I have a plug in device which you can input your tariff, it then measures the amount of electricity you use and the cost etc. For a full charge on my Volt it costs me £1.80 (inc vat) during the day. I am with Scottish Hydro on a Standard meter at the moment, they do a Powerfuel tariff which uses an E7 meter with the day rate the same as a Standard meter tariff and the night rate is cheaper for charging. With my home charger I had fitted by BG recently the full charging time is 4hrs so fits in well with the low rate tariff. At £1.80 for 40 miles driving range the day rate is still cheap motoring.