He's a student but pays council tax snd I think is a bit resentful about this
So he's trying to recoup his losses by saving on electricity? 
I once had a flatmate who was stingy like this. She'd come home with a loaf of bread that cost £1, say, and announce that we owed her 33.3p each. Since that clearly wasn't possible, she'd ask for 33p each this time, and then next time she'd ask one of us for 34p and the next time the other for 34p. This despite the fact that we both regularly bought things like milk and bread and didn't ask for the money (figuring that none of us was on the breadline, and it would roughly even out). We tried having a kitty that we'd put money into for communal stuff, but she was convinced that we'd both steal from it (despite there being no reason for her to suspect this), so it was abandoned in favour of her crazy system. Obviously, neither of us ever actually had 33/34p in change, so she would make a mental note of how much it was and add/deduct as appropriate the next time.
She also once went nuts because I had a friend round who wanted ketchup with dinner - I didn't have any as i don't eat ketchup, so used a small amount of hers instead. But she'd happily eat my biscuits, chocolates etc all the time (as I offered them around) - didn't seem to think that maybe, just this once, she could reciprocate that generosity.
We were good friends before we moved in together. We're not any more.