Total running watts
225W
Generator Wattage Calculator
Estimate the running watts, starting surge, and recommended generator wattage for the devices you want to keep powered.
This tool is for planning a load list before comparing it with a generator rating. It does not verify a specific generator model, electrical connection method, transfer equipment, fuel plan, or code requirement.
Generator sizing inputs
%
Extra sizing margin added to the peak starting estimate.
Uses the largest single extra starting surge.
Results
Estimated peak starting watts
1,275W
Recommended generator size
1,600W
Rounded up to the nearest 100 watts.
Surge mode
Staggered
Staggered mode assumes the largest starting surge is the main extra load.
Headroom included
20%
Selected device table
| Item | Value | Notes |
|---|---|---|
| Refrigerator | 1 x 150 W running = 150 W | Starting 1,200 W each; extra surge 1,050 W total. |
| Wi-Fi router | 1 x 15 W running = 15 W | Starting 15 W each; extra surge 0 W total. |
| LED light bulb | 6 x 10 W running = 60 W | Starting 10 W each; extra surge 0 W total. |
Formula
recommendedWatts = roundUpToNearestHundred((runningTotal + startingExtra) x (1 + headroomPercent / 100))Extra starting load is calculated as starting watts minus running watts, never below zero. Staggered mode assumes major motor loads are started one at a time; conservative mode assumes those surges may overlap.
- runningTotal
- The sum of quantity x running watts for every selected device.
- startingExtra
- In staggered mode, the largest extra starting load. In conservative mode, the sum of all extra starting loads.
- headroomPercent
- A planning margin added after estimating peak starting watts.
Example
This example uses staggered surge mode. Conservative mode can produce a higher estimate when multiple motor loads may start at the same time.
- A refrigerator uses 150 running watts and 1,200 starting watts.
- A Wi-Fi router uses 15 running watts and has no extra starting surge.
- A laptop charger uses 60 running watts and has no extra starting surge.
- Running load is 225 W. The largest extra starting surge is 1,050 W, so peak starting watts are 1,275 W.
- With 20% headroom, the rounded recommended generator size is 1,600 W.
How to use this calculator
- Add each device you want to run during the outage or project.
- Use the device label or manual for running watts and starting watts when available.
- Choose staggered surge mode for one-at-a-time starting, or conservative mode when loads may start together.
- Review warnings, then compare the recommended watts with equipment specifications and qualified guidance.
Input guide
- Quantity: how many identical devices are included in the load list.
- Running watts: the steady power draw while the device is operating.
- Starting watts: the short surge used by motors, pumps, compressors, or similar equipment.
- Headroom: an extra planning margin added to the peak starting estimate.
Common mistakes
- Using only running watts for devices with compressors or motors.
- Assuming every device can start at exactly the right time.
- Mixing up watts, volt-amps, amps, and watt-hours.
- Ignoring manufacturer limits, fuel safety, extension cord ratings, transfer equipment, or local code requirements.
Limitations
The estimate uses simple wattage arithmetic. It does not model power factor, voltage drop, generator altitude derating, fuel condition, motor start profiles, simultaneous thermostat behavior, or brand specific surge capability.
FAQ
Why do starting watts matter?
Motors and compressors can briefly need more power when starting than they use while running. The estimate accounts for that surge.
Is staggered starting always safe?
No. Staggered starting is only an estimate that assumes large loads do not start at the same time. Use conservative mode when that assumption is not realistic.
Should I use running watts or starting watts from the label?
Use running watts for the steady load and starting watts for short motor or compressor surge. If the label only lists amps, convert with volts x amps before using the value.
Does this calculator tell me how to connect a generator?
No. It estimates wattage only and does not provide wiring, transfer switch, interlock, backfeed, or whole-home connection instructions.
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Methodology and disclaimer
This calculator is for planning estimates only. For high-power equipment, transfer switches, whole-home connections, fuel safety, and code compliance, consult a qualified electrician, device manufacturer, or local authority.
The calculator sums steady running load, adds either the largest extra starting surge or all extra starting surges, then applies the selected headroom percentage and rounds up to the nearest 100 watts.