What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Loads: 3 appliances.
- Safety margin: 20 %.
What happens, step by step
- Appliances start one at a time, and their running watts stack up.
- Appliance 2 has no start surge: a steady 1,500 W.
- Appliance 3 spikes to 1,800 W to start, then settles at 600 W.
- The biggest surge starts last, on top of everything: 4,300 W peak.
- Choose 3,360 W running, with 20% margin, and 4,300 W surge.
The answer: recommended generator running rating is 3,360 W.
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
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.