generator tool
Generator Size Calculator
Estimate generator capacity from running loads and startup surge.
Load builder
What needs to run?
Refrigerator
Preset planning estimate
Freezer
Preset planning estimate
Use equipment label values when available. Preset appliance values are planning estimates only.
Generator sizing guide
How this generator size estimate works
The load builder keeps continuous running demand separate from starting demand. Add the appliances you expect to run, edit their values from equipment labels when available, and use the headroom field as an explicit planning assumption.
Current sizing logic
running load = running watts × quantity
total running watts = sum of all running loads
surge delta = max(0, starting watts − running watts) × quantity
running capacity = total running watts × (1 + headroom ÷ 100)
estimated peak = total running watts + largest surge delta
The current implementation uses the largest applicable row surge, rather than adding every appliance's starting wattage together. It assumes identical selected loads in a row may start together.
Worked examples
The wattages below are illustrative planning examples, not universal appliance specifications. Check actual nameplates or manufacturer documentation.
Refrigerator + lights/fans: using illustrative 180 W running/600 W starting for a refrigerator and 300 W for lights/fans gives 480 W total running, a 420 W largest surge delta, 576 W running capacity with 20% headroom, and a 900 W estimated peak.
Refrigerator + water pump + lights/fans: using illustrative 180/600 W, 1,000/3,000 W, and 300 W values gives 1,480 W total running, a 2,000 W largest surge delta, 1,776 W running capacity with 20% headroom, and a 3,480 W estimated peak.
Running versus peak ratings
| Requirement | What it represents | What to verify |
|---|---|---|
| Running/continuous watts | Normal combined load | Generator rated running output |
| Starting/peak watts | Temporary startup requirement | Generator peak output and appliance startup data |
Generator nameplates distinguish rated running output from peak or starting output. This calculator is a planning estimate and does not guarantee compatibility, safe home connection, or performance under every load combination.
Frequently asked questions
What are running watts?
Running watts are the continuous load used while an appliance operates normally.
What is the starting surge?
Starting surge is the applicable extra demand above running watts when a selected load starts. Motors, pumps, compressors, and similar equipment can require more at startup.
How does the calculator apply headroom?
It multiplies total running watts by 1 plus the entered headroom percentage. The current default is 20%.
Does the result guarantee a generator will work?
No. Verify generator nameplate running and peak ratings, appliance labels, voltage requirements, and installation details with the manufacturer and a qualified professional.
Important assumptions
This is a planning estimate, not a guarantee of performance. Real equipment varies with temperature, age, power quality, duty cycle, wiring, and manufacturer specifications.