Generator

A diesel, petrol or gas generator produces electricity with an internal combustion engine. In standbyGlossary · UPS technologyOffline (Standby) UPSA UPS that leaves the load on the mains and switches to battery only when the mains fails, with a short transfer time; the simplest and cheapest topology. installations it is connected through an automatic transfer switch (ATS), which detects the outage, gives the start command and transfers the load to the generator. This usually takes from a few seconds to about a minute.

Because a generator cannot supply power instantly, it is often combined with a UPS: the UPS bridges the gap until the generator stabilises, and the generator then provides hours of runtime as long as there is fuel.

When sizing, the generator needs ample headroom above the UPS rating, often 1.5 to 2 times, because of inrush currentGlossary · Electrical unitsInrush CurrentThe initial surge of current required when electrical equipment is first powered on., battery charging and harmonics. A small generator with unstable voltage and frequency may not be accepted by the UPS.