Check off the critical devices you want to keep running during an electrical grid failure to calculate peak inrush surge and continuous load.
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Whole-House Generator Sizer
Presets:
Selected Outage Appliances
Required Electrical Capacity
Continuous Running 0W
Peak Inrush Surge 0W
Load Tier Awaiting Selection
Recommended Generator: 0 kW
⛽ Outage Duration & Fuel Reserve Planner
⛽ Est. Gasoline Needed: 0 Gal
🔵 Or 20 lb Propane Tanks: 0 Tanks
*Tip: Store fuel with stabilizer (STA-BIL) to prevent gumming, and keep 5-gallon EPA cans ready.
Matched Power Equipment
Select appliances above to calculate sizing and live equipment matches.
🛡️ Generator Safety (25-Foot Rule):
Always operate generators at least 20 to 25 feet away from windows and doors to prevent carbon monoxide poisoning. For 25–50 ft distances, use an all-weather 10 AWG extension cord.
How the WattSpec Generator Sizing Formula Works
Electricians calculate backup generator capacity using three core principles:
- Total Running Load: The continuous wattage of all devices operating simultaneously.
- Largest Motor Startup Surge: Motors do not start at the same millisecond. Peak surge is calculated as the sum of all running watts plus the surge difference of the single largest motor.
- 20% Continuous Safety Margin: Generators should not run at 100% maximum capacity continuously. Adding a 20% cushion prevents overheating.