Please refer to section 8 of VAT Notice 700/64 – Motoring Expenses:
Motoring expenses (VAT Notice 700/64): 8. Road fuel bought for business
Paragraph 8.8 establishes that you work out your input tax by multiplying the fuel element of the mileage allowance by the VAT fraction.
Assuming that the employee has been charged VAT at the reduced rate of 5% on the supply of electricity to their home, then the VAT fraction would be 1/21.
Paragraph 8.10 establishes that you must retain invoices issued to your employees when the fuel is delivered to them. This can be a full VAT invoice or a less detailed VAT invoice.
For cars charged at home, the employee would need an invoice from their electricity supplier.
Please note that an electricity bill may not show all the information required on a full VAT invoice or a less detailed VAT invoice.