Estimate the generator size needed to run multiple loads, accounting for motor startup surge.
What it powers
Watt breakdown
Usage Tip
Stagger when you switch on big motor loads like a well pump or AC so their surges do not stack.
starting watts = running + (largest load × (surge − 1))
Size the generator to the starting-watts figure.
What size generator do I need?
Sizing a generator is not just adding up watts. You need the total running watts of everything that will run at once, plus enough extra to cover the single biggest motor as it starts — the starting surge. A fridge runs at about 700 watts but jolts to 2,200 for a moment when the compressor kicks in. Get the running figure wrong and the generator is too small; ignore the surge and it stalls every time the well pump cycles. This calculator adds the running load, finds the largest surge, and recommends a generator with the right running and surge ratings, plus the fuel and runtime.
Starting watts vs running watts
Anything with a motor — fridges, pumps, air conditioners, furnace fans — draws a brief surge several times its running wattage to get spinning. Resistive loads like lights, microwaves and heaters do not. You size the generator’s running rating for the steady load, and its surge (peak) rating for the running load plus the largest single starting surge, since they rarely all start at the same instant.
| Appliance | Running | Starting |
|---|---|---|
| Refrigerator | 700W | 2,200W |
| Freezer | 600W | 1,800W |
| Sump pump | 800W | 2,400W |
| Well pump | 1,000W | 3,000W |
| Furnace fan | 800W | 2,350W |
| Window AC | 1,200W | 3,600W |
| Central AC | 3,500W | 8,750W |
| Microwave | 1,200W | 1,200W |
| Electric water heater | 4,500W | 4,500W |
Generator size chart
| Use | Generator |
|---|---|
| Camping / tailgating | 2,000W |
| Basic home backup (fridge, furnace, lights) | 5,000W |
| Well pump plus house | 8,000W |
| Whole home | 12,000W and up |
Emergency load presets
A good starting point for what to back up:
- Essential home — refrigerator, furnace fan, a few lights, internet. Roughly a 3,000 to 5,000W generator.
- Comfort — the essentials plus microwave, TV and a window AC. Around 6,000 to 8,000W.
- Whole home — nearly everything except the EV charger. 12,000W and up, usually a standby unit.
- RV — air conditioner, microwave, refrigerator. 3,000 to 4,000W.
Fuel comparison
| Fuel | Notes |
|---|---|
| Gasoline | Cheap and available, but short shelf life and you must store it |
| Propane | Long shelf life, clean burning, slightly less power; tanks are easy to store |
| Natural gas | Continuous utility supply, no refueling, lower output; needs a gas hookup |
| Diesel | Efficient and durable with long runtime; the units cost more |
Generator types
Portable generators are the common gasoline units on wheels. Inverter generators are quieter and produce clean power for electronics, but cost more per watt. Whole-house standby units run on natural gas or propane, start automatically, and power the whole panel. RV generators are compact and built into or sized for a recreational vehicle.
Frequently asked questions
What size generator to run a refrigerator?
A fridge needs about 700 running watts but surges to roughly 2,200 to start, so a 2,000 to 3,000W generator handles it with margin.
What size generator for a well pump?
A typical 1/2 hp well pump runs near 1,000W but surges to about 3,000W, so plan for that surge on top of your other loads.
What size generator to run a house?
Essentials need 5,000 to 7,500W; a whole house with central AC usually wants 12,000W or more.
How long will a generator run on a tank?
Often 8 to 12 hours at half load for a portable, depending on tank size and fuel. Lower load means longer runtime.
Why does my generator stall when the AC turns on?
The starting surge exceeds the surge rating. Size for the running load plus the largest single starting surge.
Can one generator start two motors at once?
Risky. Stagger large motor loads, or size the surge rating to cover more than one starting at the same time.
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Estimates are for planning. Appliance wattages vary by model — check the nameplate — and motor surges, altitude, and power factor all affect real sizing. A generator must be connected through a proper transfer switch by a licensed electrician; never backfeed a panel. Follow the manufacturer’s instructions and local code.