Energycount | Dorset Sustainability & Energy Consultants | SAP Calculations | Part L Reports
Energycount | Dorset Sustainability & Energy Consultants | SAP Calculations | Part L Reports
Energycount | Dorset Sustainability & Energy Consultants | SAP Calculations | Part L Reports
Energy & Sustainability Reports to help you
comply with Building Regulations and Planning requirements.

Fuel Choice & Fuel Factor

How does my fuel choice affect the SAP calculations?

Or "Can I pass with electric heating?" is a common question we are asked...

The primary compliance target in Part L1a of the building regs is based on carbon emissions (CO2) and is referred to as DER/TER. DER (Dwelling Emission Rate) must be equal to or lower than the TER (Target Emission Rate) for the SAP calculation to pass. Both DER & TER are measured in kg CO2/m² (kilograms of CO2 per m² of floor area).

The fuel type used to provide space & water heating affects the CO2 emissions of the dwelling. If electricity or oil are used to provide space and/or water heating, the emissions of the heating will increase compared to using mains gas.

The target emissions in SAP are based on using mains gas. Oil has 1.38 times the emissions of gas, so using oil will increase your heating related emissions by 138%. The table below shows the CO2 emissions and fuel factors used in SAP for the most common fuel sources used in new dwellings.



CO2 Emissions

kgCO2 per kWh


Fuel Factor


Mains Gas









Heating oil



Wood logs



Wood pellets



Multi- fuel appliance



[1] Table 12 in SAP 2012: The Government’s Standard Assessment Procedure for Energy Rating of Dwellings

[2] Table 1 in L1A Conservation of Fuel and Power in New Dwellings 2013


The table shows CO2 emissions and the applied fuel factor used in SAP for each fuel source. You may notice that the fuel factor is not always a direct measure of how many more emissions are emitted for any given fuel. This is a way for SAP to encourage certain fuel sources over others. A good example of this was way back in the days of SAP 2005. Back then under 2006 regs it was possible to achieve a better DER/TER result if heating oil was used to fuel a boiler rather than mains gas. The fuel factor for oil was such that it was actually lower than for gas. In newer updates to SAP however, heating oil has a fuel factor that now penalises it by bringing the fuel factor closer to the actual emissions represented.

So, where does this leave electric & oil as fuel sources in SAP?

Electricity has around 2.4 times the emissions of mains gas, therefore using electricity for the space heating will result in the CO2 emissions for the dwelling to be more than double that compared to burning gas. However, like many things, it is a bit more complicated than that. Electric heaters are deemed to be 100% efficient in SAP, whereas even the most efficient gas boilers are only around 90% efficient. The greater efficiency of electric heating results in the emissions being closer to double that of mains gas.


This is where the fuel factor comes in

When SAP calculates the DER (Dwelling Emission Rate) & TER (Target Emission Rate) it applies the fuel factor depending on what fuel is used for heating. For gas the fuel factor is 1.

Where other fuel sources are used the fuel factor for that particular fuel is used when calculating both the DER and TER in SAP, effectively increasing them both (as nowadays no fuel source has a lower fuel factor than gas, i.e. 1). So, in theory at least, increasing the TER with a fuel factor should make the SAP calc easier to pass when using other fuel sources as effectively to goal posts have been widened..

However, it is not quite that straightforward. When SAP applies a fuel factor it increases the DER as well as the TER, thus it starts to 'level up' the emissions of the dwelling again. Then there is the actual CO2 emissions to consider, where not truly reflected by the fuel factor. Using electricity as an example - when using electricty heating emissions increase by 240% (2.4 times that of gas), but the fuel factor only increases the heating emissions in the TER by 155% (1.55 times), so there will need to be additional CO2 emission-saving measures in order for the SAP calcs to pass, even though electric heating is deemed to be 100% efficient..


So, why do heat pumps work so well in SAP?

Heat pumps use electricity for their fuel source but are designed to extract heat energy from their surroundings. The electrical energy put into a heat pump is usually a fraction of the heat energy returned, which in turn is used for space and water heating. A typical ASHP (Air Source Heat Pump) is around 250-300% efficient, meaning that for every 1kW of electrical energy used 25-3kW of heat energy is produced. Similarly, a GSHP (Ground Source Heat Pump) is usually between 300 & 500% efficient.

Therefore, in SAP the efficiencies of a heat pump exceeds the increased CO2 emissions applied to the TER (due to the fuel factor of 1.55). The DER when using a heat pump usually becomes around half of the TER, thus achieving an easy pass in SAP.


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