Self-employed charging an electric car at home

I have a self-employed client who has bought an electric car.  I have been trying to find out what he can claim for charging it at home, but HMRC say that the Advisory Electricity Rate (AER) of 4p per mile is for company car users only (see https://www.gov.uk/government/publications/advisory-fuel-rates/advisory-... ).

Therefore, how does a self-employed person work out what to claim?  I can see that his electricity payments have gone up, but there could be a number of reasons for this (including price increases), and the costs fluctuate a lot from year to year in any case, so it is impossible to separate out how much was used by the car.