Answer first
What this calculator tells you
Estimate the generator wattage needed from running watts and the largest starting surge. Size a portable generator so a starting motor does not trip it. Formula: Starting watts needed = running watts of everything on + the largest single starting surge; add 20 percent headroom. At the worked-example inputs, the minimum generator size (watts) is 5,000. Holding every other input steady, moving running watts of everything on together from 2,400 to 3,600 moves the result from 4,400 to 5,600.
Transparent method
The formula
Size a portable generator so a starting motor does not trip it.
Worked example
Example inputs
How to interpret the result
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.
At the worked-example inputs the minimum generator size (watts) is 5,000. It rises with running watts of everything on together and extra surge of the largest motor at start.
Real energy use depends on how the appliance or vehicle is actually run: duty cycle, weather, driving style, battery age and your utility's rate structure, including tiers and time-of-use pricing. Treat the result as a planning estimate and check it against your own bill or trip log.
Before you rely on it
What to check
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.
The common error
Where people go wrong with generator size calculator
Adding every appliance's surge together. Motors rarely start at the same instant, so only the largest single surge is added on top of the running total.
Sensitivity evidence
How running watts of everything on together changes the minimum generator size (watts)
Holding every other input at the worked-example value, moving running watts of everything on together from 2,400 to 3,600 moves the minimum generator size (watts) from 4,400 to 5,600: a spread of 1,200, or 24% of the worked-example result.
| 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 |
| 3,000worked example | 5,000 | 6,000 |
| 3,300 | 5,300 | 6,360 |
| 3,600 | 5,600 | 6,720 |
Every input, tested
Which input moves the minimum generator size (watts) most
Of the 2 inputs, running watts of everything on together moves the minimum generator size (watts) most (600 across the range tested) and extra surge of the largest motor at start moves it least (400).
| Input | Tested from | To | Minimum generator size (watts) at each end | Swing |
|---|---|---|---|---|
| Running watts of everything on together | 2,700 | 3,300 | 4,700 to 5,300 | 600 (12%) |
| Extra surge of the largest motor at start | 1,800 | 2,200 | 4,800 to 5,200 | 400 (8.0%) |
Two variables at once
Minimum generator size (watts) by running watts of everything on together and extra surge of the largest motor at start
Across the grid the minimum generator size (watts) runs from 4,000 to 6,000. Moving running watts of everything on together from 2,400 to 3,600 shifts it by 1,200 at the middle column, and moving extra surge of the largest motor at start from 1,600 to 2,400 shifts it by 800 at the middle row, so running watts of everything on together is the bigger lever here.
| 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 |
The highlighted cell is the worked example: 5,000.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Running watts of everything on together | 3,000 | Enter the running watts of everything on together used in this calculation. |
| Extra surge of the largest motor at start | 2,000 | Motors draw far more when they start. Check the label or manual for the starting wattage. |
| Minimum generator size (watts) | 5,000 | |
| With 20% headroom (watts) | 6,000 | |
Inputs, definitions and assumptions
Running watts of everything on together
Enter the running watts of everything on together used in this calculation. The prefilled worked-example value is 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. The prefilled worked-example value is 2,000.
How to use this calculator
- 1Verify the inputs. Gather running watts of everything on together and extra surge of the largest motor at start from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the minimum generator size (watts) at 5,000. Store your own version of it as Scenario A.
- 3Test one change. Start with running watts of everything on together, the input with the biggest effect here: moving running watts of everything on together from 2,700 to 3,300 takes the minimum generator size (watts) from 4,700 to 5,300, a swing of 12% of the worked-example figure.
- 4Check the extremes. At half the example running watts of everything on together (1,500) the minimum generator size (watts) is 3,500; at double (6,000) it is 8,000.
People also ask
Frequently asked questions
How do you calculate generator size?
Starting watts needed = running watts of everything on + the largest single starting surge; add 20 percent headroom. At the worked-example inputs the minimum generator size (watts) is 5,000.
What does the generator size result mean?
Size a portable generator so a starting motor does not trip it. At the worked-example inputs the minimum generator size (watts) is 5,000. It rises with running watts of everything on together and extra surge of the largest motor at start.
How much does running watts of everything on together change the minimum generator size (watts)?
Holding every other input at the worked-example value, moving running watts of everything on together from 2,400 to 3,600 moves the minimum generator size (watts) from 4,400 to 5,600, a spread of 1,200.
What are the limits of this generator size calculator?
Real energy use depends on how the appliance or vehicle is actually run: duty cycle, weather, driving style, battery age and your utility's rate structure, including tiers and time-of-use pricing. Treat the result as a planning estimate and check it against your own bill or trip log. The tables on this page test running watts of everything on together only from 2,400 to 3,600; a value outside that range is not tabulated here.
Which input moves the minimum generator size (watts) most in the generator size calculator?
Ranked by how far each moves the minimum generator size (watts) across the range tested: running watts of everything on together (600, 12%) and extra surge of the largest motor at start (400, 8.0%).
If I double running watts of everything on together in the generator size calculator, does the minimum generator size (watts) double?
Doubling it from 3,000 to 6,000 takes the minimum generator size (watts) from 5,000 to 8,000, which is 1.60 times the worked-example figure. So it grows, but by less than double. Halving it to 1,500 gives 3,500.
How much does extra surge of the largest motor at start matter in the generator size calculator?
The worked example uses 2,000. Holding every other input at its worked-example value, moving extra surge of the largest motor at start from 1,800 to 2,200 takes the minimum generator size (watts) from 4,800 to 5,200, a swing of 8.0% of the worked-example figure.
Which inputs change the with 20% headroom (watts) in the generator size calculator?
At the worked-example inputs it is 6,000. Running watts of everything on together takes it from 5,640 to 6,360 and extra surge of the largest motor at start takes it from 5,760 to 6,240.
Why do solar panels produce less than their rated watts?
Ratings come from lab conditions of strong sun and cool panels. Real roofs face heat, dirt, shade, wiring losses and an angled sun, so a planning loss of around 14 percent is a common allowance.
How long will a battery really last?
Runtime falls as the load rises and as the battery ages, cools or drains deeply. The calculator shows the best case at your chosen usable depth, so plan on somewhat less.
What is a peak sun hour?
One hour of sunlight at 1,000 watts per square meter, the standard test strength. A day with five peak sun hours delivers the same energy as five hours at that strength, even if daylight lasts much longer.
Sources and evidence
Free Calculators Online is independent and is not affiliated with or endorsed by the source organizations. Educational estimates only.