Finding the perfect balance is the tricky part. I do I.T. consulting on the side, but I am not stringing myself out for a customer. Its one of the expectations when I offer my services.

No I will not be on call, no your computer turning on isn't a emergency and I will not drop everything to hit the power button.

If they don't like the terms, they're more than welcome to pay double my asking and pay a monthly retainer.