The usual little UPS sold for home use has a a 12V motorcycle battery desisned to give your machine a message to shut down gracefully (5-15min). The circuitry is designed to give you enought amp/hrs to do that only - it cannot handle prolonged loads without overheating.
There are real UPSes that really wil give you several hours, but they are big, heavy, noisy and ferkin' expensive - PDR1's answer is theoretically correct but impracticable unless yer wants to spend mucho dinero. Better to buy a little genny and hope it starts.
My solution is relatively cheap, seems safe enough and lasts me a couple of hours. Just add batteries in series for more time, but be prepared for a l-o-o-o-n-g recharge.
Mac
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