About the Generator Size Calculator
This generator size calculator answers "what size generator do I need?" by adding up the appliances you want to run at the same time. List each item with its running watts and, for anything with a motor or compressor, its starting (surge) watts. The calculator totals the running load, adds the single largest start-up surge, applies a safety margin and suggests the next common generator size.
It suits homeowners planning backup power for outages, RV and camping users, contractors powering tools on a job site, and anyone comparing portable and standby generators. The chart shows which loads dominate so you can decide what to leave off or stagger.
Motors draw two to three times their running power for a second or two when they start. Because you rarely start every motor at the same moment, the usual sizing method adds only the biggest extra surge to the total running watts. Nameplate watts are best; the defaults are typical figures, not measurements of your equipment. For a whole-house standby unit, have an electrician perform a load calculation.
With the default inputs, the recommended generator (with margin) is 6,900 W. Change any value above to recalculate instantly.
How to use the generator size calculator
- 1List each appliance on its own line as name, running watts, starting watts (and optional quantity).
- 2Use the nameplate or manual; for resistive loads like lights or heaters leave starting watts blank.
- 3Include only the items you will run at the same time.
- 4Set a safety margin — 20% is a sensible default.
- 5Read the recommended wattage and the next common generator size.
Formula and method
Every appliance you want to run at the same time contributes its running (rated) watts. Motor-driven loads such as refrigerators, sump and well pumps, furnace blowers and air conditioners also draw a brief inrush when starting, often two to three times the running watts. Because motors are normally started one at a time, only the largest extra surge (starting minus running) is added to the running total.
A safety margin is then applied so the generator is not run flat out and to leave room for future loads, and the result is rounded up to the next common generator size. Current is running watts divided by the generator voltage.
- Σ running W
- Sum of running watts of all loads on at once
- starting W
- Surge watts a motor draws while starting
- margin
- Extra headroom, e.g. 20%
Worked examples
Essential home backup during an outage
The seven loads add up to 4,200 running watts. The furnace blower has the largest extra start-up surge, 2,350 − 800 = 1,550 W, so the minimum starting capacity is 5,750 W. With a 20% margin you need about 6,900 W, so a 7,500 W generator fits.
Job-site tools
Running watts total 1,400 + 1,600 + 500 = 3,500 W. The compressor adds the biggest surge, 4,500 − 1,600 = 2,900 W, for 6,400 W of starting capacity. A 10% margin brings it to 7,040 W; the running load pulls about 29 A at 120 V.
RV with an air conditioner
Running watts are 1,780 W and the AC compressor adds a 1,500 W surge, so the generator must start 3,280 W. With 15% headroom that is 3,772 W, just above a 3,500 W unit, so the next common size is 5,000 W — or fit a soft starter to the AC.
Frequently asked questions
What size generator do I need to run a house?+
Essential circuits — refrigerator, sump pump, furnace blower, lights and electronics — usually need about 5,000–7,500 W. Adding central air, a well pump or an electric range can push the requirement to 15,000–22,000 W or more.
What is the difference between running watts and starting watts?+
Running watts are the steady power an appliance uses. Starting (surge) watts are the brief higher draw when a motor or compressor starts, often two to three times the running figure, lasting a second or two.
Why add only the largest starting watts?+
Motors are rarely started at exactly the same moment. Adding the single biggest surge to the running total covers the worst case when that motor starts while everything else is already running.
How do I find the wattage of an appliance?+
Check the nameplate or manual. If only amps are listed, multiply amps by volts (for example 5 A × 120 V = 600 W). A plug-in watt meter gives the real running figure for 120 V appliances.
Should I run a generator at full load?+
No. Most manufacturers rate continuous output below peak output, and portable generators run cooler, quieter and more efficiently at roughly 50–80% of their rating. A margin of 10–25% is a common rule of thumb.