I use a thing called a "Far Outlet". It was sold by Galaxy Audio. It is a battery (about the size of a motorcycle battery) with a sine-wave, pure power inverter. I have done live keyboard jobs and DJ jobs with it, and it can power a keyboard, small PA etc. for many hours ...

Not sure if these are still available . . .

Jim
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Genos / Tyros5 / HK Lucas Nano 600 / FTB Maxx 40a / EV ZX1A / Rock'n'Roller cart / Hauptwerk virtual pipe organ / misc other audio & music toys