I just read last week's cover story on the BMW i3, and clearly the standard fuel economy rating of litres per 100 kilometres does not work when reporting on electric cars. Why don't car companies start reporting how much kwH it takes to fill the batteries? At about 30c per kWh, it's a much more definable economy.
We're at the dawn of the electric era and waiting for an official global standard. It's not about charging the batteries but the range you get for the charge. A simple standard is being developed in Europe. I can recall the confusion when we switched away from the imperial miles per gallon standard in Australia - the economy number got smaller instead of larger when expressed in L/100 km.