Generator Sizing Calculator (Running and Starting Watts)
List the loads you want to run at the same time with their running and starting (surge) watts. The calculator adds up the running watts, adds the single largest starting surge and a safety margin, and recommends a generator size in kW with the load current at 120 V and 240 V. Clear both boxes to leave a load out.
Your numbers
Result
Enter your numbers to see the result.
Take this to the job site
The same tools, and more, in apps that work offline when there is no signal.
Voltbox: Electrician Toolbox
NICET/ToolsVoltBox has motor and generator tools, wire and breaker sizing and a multimeter guide, all offline for the backup-power job in a basement with no signal.
BuildSmith: Construction Calc
NICET/Tools37 trade calculators, code tables and converters for the whole job site, fast and offline.
VoltPrep Electrician Exam Prep
NICET/Tools1,000 practice questions for the journeyman and master electrician exams, with the code-based reasoning behind each answer.
Read the how-to guide
How the size is figured
- Running watts = the sum of the running watts of every load you listed.
- Peak watts = running watts + the largest single (starting − running) among the loads. Motors do not all start at once, so only the biggest surge is added.
- Both figures are increased by the safety margin (20% is typical: it covers nameplate optimism, aging, altitude and hot weather, and keeps the generator from running flat out).
- The recommended size is the next common generator rating at or above the peak figure: 2.2, 3.5, 5.5, 7.5, 9, 12, 14, 18, 22 or 26 kW. Amps = watts ÷ 120 V or ÷ 240 V.
Generators are sold with a running (continuous) rating and a higher starting or surge rating. Sizing the running rating to the peak figure is the conservative choice; a unit whose running rating covers the running figure and whose surge rating covers the peak figure also works if the motors start one at a time.
Worked example
Refrigerator 700/2,200 W, well pump 1,000/2,000, furnace blower 800/2,400, sump pump 800/1,300, lights 300, microwave 1,000 and 300 W of electronics: running watts 4,900. The largest surge is the furnace blower at 2,400 − 800 = 1,600 W, so the peak is 6,500 W. With a 20% margin that is 5,880 W running and 7,800 W peak, which calls for a 9 kW generator. The peak draws 65.0 A at 120 V or 32.5 A at 240 V.
Frequently asked questions
Why only the largest starting surge?
Motor surges last a fraction of a second and the chance of two motors starting in the same instant is small, so the usual method adds only the single largest surge to the running total. If two large motors are on the same control (a well pump and a pressure booster, say), add both.
Do I need a 240 V generator?
Yes if you run anything 240 V: a well pump, central air conditioner, electric dryer, range or water heater. Those loads also need the generator connected through a transfer switch or an interlock kit at the panel.
Can I plug the generator into a dryer outlet?
No. Back-feeding through a receptacle is illegal in most places and can electrocute a line worker or destroy the generator when the utility returns. Use a listed transfer switch, interlock or inlet installed by an electrician.
What about central air conditioning?
A 3-ton condenser draws about 3,500 W running and 10,000 W or more starting unless it has a soft starter. Get the locked-rotor amps from the nameplate (LRA × volts = starting watts) and enter it as a load, or plan on a standby generator sized by the installer.
Does altitude or heat change the size?
Engine output falls about 3% per 1,000 ft above the rating altitude and about 1% per 10 °F above 60 °F. The 20% margin covers moderate conditions; at high altitude size up.
Related calculators
Video walkthroughs on YouTube. The TapForge channel posts walkthroughs of these calculators and app demos. Subscribe to the channel.
Get the next tool first.
One short email when a new calculator, guide, practice exam or app goes live. No spam, unsubscribe any time.
You'll get a confirmation email first. We never share addresses.
This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.