How do generator sizing calculators work?

A generator sizing calculator works out the size of generator needed for a particular load. The calculator typically asks about the type of appliances in use or the power demand and shows a suitable generator. Calculating these requirements in advance reduces the risk of overloading the generator.

A generator combines a small gasoline or diesel engine with an alternator and a voltage regulator to provide a mains power output. Generators are useful where mains power is unavailable or as a backup for unreliable mains power supplies.

An average portable generator supplies around 2,000 watts, which is enough to run small domestic appliances such as a TV, a refrigerator or handheld power tools. Larger portable generators can supply up to 7,000 watts, which can run a large industrial refrigeration unit or a welding set. Fixed generators can supply up to 3 megawatts, which can provide backup power for an entire factory or office building.

An important factor in sizing a generator is peak load. Many appliances require much more power to start up than to run continuously, which can sometimes overload a generator. The generator should be capable of powering all the appliances connected to it at their maximum power draw, not just the average draw. When using appliances that present a reactive load, multiply the current draw by the power factor to establish the apparent load.