[Playfair]

Mobert, I would think the pump would run for several hours, but car batteries don't like to be discharged like that. I have a 600w inverter that I can run my return pump, skimmer, ph's, and VHO lighting or heater... During a black out last year I ended up just leaving the "leased" car idling for 9 hours. The alternator puts out about 50amps, which is what I was drawing.

Mako- That device you linked to is what MADE me DIY! Same thing, for less than 1/2 the cost... And it doesn't include the battery(ies).

That unit does have 500w capacity instead of 300, but it would take a lot of juice to last very long sucking that much power. (500w=~45 amps@12v, which means one 90ah batt would only last 2 hrs). I choose to be realistic at running only my pump and cabnet light, but overrating just a little.

One other difference I see is the "delay" feature. Sounds neat, but I probably won't incorporate it right away.