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What Size Generator Do I Need? Free Calculator

Check the appliances you'll run during an outage and get your recommended generator size in watts — in seconds.

Quick answer: most homes need 5,000–7,500 watts for essentials, or 12,000–22,000 watts for whole-house backup. Use the calculator below for your exact number.

Generator Size Calculator

Step 1: check every appliance you want to keep running during a power outage. Step 2: hit calculate.

Your recommended generator size

0 W

⚠️ This is a planning estimate. For a standby installation, have a licensed electrician confirm the final size and install a proper transfer switch.

How Generator Sizing Works

The three numbers behind every correct answer.

  1. Running watts — the steady power each appliance draws while operating. Add up everything you plan to run at the same time. This is your baseline load.
  2. Starting watts — the short surge motors need to kick on. Fridges, AC units, and pumps can pull 2–5× their running watts for a few seconds. Add only the single highest starting surge to your total (appliances rarely all start at once).
  3. Safety margin — add 20% on top. It keeps the generator from running at its absolute limit, covers small loads you forgot, and leaves room for one more appliance later.
✅ Formula: (sum of running watts + highest starting watts) × 1.2 = minimum generator watts. That's exactly what the calculator above does.

Typical Generator Size by Home

Quick reference before you calculate — your exact number comes from your appliance list.

Home sizeBedroomsTypical generator sizeCovers
1,000 sq ft25–7 kWLights, fridge, Wi-Fi, small window AC
1,500 sq ft37.5–10 kW essentials
12–16 kW whole-home		Essentials + window AC, or most circuits with small HVAC
2,000 sq ft414–20 kWNear whole-home incl. central AC
3,000 sq ft+5+20–26 kWWhole-house backup, multiple systems

Ranges, not rules — climate, electric vs gas appliances, and well pumps move the number a lot. The calculator is more accurate than any table.

Common Sizing Mistakes

Avoid these and you'll buy right the first time.

  1. Ignoring starting watts. The #1 reason generators trip. A fridge that "only uses 200W" needs a 1,200W surge to start — size for the surge.
  2. Buying too small to save money. An undersized unit trips breakers, stalls under load, and can damage motors. Saving $300 upfront isn't worth it.
  3. Buying far too big. Double the needed size wastes fuel, costs thousands more installed, and runs inefficiently at light load.
  4. Forgetting future loads. EV charger, hot tub, workshop, finished basement — if it's coming in the next few years, size for it now.
  5. Skipping the transfer switch. Backfeeding through a dryer outlet is illegal and can kill utility workers. Budget for a proper switch.
  6. Running a portable indoors. Never — not in the garage, not "just for a minute." Carbon monoxide kills silently. Outdoors, 20+ feet from windows.

Generator Sizing FAQ

The questions everyone asks before buying.

Keep Reading

⚡ Generator Wattage ChartRunning & starting watts for 30+ appliances — searchable table 🔌 Portable vs StandbyCost, install, fuel and power compared — which fits your home?