Dec 17, 2012
There is some losses, every time you convert energy to another source. (First law of Thermal Dynamics). I do not know exactly how many KWhs of power it takes going into a battery, and then the next day, get 1 KWh of power out. (the units are the same, but it's more than 1:1) Although it's not too bad. The costs I read about earlier, take that into effect.
It also depends on the type of batteries used. Lith-ion, vs Lead acid, etc. Also as batteries age, they loose some efficiency too. But as an internal Combustion Engine ages, it also looses some efficiency too.