Originally Posted by roka64
Originally Posted by AndyH5512
The battery backup is a good idea. How long will one last if it is running only PH's?
That is going to depend on the Wattage of both the phs and back up.
Yup. A real real REAL rough estimate is to find out how many "amp-hours" the battery is rated for. Then take that number, and divide it by the current you're putting through it - in Amps. To get amps, divide the wattage of your load (powerheads) by the voltage of your load (most likely 120V). As an example:
Say you have two powerheads that are rated 12 Watts each.
Total current for the two would be:
2 x 12 Watts = 24 Watts
24 Watts / 120 Volts = 0.2 Amps
Say you have a 4.0 Amp-hour rated battery in your backup. Then, the absolute MOST time you could get would be:
4.0 Amp-hour / 0.2 Amps = 20 hours
Again... this doesn't account for a lot of stuff (like the inverter efficiency, the starting current when things power up, blah blah blah) but it at least gives you an absolute maximum for time to expect. To be safe, I'd cut the number in half so you wouldn't be disappointed.