Battery backup

[Dave C]

Well that's 2.5 hours at a 250w draw. To be comparable to the Vortech example, I could run a single Tunze 6100 at 1/2 power (22.5 watts) and extend this to about 28 hours. I assumed that running a return pump from my 100g sump would help oxygenate the water better then a powerhead would, and increase the volume of water and thereby conserve more heat longer.

This box can be powered by my car as well so I could get an extension cord and do the same as you. I can also purchase another deep cycle battery for another $120 and almost double that time, much like you could do with your inverter.

I agree that 2.5 hours is a short time to be out of power and if I knew that it would never exceed that length of time I'd go without a backup. But if I assume that 5 hours is the max for my part of the country then I've cut that in half and now I've only got 2.5 hours of 0 power, which as you said isn't much. Basically I was using 250w as a maximum power draw when I was choosing options for a backup.

[Dave C]

Here's a good LINK to determine the estimated time you'll get from your backup system. You enter the amp-hours of the batteries, planned depth of discharge, operating temperature and the wattage you'll be drawing and it tells you how many hours you'll have power. If I run the return pumps I'll get 4.9 hours. If instead I run 2 Tunze in my big tank at the fluctuating 30-100% and the return pump in the other tank I'll have 7.5 hours.