You stated on the ground and with the engine off. I assume that implies no ground power either and the bird is probably on alert and the customer wants a probability of scrambling with an operational system.

Given this status, the system is not operating accumulating zero hours. Mathematically, failure rate is a non-player in any operational environment, GF, AUF, or AIF since time goes to zero. Some ground time could be applied beginning at start up, taxi time to end of runway, and end at take off roll. then operational time begins and AUF and AIF become applicable.

If you want to get into the nitty gritty of down time, one could look at dormant time with some adjustments for storage environment such as humidity, temperature excursions, etc. That would involve some detailed analysis at the component level.