You can do it a number of ways,
You can claim the standard £2 a week I think and they don't question anything.
Work out how much it costs you ie how many watts the Charger takes, you PC uses and printer and then times that by your charges from electric company
Or
Work out the square meterage of your house, work out the area you use for business like garage and office then take that percentage and times that by your bills.
Each one is allowable through hmrc although some may work out more or less than each other