Generator calculator
Generator Size Calculator
Turn your load into a minimum generator rating, with a sensible reserve and the starting surge of your biggest motor.
- Free to use
- Formula shown
- Worked example
A generator has to cover two numbers: the running watts of everything you power at once, and the brief starting surge when a motor or compressor kicks on. This calculator takes both, adds a sensible reserve and tells you the minimum generator rating to shop for.
Generator Size Calculator
Estimate generator capacity from running load, additional startup surge, and a capacity margin.
Find the minimum running and starting capacity your generator should provide based on the loads you plan to run.
Enter your load values and calculate to see the minimum running and surge capacity to look for.
Don’t know your totals yet? Build your list in the Generator Load Calculator first. It passes your running load and largest surge straight into this tool.
How the size is calculated
The calculator works out two requirements and uses whichever is larger:
- Starting requirement = total running watts + the largest single additional starting surge. Motors rarely start at exactly the same moment, so only the biggest surge is added.
- Running requirement with reserve = total running watts × (1 + reserve %). A 20% reserve keeps the generator out of its upper range, where it runs hot, loud and thirsty.
The larger figure is rounded up to the next 500 W, which is roughly how generators are sold.
Worked example
A refrigerator (700 W running, 2,200 W extra to start), a 1/2 HP sump pump (1,050 W running, 2,200 W extra), lights and internet (300 W) and a furnace blower (800 W running, 2,350 W extra) add up to 2,850 W running. The largest extra surge is the furnace at 2,350 W.
- Starting requirement: 2,850 + 2,350 = 5,200 W
- Running + 20% reserve: 2,850 × 1.2 = 3,420 W
- Recommendation: the larger, 5,200 W, rounded up to 5,500 W
Look for a generator with at least 3,420 running watts and 5,500 starting (surge) watts. Many “5,500 W” portables are rated by surge, so check that the running figure is high enough too.
Running watts vs starting watts
| Rating | What it means | What to compare it with |
|---|---|---|
| Running (rated) watts | Power the generator can supply continuously | Your total running load plus reserve |
| Starting (surge, peak) watts | Short burst for a few seconds while motors start | Running load plus the largest starting surge |
Resistive loads such as heaters, toasters and incandescent bulbs have no surge. Anything with a motor or compressor (refrigerators, freezers, pumps, air conditioners, furnace blowers, power tools) needs two to three times its running watts for a moment at start-up. See typical values in our appliance wattage chart.
Tips for choosing the right size
- Check voltage. Well pumps, central air and electric dryers need 240 V. Confirm the generator has a 120/240 V outlet such as an L14-30.
- Stagger starts. Turning large motors on one at a time keeps the surge requirement down to the single largest motor.
- Account for altitude and heat. Engine output falls by roughly 3.5% per 1,000 ft above sea level and in high temperatures. Add margin if you live at altitude.
- Sensitive electronics run best on an inverter generator or a conventional one with low total harmonic distortion.
Frequently asked questions
What size generator do I need to run a house?
Essentials only (refrigerator, freezer, sump pump, lights, internet and a gas furnace fan) usually need 5,000 to 7,500 W. Adding central air or an electric water heater pushes most homes to 12 to 22 kW, which is home standby territory.
Is a bigger generator always better?
No. An oversized generator burns more fuel at light loads and costs more to buy. Aim for your load to sit between about 50% and 80% of running capacity most of the time. Use the fuel consumption calculator to compare sizes.
Why is the reserve 20%?
It is a common planning margin, not a code requirement. It covers appliances drawing a little more than their label, future additions and derating. Lower it if your load list is precise, or raise it if you expect to add equipment.
Keep planning
The next calculators most people need after this one.
Not sure where to start?
Begin with the load calculator: it totals what you want to run and passes the numbers on.
Calculators give planning estimates from typical values. Check your equipment’s nameplate and manual, and have a licensed electrician confirm anything connected to your home’s wiring. See our terms.