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Generator Size Calculator: Running Watts and Starting Surge
Build an editable appliance list from typical examples or your own nameplate values. Estimate running load plus the largest extra starting surge and add a margin.
Generator Size Calculator: Running Watts and Starting Surge: Sums running watts times quantity, adds the largest positive difference between one device's starting and running watts, then applies the margin. It assumes one device starts at a time while the others run. It reports both a continuous rating with margin and a peak rating with margin. Compare stored-energy backup with the [battery runtime calculator](/battery-runtime-calculator). Runs 100% locally in your browser with zero server file uploads.
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Typical appliance examples are illustrative planning values. Replace them with nameplate ratings and measured starting watts; they are not product specifications.
| Appliance | Running W | Starting W | Add to list |
|---|---|---|---|
| Refrigerator | 150 | 600 | |
| Water pump | 800 | 2400 | |
| Lighting | 60 | 60 | |
| Kettle | 1500 | 1500 | |
| Laptop | 60 | 60 |
Formula: running total = Σ(quantity × running watts); extra surge = max(starting watts − running watts, 0) among active devices; peak = running total + largest extra surge; recommended rating = peak × (1 + margin/100).
Unmargined peak: 660 W. Starting watts means total draw during start, not extra draw. Assumes one device starts while the rest run; simultaneous starts require a larger allowance. Choose a machine meeting both continuous and surge ratings, with suitable surge duration, voltage and outlets. Watts do not capture apparent-power requirements or altitude derating.
Operate portable generators outdoors, at least 20 ft (6.1 m) from the home, with exhaust away from openings. Never use indoors or in a garage. Use carbon monoxide alarms and an electrician-installed transfer arrangement for household circuits.
Original planning examples
The editable appliance scenarios are original illustrative values: refrigerator 150/600 W, water pump 800/2400 W, lighting 60/60 W, kettle 1500/1500 W, laptop 60/60 W, each running/starting. They are not manufacturer specifications or population averages. Replace them with measured or nameplate values.
Sizing and safe operation
Running total = sum of quantity times running watts. Peak = running total plus max(starting minus running, zero). Apply the chosen margin to both continuous and peak ratings. Simultaneous starts and apparent power are excluded.
US Consumer Product Safety Commission: operate generators outdoors at least 20 feet from homes and direct exhaust away from openings. Source: https://www.cpsc.gov/Newsroom/News-Releases/2026/CPSC-Warns-of-Generator-Carbon-Monoxide-and-Fire-Hazards-Ahead-of-Hurricane-Season
How to use it
- Add appliances with quantities, running watts and total starting watts.
- Replace example values with nameplate or measured loads and set a margin.
- Check suggested continuous and peak ratings, surge duration and safety requirements.
Privacy & limitations
Your appliance list stays in your browser.
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Frequently asked questions
Are starting watts extra watts?
Enter total starting draw. The calculator subtracts running draw to find the extra surge, so the running portion is counted once.
Can two motors start together?
This estimate assumes one starts at a time. Coincident starts, poor power factor, altitude derating or short advertised surge duration can require a larger machine. Check manufacturer requirements.
Where can I operate a portable generator?
Outdoors at least 20 feet (6.1 metres) from the home, with exhaust away from openings. Never indoors or in a garage. Use CO alarms and an electrician-installed transfer arrangement for household circuits.
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