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.
About this calculator
Choosing the right generator size requires accounting for two distinct power demands: running watts and startup (surge) watts. Most motor-driven appliances—refrigerators, sump pumps, air conditioners—draw two to three times their running wattage for the brief moment they start up. The formula is: required generator size = essentialLoad × startupFactor. The essential load is the sum of running watts for all appliances you need during an outage. The startup factor (typically 1.25 to 3.0) ensures the generator can handle the largest motor surge without stalling or tripping. Undersizing a generator leads to voltage drops, motor damage, and generator overload. Manufacturers typically rate generators at both running watts and peak (surge) watts; your calculated value should not exceed the generator's running watt rating.
How to use
Example: you need to power a refrigerator (150 W), sump pump (800 W), and several lights (200 W), totaling 1,150 W. Your sump pump is the largest motor load with a startup surge, so you choose a startup factor of 2.0. Step 1 — Essential load: 150 + 800 + 200 = 1,150 W. Step 2 — Apply startup factor: 1,150 × 2.0 = 2,300 W. You should select a generator rated for at least 2,300 running watts—in practice a 3,000 W unit to provide a safety margin and room for additional small loads.
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 a generator can sustain for only a few seconds to start motors. Always size your generator so the running watts rating meets or exceeds your essential load calculation. Using peak watts as your baseline risks overloading the generator within minutes of operation, causing engine damage or automatic shutdown. A reliable rule is to target 80% or less of the generator's running watt rating under normal load.