Seriously, this sounds like mother nature's nightmare to me.
Each to their own of course, but please be aware that 'energy costs' has at least two meanings, a financial and an ecological one.

So if the thing doesn't fold proteins over night or something, the ecological aspect will thank you if you e.g. use time compression, save, turn off and continue later, with you actually sitting in front of your hobby.  :)
Hobbies are important, so nobody speaks of leaving them for the sake of energy saving, but with the compression working and also the panel state save (works well on the 767), you won't miss a thing.
I'm also flying online in this config if the skies a rarely populated and/or ATC allows.

FSX capable systems aren't energy savers, expect some 250+ watts on an i7 + GTX4xx config, without the screen turned on!. FSX running doesn't mean that the components idle or get anywhere close to power safe states.
8 hours of sleep give you some 2 kilowatt-hours each day you do this. Three times a week and/or longer sleeps easily sum up 'nicely' just for the sake of not saving a flight.