During the recent power failure, our basement was in danger of flooding without its sump pump. We had a small generator which could be connected into the basement with an extension cord through the garage. However, we had not run it for many years. Not surprisingly, it would not start. (Instructions with backup generators suggest running them briefly once month to keep them operational.)
Our 1/3 hp sump pump draws 640 watts. We had a modest 760-watt inverter in our camper to run a computer. That seemed enough for the pump. So, we backed our small car out of the garage, connected the inverter to its battery, and plugged the pump into the extension cord. It worked. Idling all night at about 700 rpm, the 4-cylinder, 2.5 liter engine used about 10 liters of gas. When not running the pump, we also found this could run our oil furnace, which draws 320 watts when operating both burner and 3-zone circulation pumps.