Almighty Calculator

Generator Sizing Calculator

Add up the running watts of everything you will power at once, then add the highest starting watts of any one motor; that total is the surge rating you need.

Estimate only. Results are estimates. Verify against the applicable code and manufacturer specifications before relying on them for safety-related work.

W
Lights, heaters, electronics: running watts add up with no surge.
W
Refrigerator, pump, furnace fan, tools: sum of running watts.
W
From its nameplate or manual: the highest starting watts of any one motor. Only one motor starts at a time here.
%
Your choice; running near the limit is hard on a generator.
Continuous rating needed
3,600W
Surge rating needed6,000 W
Total running watts3,000 W
Running current at 120 V25 A
Running current at 240 V12.5 A

The surge figure adds the full starting watts to the running total, the method manufacturers publish. It is slightly conservative, since that motor’s own running watts are counted twice.

Show the math

Running total = 2,000 + 1,000 = 3,000 W
Surge = 3,000 + 3,000 = 6,000 W
Continuous rating = 3,000 × 1.2 = 3,600 W
Amps = 3,000 ÷ 120 V = 25 A; 3,000 ÷ 240 V = 12.5 A

Rounded the same way as the result above.

How it works

A generator has two ratings: running (continuous) watts and a higher starting (surge) watts it can deliver for a few seconds. Electric motors draw a large surge when they start, often several times their running watts.

Add the running watts of everything you will operate together. Then add the highest starting watts of any one motor, since motors rarely start at the same instant. That gives the surge rating, and it is slightly conservative because that motor’s running watts are counted twice. Apply your headroom to the running total for the continuous rating.

Running total = resistive + motor running watts Surge = running total + largest starting watts Continuous rating = running total × (1 + headroom) Amps = watts ÷ volts

Worked example

2,000 W of lights and electronics and 1,000 W of motors, the largest of which starts at 3,000 W:

  1. Running total = 2,000 + 1,000 = 3,000 W
  2. Surge = 3,000 + 3,000 = 6,000 W
  3. Continuous rating = 3,000 × 1.2 = 3,600 W
  4. Amps = 3,000 ÷ 120 V = 25 A; 3,000 ÷ 240 V = 12.5 A
InputValue
Resistive and electronic loads (running watts)2000 W
All motor loads, running watts total1000 W
Starting watts of the biggest motor3000 W
Headroom on the running load20 %
ResultValue
Continuous rating needed3,600 W
Surge rating needed6,000 W
Total running watts3,000 W
Running current at 120 V25 A
Running current at 240 V12.5 A

Assumptions and limits

  • Starting watts come from nameplates or manuals; this calculator has no built-in appliance list, which would be guesswork.
  • Assumes only one motor starts at a time. If several start together, add their starting surges.
  • Not sized for altitude, temperature, or fuel; manufacturers derate for those, and the headroom setting is your choice.

Common questions

What is the difference between running and starting watts?

Running watts are what a load draws continuously. Starting watts are the brief surge a motor pulls to start, often 2 to 3 times higher or more.

How do I find the starting watts?

Check the nameplate or manual. If only amps are listed, watts are volts times amps; motors also list locked-rotor amps.

Do I need a transfer switch?

Yes for hooking a generator to your home wiring. Use a properly installed transfer switch or interlock; never backfeed through an outlet.

Where should I run a generator?

Outdoors and well away from openings, because engine exhaust contains carbon monoxide.

Sources

Updated 2026-09-30