Generator Size Calculator

Motors (refrigerators, pumps, AC) draw several times their running power for a moment when starting. Only the largest single starting surge is added, assuming motors start one at a time.

Important: Connect a generator to house wiring only through a listed transfer switch or interlock installed per code. Never run a generator indoors or in an attached garage — carbon monoxide is deadly.

Tapeline — Generator Size Calculator

Printed September 29, 2026

Loads700 × 2200; 1500 × 1500; 600 × 1800 W
Safety margin20 %

Results update as you type.

Loads
Appliance 1
Appliance 2
Appliance 3
%

How it’s calculated

  • Running = Σ running watts
  • Peak = running + largest (starting − running) of any one appliance
  • Recommended running rating = running × (1 + margin)

Worked example

Refrigerator (700/2,200 W), space heater (1,500 W), sump pump (600/1,800 W), 20% margin.

Inputs

Loads
700 × 2200; 1500 × 1500; 600 × 1800 W
Safety margin
20 %

Results

Recommended generator running rating
3,360 W
Total running watts
2,800 W
Peak watts (largest start)
4,300 W

Load this example into the calculator

Assumptions and limits

  • Motors are started one at a time.
  • Use nameplate or manufacturer data; starting watts vary widely.

Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.

Questions

What size generator do I need to run a refrigerator?

A typical refrigerator runs at a few hundred watts but may need 1,500–2,200 W to start. Check the label or manufacturer data.

What is the difference between running and starting watts?

Running watts are continuous; starting (surge) watts are the brief peak when a motor starts.