Backup Generator

A backup generator is an engine that burns fuel and turns a small electrical generator, so that a household or a farm can keep a few important things running when its usual power supply stops. It is not meant to power everything. The useful question is which loads fail badly within hours - a freezer full of food, a fan that ventilates an animal building - and which can simply wait until the power comes back. It is sized for the first group only.

What decides whether one works is almost always the starting surge of the largest motor on that list. A motor draws about four times as much power to start as it does to keep turning, so a single well pump can set the size of the whole unit. The engine is matched to the electricity it delivers, at roughly 1.5 to 1.7 kilowatts of engine power (2 to 2.25 horsepower) for every 1000 watts produced; air cooling serves up to about 15 kilowatts of output and a liquid-cooled engine is needed above that. Some units carry their own engine, burning gasoline, propane or diesel; others are turned by a tractor's power take-off shaft.

Wiring one into a building is licensed electrical work: it needs a double-throw transfer switch that disconnects the building from the utility line before it connects the generator. Without one the machine feeds power back down that line and can electrocute the crew working to restore it. Exhaust carries carbon monoxide, a gas with no smell that is deadly in an enclosed space, so the unit runs outdoors or vents well clear of any room, with the pipe at least 150 mm (6 in) from anything that can burn. The usual failure is not starting on the day it is wanted, which is why stored fuel is changed about every six months with a stabiliser added and the engine is run briefly at intervals. Its redundancy lasts only as long as that fuel store, so it belongs beside a primary supply, not instead of one.