Generator Sizing Calculator
Size a portable or standby generator from your actual appliance list, not one flat number. Enter each load's running watts and (for motors) starting watts, add a safety margin, and get the total running load, the worst-case starting surge, a recommended generator size, and the amp draw at 120V/240V. Runs entirely in your browser.
Input
One load per line: name, running watts[, starting watts]. Starting watts only matters for motors — pumps, compressors, fridges, AC units. Leave it off for lights, electronics and other non-motor loads.
Extra headroom above the calculated requirement — 20% is a common rule of thumb.
Output
| Voltage | Amps |
|---|---|
| No data yet | |
| Appliance | Running (W) | Starting (W) | Extra surge (W) |
|---|---|---|---|
| No data yet | |||
Guides
Figure out what size portable or standby generator you actually need, from your real appliance list — not one guessed-at "total load" number.
Most quick sizing tools ask for a single wattage figure and call it done. That misses the thing that actually determines generator size: a generator doesn't just need to carry your steady running load, it needs to survive the surge of starting a motor. A refrigerator, sump pump or AC compressor can pull 2–3× its running watts for the first second or two of starting — and that spike, not the running total, is usually what a cheap generator trips on.
How to use it
- List your appliances, one per line:
name, running watts[, starting watts]. Running watts is what the label/nameplate lists as continuous draw. Starting watts only matters for motors — fridges, freezers, sump/well pumps, air conditioners, power tools. Leave it off for lights, electronics, chargers and anything else without a motor. - Set a safety margin — 20% is the common rule of thumb, giving headroom for voltage sag and appliances running slightly hotter than nameplate.
- Read the results: total running load, worst-case starting load, a recommended generator size rounded up to a standard nameplate wattage, and the amp draw at 120V and 240V so you can check it against your panel or extension cords.
How the calculation works
Total running watts is just the sum of every load's running watts — this is the sustained load the generator has to carry indefinitely.
Peak starting watts assumes only one large motor starts at a time (the normal case — you don't start your fridge and your sump pump in the same instant): it's the total running load, plus the extra draw of whichever single motor has the biggest gap between its running and starting watts.
Recommended size takes the larger of the (margin-adjusted) running and starting requirements and rounds up to the nearest common generator nameplate size, since generators are sold in fixed sizes, not built to order.
Why does starting watts matter more than running watts?
Because a generator that can carry your running load all day can still stall or trip its breaker the instant a motor's inrush current hits it. Running-watts-only sizing works fine for a load of lights and electronics; it fails as soon as anything with a compressor or pump is on the list.
What if I might start two big motors at once?
This calculator assumes the common case — one motor at a time. If your setup could genuinely start two large motors simultaneously (e.g. staggered-start isn't an option), size for their combined starting surge instead of just the largest one, which means sizing up from what's shown here.
Is this a substitute for an electrician's load calculation?
No. This is a rule-of-thumb sizing estimate for choosing a generator. Before permanently wiring a standby generator through a transfer switch, get a licensed electrician to do a proper load calculation and confirm the installation meets your local code.
Related tools
Working out the breaker or wiring on the other side of the panel? The Circuit Breaker Sizing Helper turns a load current into a recommended breaker rating, and the UPS Battery Backup Time Calculator estimates how long a battery backup — rather than a fuel generator — would carry the same load.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/generator-sizing-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"loads": "Refrigerator, 700, 2200\nSump Pump, 800, 2000\nLig…",
"safetyMargin": "20"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `generator-sizing-calculator` tool (Generator Sizing Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.