"I would love to meet up for coffee, but the numbers on my electricity meter are about to get round to zero again, and I want to be there when it happens."
@SW-User In real life at school I did use the even more devastating "I'm washing my hair."
To which my friend tactfully added (points to the guy asking me out when there was another girl present, in retrospect), "And she's washing it every other evening you'd like her to go out, too."
