Home Generator Size Calculator
Find the minimum wattage generator needed to power your essential home appliances during an outage. Accounts for motor startup surges that can trip or damage undersized generators.
Last updated: September 2026
Formula below · 2 sources (nfpa.org, Wikipedia) · Updated Sep 2026
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About this calculator
A generator must meet two demands: the running watts of everything you power, and the brief starting surge when a motor (refrigerator, sump pump, well pump, air conditioner) starts. Only one motor normally starts at a time, so the standard sizing rule is: surge capacity needed = total running watts + the largest motor’s running watts × (startup factor − 1). Enter the running watts of the largest motor to use this rule. If you leave it blank, the calculator applies the startup factor to the whole load (essentialLoad × startupFactor), which is a conservative upper bound. Choose a generator whose surge (peak) rating is at least this result and whose running rating is at least your essential load, ideally with the load at 80% or less of the running rating. If several motors can start at the same moment (for example when an automatic transfer switch reconnects the whole house), add their surges together or use a load-management module.
How to use
Example: a refrigerator (150 W), sump pump (800 W) and lights (200 W) total 1,150 W running. The sump pump is the largest motor, with a startup factor of 2.0. Surge needed = 1,150 + 800 × (2.0 − 1) = 1,950 W. You need a generator with at least 1,950 surge watts and 1,150 running watts; a 2,000-3,000 W unit gives margin. Leaving the largest-motor field blank gives 1,150 × 2.0 = 2,300 W, the conservative whole-load estimate. The default 5,000 W with a 1.5 factor and no motor entered gives 7,500 W.
Frequently asked questions
What startup factor should I use when sizing a home generator?
The startup factor depends on the type of motors in your essential appliances. Resistive loads like lights and heaters have a factor of 1.0 since they draw no surge current. Small induction motors such as refrigerators and sump pumps typically use a factor of 1.5 to 2.5. Central air conditioners and well pumps can require factors of 2.5 to 3.0 or higher. When in doubt, use 2.0 as a conservative general estimate, or check the appliance nameplate for locked-rotor amperage data.
How do I calculate the total wattage of appliances I need during a power outage?
List every appliance you consider essential—typically a refrigerator, heating or cooling equipment, a sump pump, lights, and phone/device chargers. Find the running wattage on each appliance's nameplate or in the owner's manual; if only amps are listed, multiply by the voltage (usually 120 V). Add all running watts together to get your essential load. Be realistic about what you truly need versus what would be convenient, as smaller generators are cheaper, quieter, and more fuel-efficient.
What is the difference between running watts and peak watts on a generator?
Running watts (also called rated watts) is the continuous power a generator can supply indefinitely. Peak watts (surge watts) is the higher output it can sustain for a few seconds to start motors. Size the generator so its running rating covers your total essential load (ideally at 80% or less of the rating) and its surge rating covers this calculator’s result. Do not compare your running load against the surge rating, or the generator will be overloaded in normal operation.
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