Battery Back-up ???

[ALang]

I just bought a new Garrison backup power system, 400W.
It has: 2 sealed, 20Ah lead-acid batteries
Built-in transfer relay
BUT, the output is a non-sinusoidal modified sine wave

Questions:
Before I go ahead and plug this thing in, is this something that I can rely on for the occasional power outages to run my 300W heater plus one or two Korallia-4 power heads?? I had read here that maybe non-sine wave isn't good for the Power heads?
My tank is a 6', 130gal mixed reef, pretty heavily stocked.

I was thinking since the house is warm, and the tank is 130gal, I can maybe do away with the heater if it helps.

Thanks for all your input. Lenny.