Outage load planning
Backup generator sizing calculator: running and peak watts
What size generator do you need? Enough running watts for everything on at once, plus a 20% margin, and enough peak watts for your biggest motor start. Enter both below to get the rating pair to shop for and the rated generators whose stated output covers it.
Recommended ratings
- Running rating
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- Peak coverage
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- Matching ranking
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Rated generators whose stated output covers it
The five best-rated generators whose listing or manual states at least the recommended running and peak watts (gasoline rating) and lists the fuel you chose, one per listing family.
| Enter your values above. |
How to read the recommendation
Recommended running rating = your running total × 1.2, rounded up to the next 500 W. Recommended peak coverage = running total + largest starting surge, rounded up to the next 500 W. At 4,000 W running and a 1,500 W surge, that is 5,000 W running and 5,500 W peak.
The 20% margin keeps the engine off its continuous limit while compressors cycle. Actual demand varies with weather, appliance condition and what your household switches on; dual fuel models also rate lower on propane than on gasoline — plan around the rating of the fuel you will depend on.
Before choosing a generator
- List essential loads and read their nameplate running watts.
- Identify the largest motor-start surge (well pump, sump pump, AC).
- Check both figures against the exact listing and manual, per fuel.
This tool does not establish equipment safety or electrical compatibility. Do not use it to qualify backup power for medical or life-support equipment; follow the equipment provider's requirements and a qualified backup plan. Any connection to household wiring belongs to a licensed electrician.
Read our research methodology and the starting vs running watts guide.
Frequently asked questions
How many watts does my generator need?
Add up the running watts of every appliance that will run at the same time (nameplate or manual), then find the largest starting surge above running, such as a well pump, sump pump or air conditioner. The calculator adds a 20% margin to the running total and rounds up to the next 500 W.
Why does running watts matter more than peak watts?
Peak (starting) watts are available only for a few seconds while a motor starts. Over hours, a generator can only carry its running watts; running at that limit leaves no headroom when compressors cycle.
Does the tool pick a generator for me?
No. It lists rated generators whose listing or manual states at least the recommended running and peak watts, with the fuel you plan to use. Ratings are seller figures, usually for gasoline, not our measurements; propane and natural gas ratings are lower.
Can I use it as a backup generator sizing calculator for my house?
Yes, for a portable on essential circuits: list what must run together during an outage (fridge, freezer, sump or well pump, furnace blower, lights, internet), enter the running total and the biggest starting surge, and read the recommended ratings. For hardwired circuits, your electrician sizes the transfer switch or interlock with you.
Is this a standby or whole house generator sizing calculator?
It uses the same arithmetic, but this site covers portable generators only. A permanently installed standby unit is sized by the installer, who also handles the gas line and the automatic transfer switch. Whole-house sizing on a portable usually lands in our 15,000 watt generators or 10,000 watt generators rankings.
Is there a generator sizing chart?
Generic wattage charts are a rough guide at best: a modern fridge can run far below the chart figure and a deep-well pump far above it. Read the nameplate of each appliance instead. Our guide on sizing a backup generator walks through it load by load.