I had exactly the same... walked up to our joliday cottage to let the gursts know that a rare owl had been seen regularly hunting in our fields over the previous days to find a very beefy cable trailing out the front door plugged into a standard 13 amp socket within the house. I asked to "discuss" the matter. aThe car owner explained that his (large) car was a hybrid with a small battery that cost about £1 to charge.. I explained that the hire rates did not include an allowance for guests to fuel their car, and as I really had no idea how much "juice" he was taking, I suggested that he left a "contribution" to cover the cost. He left £5... I was annoyed that he accepted fully that he should pay for it,
yet did not think to ask permission or offer payment in the first place...
My conclusions
a) no point in getting annoyed, we'll all be seeing more electric cars and have to put in policies and if neccessary, the infrastructure to deal with it.
b) I'll be recording the electricity meter from now on... with a small allowance for tupical use. any extra will be charged.