Generator Size Calculator
Calculate the recommended size for a fuel-powered backup generator based on appliance running and surge watts, with an adjustable safety margin.
Appliances / Circuits
#1
#2
#3
Typically ~20% headroom on top of total running watts
Recommended Generator Size
2,300 W
Total running load: 1100 WWorst-case with one motor starting: 2300 WHow to Size a Backup Generator
List every appliance or circuit you want the generator to power, along with each one's running watts (steady-state power draw) and starting/surge watts (the brief extra power a motor draws when it first kicks on). A fuel-powered generator needs to be sized for the total steady load plus a safety margin, or for the moment any single motor-driven appliance starts up — whichever is higher.
This is different from sizing a battery inverter or UPS-style backup — see our separate Inverter Size Calculator for that. A fuel generator's surge capacity only needs to cover one motor starting at a time (since appliances rarely all start simultaneously), which this calculator accounts for by testing each appliance's surge against the others' steady running load, rather than summing every appliance's surge watts together.
Example
A refrigerator (200W running / 800W surge), a sump pump (800W running / 2,000W surge), and LED lights (100W running) total 1,100W running. With a 20% margin, that's 1,320W. But if the sump pump starts while the fridge and lights are already running, the load spikes to 2,000 + 200 + 100 = 2,300W — higher than the margin-based figure, so 2,300W is the recommended generator size.
Common Use Cases
- Choosing a portable or standby generator for home backup power during outages.
- Sizing a generator for a job site, RV, or off-grid cabin.
- Checking whether an existing generator can safely handle a new appliance.
- Planning which circuits to prioritize if a smaller generator is preferred.
FAQs
Why doesn't this add up every appliance's surge watts?
In practice, appliances rarely start at the exact same instant, and most generator sizing guidance assumes only one motor-driven appliance is starting up at any given moment while the others are already running steadily. Summing every appliance's peak surge watts together would significantly oversize (and overspend on) the generator.
How is this different from sizing a battery inverter?
A battery inverter or UPS-style backup draws from stored battery energy and is typically sized for a specific, often smaller, set of essential loads with attention to how long the battery will last. A fuel-powered generator burns fuel continuously and is generally sized around surge/running wattage rather than a battery runtime budget — use our separate Inverter Size Calculator if you're sizing a battery-based backup instead.
What if I don't know an appliance's surge watts?
Check the appliance's nameplate or manual — motor-driven appliances (refrigerators, pumps, air conditioners, power tools) usually list a starting or locked-rotor amperage you can convert to watts. As a rough estimate, surge watts are often 2-3x running watts for motor-driven appliances, while non-motor loads like lights and electronics have little to no surge.