Enter the total simultaneous running load and the largest additional motor-starting load above its running watts. This shortcut assumes staged starts. If motors can restart together, their overlapping surge requires a separate calculation.
Compare continuous and surge ratings on the actual generator, at the installation’s conditions.
Generator Size: worked example
| Running watts of everything on together | 3,000 |
|---|---|
| Extra surge of the largest motor at start | 2,000 |
| Minimum generator size (watts) | 5,000 |
| With 20% headroom (watts) | 6,000 |
Generator Size: changing Running watts of everything on together
| Running watts of everything on together | Minimum generator size (watts) | With 20% headroom (watts) |
|---|---|---|
| 2,400 | 4,400 | 5,280 |
| 2,700 | 4,700 | 5,640 |
| 3,000 (example) | 5,000 | 6,000 |
| 3,300 | 5,300 | 6,360 |
| 3,600 | 5,600 | 6,720 |
Generator Size: compare input ranges
| Input | Tested from | To | Minimum generator size (watts): low input | Minimum generator size (watts): high input | Swing |
|---|---|---|---|---|---|
| Running watts of everything on together | 2,700 | 3,300 | 4,700 | 5,300 | 600 (12%) |
| Extra surge of the largest motor at start | 1,800 | 2,200 | 4,800 | 5,200 | 400 (8.0%) |
Generator Size: individual input comparisons
| Input | Value | Minimum generator size (watts) | With 20% headroom (watts) |
|---|---|---|---|
| Running watts of everything on together | 2,400 | 4,400 | 5,280 |
| 2,700 | 4,700 | 5,640 | |
| 3,000 | 5,000 | 6,000 | |
| 3,300 | 5,300 | 6,360 | |
| 3,600 | 5,600 | 6,720 | |
| Extra surge of the largest motor at start | 1,600 | 4,600 | 5,520 |
| 1,800 | 4,800 | 5,760 | |
| 2,000 | 5,000 | 6,000 | |
| 2,200 | 5,200 | 6,240 | |
| 2,400 | 5,400 | 6,480 |
Generator Size: combined input changes
| Running watts of everything on together / Extra surge of the largest motor at start | 1,600 | 2,000 | 2,400 |
|---|---|---|---|
| 2,400 | 4,000 | 4,400 | 4,800 |
| 2,700 | 4,300 | 4,700 | 5,100 |
| 3,000 | 4,600 | 5,000 | 5,400 |
| 3,300 | 4,900 | 5,300 | 5,700 |
| 3,600 | 5,200 | 5,600 | 6,000 |
Generator Size: which assumptions move the result?
Generator Size: values and definitions
| Quantity and definition | Value used |
|---|---|
| Running watts of everything on together | 3,000 |
| Extra surge of the largest motor at start — Motors draw far more when they start. Check the label or manual for the starting wattage. | 2,000 |
| Minimum generator size (watts) | 5,000 |
| With 20% headroom (watts) | 6,000 |
Generator Size: half and double Running watts of everything on together
| Running watts of everything on together | Minimum generator size (watts) |
|---|---|
| 1,500 | 3,500 |
| 3,000 | 5,000 |
| 6,000 | 8,000 |
Starting capacity depends on the equipment and allowed voltage dip, as well as fuel, temperature and altitude. Use manufacturer sizing guidance for the actual loads. House connections require appropriate transfer equipment.
Operate fuel-burning portable generators outdoors away from openings according to the manufacturer’s carbon-monoxide precautions.
Generator Size: formula
U.S. Energy Information Administration: electricity data · Generac: Starting and running wattage
Common error: Assuming motor starts never overlap. After an outage, several automatic loads can restart together; the largest-single-surge shortcut only covers staged starts.
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A generator must handle two loads: everything running together, and the extra kick when the biggest motor starts. A refrigerator or air conditioner can pull several times its running watts for a moment. With 3,000 running watts and a 2,000 watt surge, you need at least 5,000, and 6,000 with headroom keeps the engine out of its overload range.
Find the starting watts on each motor's label or manual. Estimating them is how people end up with a generator that trips the moment the compressor kicks in.
Estimate battery backup on the battery runtime calculator, the daily energy use on the electricity cost calculator and solar on the solar panel output calculator. The watts to amps and three-phase power pages size wiring, and the horsepower calculator relates to engine size.
How do I size a generator for my house?
Simultaneous loads contribute to the combined running-watt requirement. Obtain their running requirements. Obtain their starting requirements too.
Twenty percent reserve is an assumption. It is not a universal sizing rule. Check coincident motor starts, load management and generator capability; total starting watts already include a running component, so adding them as extra surge can count a load twice.
Why do motors need extra starting power?
Motor starting can require more power than steady operation. Use the appliance or motor’s actual starting requirement and the generator’s surge capability. A two- or three-times multiplier is only a scenario; motor type, starting method and simultaneous loads determine whether that approximation is adequate.
Can I plug a generator into a wall outlet?
No. Backfeeding is dangerous and illegal, so use a transfer switch or interlock installed by an electrician.