Generator Size Calculator
Find the right generator size in watts and kilowatts based on what you need to power during an outage.
Sum of everything you would run at the same time — not your whole house. Use the wattage table below.
Extra starting watts of your largest motor (AC, well pump, fridge) above its running watts. Optional.
Your Generator Estimate
- List what you must run — use the wattage table below; add only items running at the same time during an outage.
- Enter total running watts — the sum of those simultaneous loads — not everything in the house.
- Pick the load type — motor-heavy loads (AC, pumps) lower the power factor and raise the kVA the generator must supply.
- Add motor surge (Advanced) — motors start at 2–3× their running watts; enter the largest so the surge capacity is covered.
- Round up to a standard size — generators come in fixed steps; the result rounds up to the next common size.
Running Watts vs Starting Watts — the Number Most People Miss
Generators have two ratings: running (rated) watts they supply continuously, and starting (surge) watts they deliver for a few seconds. Anything with a motor — a fridge, air conditioner, well pump, power tool — draws two to three times its running watts at the instant it starts. Size only for running watts and the generator will stall or trip the moment a big motor kicks on.
The right approach is to total the running watts of everything you will run at once, add headroom so the generator is not pinned at 100%, and make sure the surge capacity covers your single largest motor starting while the rest is already running. This calculator does all three: it applies your headroom, reports the kVA the alternator must supply at your power factor, and flags the surge demand so you can match the generator's starting-watt rating.
Typical Appliance Wattage
Running and starting watts for common backup loads. Add the running watts of what you will run together:
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator / freezer | 600–800 | 1,200–2,200 |
| Furnace fan (1/2 hp) | 800 | 1,600–2,350 |
| Well pump (1 hp) | 1,000–2,000 | 2,000–4,000 |
| Sump pump | 800–1,050 | 1,300–2,150 |
| Window AC (10k BTU) | 1,200 | 1,800–3,000 |
| Central AC (3 ton) | 3,500–4,000 | 8,000–12,000 |
| Microwave | 1,000–1,500 | 0 |
| Lights / electronics | 300–600 | 0 |
| Electric water heater | 4,500 | 0 |
Common Generator Sizes
What each size class typically covers in a home:
| Size | Class | Typically powers |
|---|---|---|
| 2,000–3,500 W | Recreational / portable | Fridge, lights, phones, a few outlets |
| 5,000–7,500 W | Mid portable | Fridge, furnace fan, well pump, lights |
| 8,000–10,000 W | Large portable | Above plus a window AC or small range |
| 12,000–20,000 W | Standby | Partial-to-whole home incl. central AC |
| 22,000+ W | Whole-home standby | Entire typical home automatically |
Generator Sizing Formulas
How the recommendation is built:
Surge capacity needed = Total running watts + largest motor starting watts
Apparent power (kVA) = (Running watts required ÷ 1000) ÷ power factor
Recommended size = Next standard size ≥ the greater of required running and surge
Running a generator continuously at 100% shortens its life, so the 25% headroom is a real operating guideline, not padding.
Related Calculators
Sizing the transfer switch and feeders too? Browse the full electrical calculator collection.
Frequently Asked Questions
What size generator do I need to run a house?
For essentials — fridge, furnace fan, well pump, lights, and some outlets — a 5,000 to 7,500-watt generator is typical. To add central air conditioning or an electric range, step up to 10,000 watts or more. Whole-home automatic backup usually means a 12,000 to 22,000-watt standby unit. Total the running watts of what you will run at once and let the calculator round up.
What is the difference between running and starting watts?
Running (rated) watts are supplied continuously; starting (surge) watts are delivered for a few seconds when a motor starts. Motors draw two to three times their running watts at startup, so the generator's surge rating must cover your largest motor starting while everything else runs. The calculator reports both numbers.
Why add 25% headroom?
Running a generator at 100% continuously overheats it and shortens its life, and leaves no margin for a motor surge. Sizing to about 75–80% of rated capacity keeps it in its efficient, durable range, which is why the headroom option is on by default.
Does power factor matter for a generator?
Yes. Generators are rated in both watts (kW) and apparent power (kVA). Motor-heavy loads have a power factor below 1.0, so the alternator must supply more kVA than the watt figure alone suggests. Selecting a motor-heavy load type raises the kVA the calculator reports so you do not undersize the unit.
Can one generator start my air conditioner and run the fridge?
Only if its surge rating covers the AC starting while the fridge and other loads are already running. A 3-ton central AC can surge to 8,000–12,000 watts, so it usually needs a larger standby unit or a soft-start kit on the compressor. Enter the AC surge in Advanced to see the required surge capacity.