Sunday, 5 February 2017

What difference will electric cars make to our electricity demand

Our UK electricity generation capacity margin is getting thinner and thinner and I frequently hear of PV projects cancelled or curtailed for lack of grid capacity. Will electric cars, promoted as being kind to the planet, tip us over the edge? In this post I have done some simple calculation as to the impact on electricity demand as we progressively switch over to electric cars.

National Grid estimates up to 7% increase in electricity demand by 2040 for EVs
There are around 30 million cars on the road today [1]. National Grid's Future Energy Scenarios 2016 presents 4 scenarios - in the most ambitious 'Gone Green' scenario there are 9.7 million electric vehicles (EV) on the roads in 2040 using an extra 24 TWh/year. Relative to current electricity demand that is an increase of 7%[2].

But the impact on peak demand will be minor, if drivers respond to time of use pricing.
However, this demand is highly unlikely to be spread evenly through the day and year. Firstly, people tend to drive more in the summer, presumably because it is considerably more pleasant than driving in the winter [3]. Fortunately our peak electricity demand is in the winter so this helps to even out the load.

Secondly, assuming time of use pricing is widely adopted drivers will try quite hard to charge their vehicles at times of the day when the electricity is cheap. Most people will have quite a lot of flexibility and during the peak time a lot of drivers are on the road anyway. This means electric car charging will increase non-peak demand much more than peak. The chart below shows a typical January day. If electric car charging is spread over non-peak times it will start to fill in the yellow area but not impact on the peak demand. 24 TWh/year is 66 GWh/day so it could fill in just over a quarter of the yellow area.
A typical day's UK electricity demand in January

If all cars were electric there would be more impact on peak demand
However, 66 TWh/day is from only 9.7 million electric vehicles. If we replaced all 30 million cars, it would be nearer 200 GWh/day. This does still fit into the yellow area - just - but it would be quite a challenge to ensure that there was no 'leakage' and it did not impact on the winter peak at all.

Peak demand drives requirements for generation capacity and grid strength
Why is the peak so important? Peak demand is critical because it determines how much total generation capacity we need and also how 'strong' the grid has to be to take it. If it is strong enough to handle 48 GW at peak times then it is also strong enough to handle anything less.

Electric charging could overload the distribution grid in some places.
However, there still could be problems with the distribution grid in local areas. It is weakness in the distribution grid that means PV projects have to be curtailed. Local generation is a particular problem for the distribution system because it tends to increase voltage whereas the grid was originally designed to handle loads that reduce voltage (see 'How much is too much PV'). EV charging is a conventional load so easier to handle. However, EV chargers are power hungry (especially fast chargers) and that could be a problem if there were many in a small area.

If people charge their cars using PV power that reduces the load on the grid from both the PV and the charging!

Electric heating is going to be a much bigger headache for the grid than electric cars
Unfortunately there is another big factor contributing to electricity demand in the 'Gone Green' scenario and that is heat pumps for heating our buildings. We need to switch away from gas for heating to reduce carbon emissions and the best alternative in many cases is electric heat pumps. Unfortunately, this heating demand is greatest in winter and is much harder to shift away from peak times. The Gone Green scenario includes switching 9 million homes to using heat pumps (some hybrid) increasing peak demand by 12 GW. It looks as though electric cars are not such a big deal for the grid but heat pumps could be.

[1] Transport Statistics Great Britain table VEH0102.
[2] Future Energy Scenarios (National Grid) 2016
[3] Latest UK fuel consumption figures (AA) Aug. 2015


  1. Given plenty of thermal mass, there is significant scope for demand time-shifting for heat-pumps too. Hot water heating can happen at any time in 24 hrs and should work (but that can be tricky to integrate with solar thermal).

    Space heating can certainly be shifted by 3 hours or so and still work, but you need the right sort of building for it to be longer. And if you have the wrong sort of building (draughty, badly insulated, no thermal mass: the 'plasterboard tent' some developers used to be fond of putting up) then you can't really shift at all.

    I do think that more effort on building fabric is a better plan than trying to build enough generation to heat-pump al l the existing stock. In the long term it should be cheaper.

    Cambridge should be taking a lead an insisting on only building passivehouses so that at least we aren't making the problem worse.

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