You can use the UPS to keep power on your tanks very cheaply!. Even the puniest UPS (Battery Backup ~ $30) at Staples/ Office Depot can keep your tanks life support running for days. Please keep in mind that if you have major expensive filters, then it is FAR BETTER to use somewhat costly PURE SINE WAVE UPS that start at ~ $100.
I refer to your tanks life support as Water Filters and air pumps, which use extremely low wattage in their operation. Do not plug your Heaters and Lights into the battery backup plugs on a UPS though. They require much higher wattage and the UPS will not support these for a vlong time. Most UPS have a bank of surge protected plugs to go along with the battery backup bank of plugs. This is where you should plug in your UV's, heaters, lights etc...
The ambient temperature of your home should be able to withstand any power outage outside of the winter months, and lack of light exists everywhere in nature. It would just seem like a prolonged night to your fish!