the role of this figure in government policy.

Is it a little grand to call this tweak to a tiny number, buried in the back of some spreadsheets and specialist software a significant policy shift? This tiny number is effectively an important weighting factor that drives how house builders will choose to deliver heat into the dwellings they build. It’s therefore not simply a figure that may or may not need to be ‘accurate’ to reflect reality, it’s an important policy tool, and should be set according to how we as a country want our housing stock to function. It must of course make reference to the reality of what happens on the grid, but it must also consider a plethora of other factors, not least the interaction with other parameters of the SAP model, the cost of heating to home owners and occupiers, and even the resulting comfort levels in our buildings.

Last week, the CCC have published a report, a key finding of which is that “emissions reductions from the UK’s 29 million homes have stalled, while energy use in homes – which accounts for 14% of total UK emissions – increased between 2016 and 2017.” This issue is not small fry.

Show me the money

An important aspect that the carbon factor is blind to is the cost of energy. Generally, the people or organisations that pay to build our homes, are not the same people who pay the operation energy bills. They therefore have no interest beyond compliance with how much a system will cost to run, or a home to heat. EPCs should in theory stimulate home buyers or renters to select for running costs, but the reality is that most buyers or renters either have no choice, or prioritise other factors above running costs, such as proximity to schools. A heating system comprising of electric panels or electric UFH will clearly be significantly cheaper to install than a radiator or wet UFH system with a gas boiler, so it seems likely that given the opportunity, developers will opt for the former. However, the house market being what it is, we would question whether this saving will get passed on to home buyers. Heat pumps will also be a much more favourable option from a carbon point of view, given they run on electricity. The way they leverage the input energy makes them a great option from a carbon-saving point of view in light of the greatly improved gird factor (assuming they achieve a good SCOP), but even if costs can be reduced to come closer to gas boilers, they will never compete with direct electric heating.

As a rule of thumb, electricity is roughly three to four times the price of gas. About ten years ago, it was also about three times the carbon intensity. Although the carbon ratio has improved significantly, the price ratios has remained relatively constant. The following graph shows retail gas and electricity prices for 2002 – 2017, as published by BEIS.

We are therefore concerned that the rush to panel heaters that the reduced SAP10 grid emissions factor may unlock, will have serious implications on the cost of heating for owners of new homes built to that standard.


This change within SAP will make it more likely that direct electric heating will be installed in new UK homes when the calculation comes into force. (It already has under the London Plan, but you still have to demonstrate Part L compliance via SAP 2012 as well – for now at least). In isolation, this is a questionable move. The carbon factors may not be accurate, so unless coupled with other enhancements we may end up with more carbon intensive homes than we expected – we are of course used to this phenomenon, known as the performance gap. On the other hand, the carbon factor is a moving target that is very hard to hit – it changes across the seasons, over each day, and with the varying contributions of renewable electricity sources.

There are, however, other major policy-level implications. The impact on the grid could be significant; as homes built to the standard are rolled out, the winter and peak-time loading of the electricity grid may increase significantly to supply this electric heating. Its easy to forget the costs of upgrading an maintaining the infrastructure. It could increase the cost of heating for consumers, and fail to achieve the real-word carbon performance hoped for because the carbon factor tends to be worse, exactly when we want to heat our homes.

This blog presents a heavily simplified analysis – we’ve not addressed issues such as the phase-out of coal stations and what will replace them, the relationship between gas fired power stations and renewables, the impact of other major new demands on the grid such as electric vehicles, or the role of short term and long term storage, either centralised or decentralised. But there are issues with encouraging electric heating, particularly direct electric, that need to be considered, and we worry that this policy change has not taken everything into account.

There are, of course, significant benefits to direct electric heating. It is cheap and simple to install. There are no moving parts, so its silent and reliable. It’s simple to control, and when its off, doesn’t dump any waste heat into our homes, so could be a factor in reducing summer overheating risk. It doesn’t generate particulates or pollutants at the point of use (but they are still created by some types of power station).

Can we have our cake and eat it? Well, maybe. If we can get the building fabric right, and drive down the space heat demand, there is a much stronger case for direct electric. Considering, for example the uplift in cost from gas to electric, three times a small amount of money is still a fairly small amount, but three times a bigger amount is, well… more! High performance building fabric also tends to increase thermal comfort, and I’ve yet to meet anyone who isn’t in favour of that. It also future-proofs the building – if an even better heating technology comes along in 20 years, a building with a low heating demand is likely to be better placed to incorporate it.

So this innocuous number affects much more than just how house builders meet their legal obligations but, I have yet to find a Policy Impact Assessment that shows how the decision to adjust the carbon factor will affect UK carbon emissions, as well as other areas; do get in touch if you know something I don’t.

Other blogs for reference:








How Carbon intensive is it to use electric heating?






Recommended Posts