Question about battery and power inverter

[FIXXERVI6]

I have a 2000 watt square wave and have yet to find anything it won't run

 

[nexium]

Back of the napkin type calculations... assuming 90% efficiency in your inverter.

120V, 90W = 0.75A. Allowing for the inverter inefficiency, calculate for 0.8333A at 120V (0.75 / 0.9).

120V / 12V * 0.8333A = 8.33A. So for two hours use, you want 16.67Ah available. Assuming you're buying a deep cycle battery, at 50% use, you could get away with as small as a 33AH battery. Kind of rules out the Radio Shack variety - you can get very small 12V/18AH deep cycle SLA batteries here in Oz for example - this usage could be drawn from the battery, but flatten it in the process and shorten it's lifespan.

375W inverter sounds fine - if you get a smaller inverter, eg 100W, they get hot and they can cut out even though they're "rated" for the load.

Charging deep cycle batteries is another issue, which is why I mentioned AGM batteries - they're much more tolerant to casual charging without the need for stepped voltages. Might be a good idea to drop into a solar forum and put the same question to them and see if you get a similar answer - this is an area I've done a stack of research on, but haven't actually laid out any cash yet (was looking at dual batteries in a 4WD for running a camping fridge etc). You could also get better advice about whether modified-sine is appropriate for the TV/DVD.

I bought a pair of 26 amphour batteries from a local surplus store. They worked good in our scooter which is designed for 37 amphour batteries. I'd guess 500 watts for up to 3 minutes, with a few seconds pause before full throttle again. They measured 13 volts repeatedly right after charging. I loaned one of them to a friend who was without power for 3 days. He used a 70 watt inverter powering a portable phone base station for a total of 8 hours = perhaps 8 amp hours. The battery voltage dropped to 11.3 volts, so 8 amphour spread over 2 days was dammaging to the battery which is now significantly inferior to it's twin. Even 50% may be optimistic for long battery life. Neil