Generator sizing is the process of calculating the electrical load that a generator must supply and selecting a generator with sufficient capacity to handle that load safely. Undersized generators are overloaded, causing voltage drop and potential damage to both the generator and connected equipment. Oversized generators operate inefficiently and waste fuel. Proper sizing accounts for both running watts and starting surge watts, the latter being 2 to 3 times the running wattage for motor-driven equipment. Generators covers the types and fuel options available once the size is determined.
The total running load is calculated by adding the wattage of all devices that will operate simultaneously. Common homestead loads include refrigerators (600 to 800 running watts, 2000 to 3000 starting watts), well pumps (750 to 1500 running watts, 2000 to 5000 starting watts), freezers (500 to 700 running watts), furnace fans (800 to 1200 running watts), and lighting (100 to 500 watts total). Motor loads, including pumps, compressors, and fans, require the highest starting surge and dominate generator sizing decisions.
Generator specifications list both continuous (rated) power and surge (maximum) power. Continuous power is the wattage the generator can supply indefinitely under normal load. Surge power is the short-term capacity, typically for 5 to 30 seconds, required to start electric motors. A generator with a continuous rating of 5000 watts and a surge rating of 6250 watts can power loads up to 6250 watts during startup but must not exceed 5000 watts in steady operation. Inverters and Batteries provide alternative approaches to managing surge loads.
For portable backup power, a 5000 to 7500 watt generator powers essential circuits during an outage. For construction and job site use, 3000 to 5000 watts run power tools. For whole-house standby systems, 12000 to 22000 watts are typical, depending on home size and whether electric heating or air conditioning is included. Solar Sizing uses comparable load calculation methods for off-grid solar systems.