Bill Stock wrote:
I had another power outage last weekend (4+ hours), so I finally broke down
and bought a couple of refurbed UPSs. They should last a while as they are
36 amp hours each. The Goldfish tank only uses 24 watts (2 Fluval 304s), as
I won't connect the heater or lights. But I was wondering about the Tropical
tank, since batteries last exponentially longer under reduced load. Would I
not be better to have one 100 watt heater running twice as long, rather than
the normal two 100 watt heaters. In fact I was thinking about replacing the
two 100 watt heaters with a decent digital heater and leaving one of the 100
watt heaters in the tank as a backup. I would set the backup below the
threshold of the main heater (say 75?F) and connect it to the UPS. This way
the heater would not come on until the power had been off for a while.

100, 200, or 400W heaters will use the same amount of energy at the same
temperature, if they can maintain the temperature at all.
100 gallons of water maintains its temperature for a long while, and most
fish are fairly tolerant if it's once.
I would not have used UPSs, they arn't really the right tool, for small loads,
as the battery is sized to discharge through the inverter in some half an
hour, but the inverter will typically use 5%-10% of its nameplate power as
parasitic power.
So, with a 600W UPS, you may be looking at 80W draw from the battery,
rather than 30W, which you might get with a small inverter.

I'd have gone with a deep-discharge rated battery, and a small inverter,
combined with a small battery charger to keep it charged.

Not to say it won't work of course.
A tip, you may well find that you get better run-time if you daisychain the
UPSs, rather than having both powering seperate stuff.