24/7 Support reimbursement percentage?

I’m an IT Manager that has to start offering 24/7 support to my Enterprise. My guys have the ‘do whatever it takes’ mentality and will do the support for free. However, they should be compensated for those nights where they’re gonna get called at 3AM.

The problem is those calls will be relatively infrequent and I can’t put a $ amount on it.

Do other Managers reimburse their 24/7 IT staff by a straight $ figure or some %?

If a percentage is fair then how much?

Help will be greatly appreciated.