Last updated June 18, 2026 by our expert review team
What Size Generator Do I Need?
Add up the running watts of everything you'll run at once, then add the single largest startup surge. Outage essentials need a 3,000–5,000 W generator; running most of a home (no central air) takes about 7,500 W; whole-house with central AC is 10,000–12,000 W standby territory.
Typical backup scenarios:
| What you're running | Generator |
|---|---|
| Phones, lights, Wi-Fi | 2,000 W |
| Fridge + lights + furnace | 3,000–4,000 W |
| Above + well & sump pump | 5,000 W |
| Most of a home (no central AC) | 7,500 W |
| Whole house with central AC | 10,000–12,000 W |
Running watts shown; the generator's starting rating must also clear the largest motor surge. The calculator does both.
Expert Contributors
Ehsan Ghazanfari
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FISE-certified structural engineer with 11+ years designing bridges, retaining walls, and foundations. MSc from Aalto University.
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Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
See full profileUpdated June 2026
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How the Generator Size Calculator Works
Two numbers size a generator. Running watts is the steady draw of everything on at once. Starting watts is the brief surge when a motor kicks on — and since motors don't all start at the same instant, the worst case is your total running load plus the single largest surge.
The calculator sums the running watts, adds the biggest surge, and recommends the next standard generator size whose running and starting ratings clear both. See DOE backup-power guidance.
Formulas
Running watts = sum of all appliances on at once
Peak watts = running total + largest single surge
Generator: running rating ≥ running, starting rating ≥ peak
Quick Reference
- Essentials
- 2,000–3,000 W
- Fridge + furnace
- 3,500–5,000 W
- Most of a home
- 7,500 W
- + Central AC
- 10,000 W+
- Surge rule
- +largest motor only
Pick your type
Inverter, portable, or standby?
Once you know the wattage, the type comes down to how much you're powering, how quiet and clean you need it, and whether you want it automatic.
Portable inverter
2–4 kW
Choose it if: Outage essentials, RVs, and electronics. Sips fuel at part load and runs quietly — but limited capacity.
quiet, clean power
Portable conventional
4–10 kW
Choose it if: Fridge, furnace, pumps, and tools during outages. Best value; louder, manual start, refuel by hand.
most watts per dollar
Standby (whole-house)
10–24 kW
Choose it if: Hands-off whole-home backup including central AC. Auto-starts on a transfer switch; permanently installed.
auto, highest cost
Appliance Wattage Reference
Typical running and starting watts. Motors surge at startup; resistive loads (heaters, bulbs) don't:
| Appliance | Running |
|---|---|
| Refrigerator / freezer | 700 W |
| Lights (10 bulbs) | 400 W |
| Phone / laptop charging | 100 W |
| TV + Wi-Fi router | 300 W |
| Furnace blower (½ HP) | 800 W |
| Window AC (10k BTU) | 1,200 W |
| Central AC (3 ton) | 3,500 W |
| Space heater | 1,500 W |
| Sump pump (⅓ HP) | 800 W |
| Well pump (½ HP) | 1,000 W |
| Electric water heater | 4,000 W |
| Microwave | 1,000 W |
| Coffee maker | 1,000 W |
| Washing machine | 1,200 W |
Representative values — always confirm on the appliance's nameplate or manual.
Generator Sizing Examples
Outage Essentials
4 appliancesThe fridge's startup surge sets the peak — everything else just adds running watts. A 2,000–3,000 W inverter covers this quietly.
Heat + Water Backup
5 appliancesTwo motors (well pump, furnace) mean real surge headroom. A 5,000 W generator handles staggered startups.
Whole House + Central AC
6 appliancesCentral AC is the big load. This is standby-generator territory — 10,000 W+ with a transfer switch.
Avoid these
Generator Sizing Mistakes
Sizing by running watts only
Motors surge at startup. The generator's starting rating must clear your biggest surge on top of the running load.
Adding every appliance's starting watts
Only the single largest surge stacks on the running total — motors don't all start at once. Adding them all wildly oversizes.
Backfeeding through a dryer outlet
Dangerous and illegal. Use a transfer switch installed by an electrician to power house wiring.
Running it in the garage
Carbon monoxide is fatal fast. Outdoors only, 20+ ft from the house, exhaust pointed away from openings.
Generator Size Calculator FAQs
What size generator do I need for a house?
What's the difference between running and starting watts?
Why do I only add the largest starting surge, not all of them?
What size generator runs a refrigerator and furnace?
What size generator do I need for central air conditioning?
Do I need a transfer switch?
Can I run a generator indoors or in the garage?
Working out a circuit? Convert power with the amp calculator, or size heating load with the BTU calculator.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.