I don't ADVERTISE 24/7 service, but I've been known to work at just about any and all times, and on just about every day of the year.
As Reno states, this is an outstanding way to maintain a customer base. You will acheive an outstanding bond with people when you come through in a pinch. Also, it always feels good to be the hero.
As far as billing, do whatever you think is fair and makes it worth your loosing sleep and being tired the next day. Just be SURE to tell them your after-hours rate BEFORE you leave the house. I usually charge a minimum, like travel plus a 2 hr minimum. If it's just flipping a breaker and running an ammeter on a circuit, I get 2 hours. I have never had a complaint, and a call at an odd time has never been the last call from that customer.
Good Luck!
