What do you charge as a minimum rate for a service call?

If I'm on a series of calls and my schedule is full for the day, I really don't use a minimum, but an hour is the least that I will charge.

If, on the other hand, I'm done for the day or it's the weekend, and I get an "emergency call", It's a 2 hour minimum, even if it's just flipping a breaker and takes me the drive + 5 minutes. I don't think it's asking too much, as the vast majority of the people who have these "emergencies" would never even consider going into work after hours or on the weekends, and I haven't had an argument yet.

Any thoughts or different policies out there?