Your carbon footprint depends on how much energy you use, when you use it and what the source of that energy is. Fossil fuels like oil, gas and propane are high carbon and always will be, but electricity is getting greener and greener over time. The less energy you use, and in particular the less gas you use, the lower your carbon footprint will be.
Moving electricity demand to times of the day when the national grid is 'cleaner' or selling clean energy back to the grid (e.g. from solar panels) will also reduce your footprint.
First we figure out how much gas and electricity your home uses. If you have a Sero BEE then we can measure that energy use directly and give a more accurate answer. If not, then we use survey data to build a granular energy model of your home that includes all energy uses, from heating and hot water to lighting and plug loads.
The only thing we don't include is any electricity used for charging an electric car, as we class that as part of your transport emissions rather than your home emissions. We model your home's energy use at 15 minute intervals so we can accurately predict how much of your solar you can use directly rather than selling back to the grid, and how much you can save by charging your battery when prices are low.
Your energy use will decrease and energy use patterns will change as you progress along your pathway. We use our energy model of your home to estimate the impact of each measure in your pathway on your energy use. If your pathway includes installing solar PV then we estimate how much electricity this will generate and at what time of day, and account for how much of this you will use yourself vs. selling back to the grid. If your pathway also includes a battery then we account for the fact that you will be able to use more of your solar generation yourself and shift some of your energy demand to cheaper/lower carbon times of the day.
Once we understand how much energy your home uses we apply carbon intensities which describe how much carbon is emitted for each kWh of energy you use.
For gas that carbon intensity is relatively fixed because it is predominantly a result of the chemistry of natural gas. For electricity the carbon intensity depends on what is used to generate that electricity. If electricity were produced from coal fired power stations only then it would be very high. If it were produced from 100% renewables or nuclear then it would be zero. In the UK electricity is already lower carbon than gas and it's getting cleaner very quickly as we connect more and more renewables to the grid.
We use data on the current carbon intensity of electricity, and estimates from National Grid to predict how that is expected to change into the future. We also account for the fact that the carbon intensity changes throughout the day, with lower demand times typically having a lower carbon intensity than high demand times. We account for this continuous fluctuation so that we can give you the best estimate of your carbon footprint, and so that we can help you understand how shifting your energy use can allow you to use lower carbon electricity.
How do we forecast electricity emission factors?
We started with National Grid's 'Consumer Transformation' scenario from their Future Energy Scenarios which you can find here. However real emissions have been above this scenario for the last few years. As a result we scaled the National Grid estimates to match real values now and to then gradually trend back towards the National Grid forecast, eventually rejoining it in 2040. The resulting estimates are shown on the graph below, with historical data and oil and gas carbon intensity data included for reference.
So we can account for the granular hour-by-hour fluctuations of electric carbon intensity into the future, we use estimates from the Active Building Centre which are based on the expected mix of energy sources used to generate grid electricity over time.