I would ask them for an offer and see what they are thinking.
Assuming you only get paid when they use you, I would charge significantly more for consulting than I would for an employee type position. Roughly double because:
A consultant picks up all their own overhead (insurance, retirement, etc.).
An consultant that only gets paid when needed has zero guarantee of income.
If you are "on call" and have to respond quickly when called, then you're free time is not truly free time as you must remain "ready" at that time.
The company saves by not having to pay the employee overhead (which can be huge).
The company gets the benefit of having your known/reliable services when they need them, but only having to pay when they use you. If they had to keep an employee or had to go scout out a new consultant every time they needed work done, it would cost them a lot more.
Bottom line, companies pay consultants more per hour than employees because consultants are taking all the financial risks and need to stay in business when times are slow to be available to companies when those companies want/need them. The hourly rate is more but it costs the employer much less to use consultants for sporadic work. Everyone wins.
If this is more like a part time job with a guaranteed amount of work, then I would probably charge 25% more than I would as an employee. They're still getting the benefit of not having to pay overhead and the fact that you don't "need" the insurance does not mean your hour is not worth that amount of money. It just means you must maintain a full time position in order to be able to offer them the services they want.
One caveat - if you really want to break into consulting and you think this will help you and they won't pay the rate you want, you might lower the rate to get your foot in the door. But only if you see this position actually helping you get other consulting clients.