Cool!!!! Save some $$$.
My experience with APU's is:
1) F-4 - cute little prop driven motor that popped out into the airstream when the engines could no longer provide hydraulic pressure...
2) F-16 - the electric jet - tank full of 6.7 gallons of a nasty chemical called Hydrazine. The chemical burned for approx 10 minutes to provide juice to the flight controls. Many inadvertant "APU power up's" on the ground....run away....run away.....
The F-4 APU was much more user friendly!