Backup Generator Size Calculator

Add the running wattage of all loads you need to power simultaneously. Add the starting surge wattage of the largest motor (typically 3-6x running watts for HVAC compressors). The generator kW rating must exceed this total with a safety margin of at least 20%. Most homes need 7,000-20,000 watts for partial backup and 20,000-25,000 watts for whole-home backup.

Running watts is the continuous power a load consumes during normal operation. Starting watts (also called surge or peak watts) is the brief burst of power needed to start electric motors, such as air conditioner compressors, refrigerators, and well pumps. Starting watts are typically 2-6 times the running watts and last only for a fraction of a second.

Natural gas standby generators connect directly to the utility gas supply and start automatically during outages. Propane standby generators are similar but require a propane tank. Gasoline portable generators are lower cost but require manual operation and refueling. Diesel generators are fuel-efficient and long-running but require on-site fuel storage and maintenance.

A transfer switch disconnects your home from the utility grid before connecting the generator, preventing back-feed that could electrocute utility workers. A manual transfer switch requires you to flip breakers when switching power sources. An automatic transfer switch (ATS) detects a power outage and starts the generator and switches loads automatically. Most standby generator installations require an ATS.

Portable gasoline generators typically run 6-18 hours on a full tank, depending on load and tank size. Natural gas and propane standby generators can run continuously as long as fuel is supplied, typically 500 hours between maintenance intervals. Standby generators are designed for extended outages and include automatic exercise cycles to maintain readiness.