Sunday, 20 February 2022

Estimating carbon savings for switching to EV or heat pump

Suppose you want to invest some cash in reducing your personal carbon emissions - can you save more carbon by replacing your car with an EV or by swapping your boiler with a heat pump (and how much would this cost?). Unfortunately, the answer depends on lots of variables such as how far you drive and how much is your heating bill, do you have solar panels – if so how much power are you currently exporting? So I have prepared a tool to help you estimate your savings and in this post I use it to illustrate some examples. You probably know that energy costs for an EV are lower than for a diesel or petrol car. You may be surprised that the heat pump can reduce your energy bills too, using prices from April after the cap changes.

Click here for the EV tool and here for the heat pump one. These are only estimates! If you find them useful let me know.

The tools have default electricity and gas prices estimates based on the OFGEM price caps from April. (Electricity is 29p/kWh or 16p/kWh off-peak, gas is 7.5p/kWh and 30p/day for the standing charge. You can adjust these to reflect your tariff.

Carbon savings for buying an EV, default assumptions

For the EV case, in terms of the carbon savings, the main parameters are how far you drive each year, the efficiency of the vehicles, and the embodied carbon of the vehicles – the emissions from making it before you get to use it. The tool starts with default values that you can adjust. The defaults for the EV are 8000 miles/year driven in a typical diesel. This chart shows the carbon break-even point comes at about four years. The discontinuity in the blue line is from replacing a diesel car with a new diesel after five years. You can change when this happens. If you move it to the start then break-even occurs before year two.

Default parameters: 8000 miles driven, buy a new EV now or a new diesel after 5 years.

If you have a petrol car rather than a diesel, the savings are a little more and the break-even is earlier because the petrol car is less efficient.

If you drive less, the savings are less and carbon payback takes longer.

A lot of my friends drive a lot less than 8000 miles/year, (both to reduce emissions and also because the traffic congestion in the Cambridge area is terrible). Suppose you only drive 2000 miles each year. The break-even point is now around year eight. If you only drive 1000 miles per year it is not until after year fifteen.

2000 miles driven, buy a new EV now or a new diesel after 5 years.

The problem is that the less you drive, the less the annual carbon savings so the longer it takes to pay back the embodied carbon. You can reduce this by having a smaller car with less embodied emissions – or by buying second hand.

Buying secondhand shortens the payback time, depending on how you count the embodied emissions.

If you buy second hand, how much of the embodied emissions should you take into account? The simplest approach is to say none – they happened long ago and this is now ‘sunk’ carbon. Then you make carbon savings from year 0. If you would have bought a second hand diesel car too, this is what the chart looks like (2000 miles per year).

2000 miles driven, buying second hand with no embodied emissions

An alternative approach would take into account how much of the car’s lifetime you have used up. You could argue that if you take the car after 5,000 miles of a 10,000 lifetime, then half of the embodied emissions are yours. In that case you should adjust the embodied emissions factor down by a half and the break-even point is back to 5 years. This is not unreasonable, considering that the person selling you their car probably bought it with the intention of selling it on after a few years, and if no-one did this there would not be any second hand cars to buy.

2000 miles driven, buying second hand with half the embodied emissions

By default, adding in solar panels makes no difference to the carbon emissions.

Now what if you have solar panels and you use these to charge your car? By default, this tool takes the view that power from your solar panels is part of the grid and so using your own power makes no difference to overall carbon emissions. However you may prefer to think of power from your panels as zero emissions. This makes some sense if you bought your panels with charging your EV specifically in mind and otherwise would not have had them installed. If you tick the box ‘Treat solar power as zero emissions’, you will find the carbon emissions drop and the green line is even flatter than before. However, the embodied emissions are unchanged.

Using your solar panels for charging does save money.

Whichever way you count the carbon, using power from your own solar panels saves you money, though perhaps less than you might think. The default assumptions are that you drive 8000 miles/year requiring 2400 kWh/year. By charging overnight at a cheap rate 16p/kWh (an estimate for prices from April) you only pay £380/year for energy. (You may be able to get even lower than this with a special EV tariff – you can adjust the prices if you like). Anyway, the default solar assumptions are a 4 kWp array delivering 3200 kWh/year, of which you already use about a third. The remainder is available for your car, (using a smart charger) saving you approximately £270/year, or £4,000 over 15 years.

Default assumptions (8000 miles driven) plus solar panels.

The default assumption for the EV tool is that all of your current PV export would be available for charging - you would have a smart charger such as those listed by Naked Solar here. However, if you expect to charge away from home some of the time, or your car will be not be at home when the PV power is available, you can specify a lower proportion of charging from the panels.

To estimate your heat pump savings you need to know your heating bill.

The heat pump tool is similar. You should check your bills to see how many kWh of gas you use each year or litres of oil for an oil boiler. Here are the carbon and cost charts for default assumptions. The break even point for carbon is only about 2 years in.

Heat pump replacing a boiler, default assumptions (12,000 kWh gas/year).

In the typical case, a heat pump costs more to install but your annual bill is hardly changed.

The default cost assumption for the heat pump installation is £5,000 allowing for another £5,000 from the Boiler Upgrade Grant that should replace the RHI in a few months. Even with the grant this is significantly more than a boiler replacement. However the annual costs are slightly lower than before (£950/year, down from £1,010) because the efficiency of the heat pump makes up for the higher price compared to gas. 

Heat pump costs under default assumptions.

In the cost calculation, the default assumption is that you will disconnect completely from gas so you no longer have to pay the standing charge for gas. If you decide to stay on gas, then the annual bill is £50 higher than before. 

You will not get a large fraction of your heating from solar panels - but even so they can reduce your bills.

The calculations for solar panels are more complicated because they must take into account variable heating demand and solar generation through the year. You may be surprised at how little you can reduce your bills using the PV array. Under default assumptions, you may only get around 17% of your heating electricity from the panels. This improves if you use less energy for heating or if you have a battery. However your battery can only store energy over a day or two, not from summer to winter. Unless you have a very large array you will not get much of your winter heating from the solar power. 

Still, every little helps when it comes to the costs. Even under default assumptions with a 4 kWp array and typical energy demand your annual heating bill reduces by £220 compared to the gas bill (including the standing charge saving). However this estimate is more uncertain than the rest. A better calculation would require at least your smart meter data to get a better understanding of your current pattern of electricity use.


  • Getting an EV saves lots of carbon and saves money too if you drive the typical 8000 miles/year.
  • If you drive a lot less, your savings (carbon and £) will be much reduced unless you get a second hand car instead of a new one. There are choices in how you count the embodied carbon for a second hand car.
  • If you have solar panels you can reduce your charging costs through much of the year.
  • Switching to a heat pump saves you carbon under practically all circumstances.
  • With default assumptions your annual heating bill decreases slightly. This change is helped by disconnecting from gas as you do not have to pay the standing charge. 
  • If you keep your gas connection then your bill will probably increase slightly.
  • Solar PV panels also reduce your solar heating costs but not as much as with the EV as there is little demand for heat in the summer when you have most solar power.

Do try the tools yourself: EV tool and heating tool.

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