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Whole House Generator Size Calculator

Plan your whole-house standby generator. Select every appliance in your home to get an accurate NEC-compliant generator size and monthly fuel cost estimate.

🏠
Full Home Coverage
Power everything including central AC, electric dryer, oven, and well pump as if the grid never went down.
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Automatic Transfer
Standby generators with ATS detect outages and start in 10-30 seconds, even when you are away.
📈
Home Value
A whole-house generator adds 3-5% to home value and is increasingly expected in outage-prone areas.
🛡️
Extended Outages
With a large propane tank or natural gas line, run for weeks without refueling.
W
NEC 80% Rule: Per NEC 210.20(A), continuous loads (running 3+ hours) should not exceed 80% of rated capacity. Your 22,000W generator safely handles 17,600W continuous load.

Your appliances

Running W = normal draw · Starting W = motor startup surge · Contruns 3+ hours (NEC continuous load) · Non-Contshort duty

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4 appliances · 2,650W running
Your Generator Capacity
22,000W
watts
Load Status16%
NEC Load: 2,888WUsable: 17,600W
Gas Furnace BlowerMAX SURGE800/2,000WCont
Refrigerator150/600WCont
Microwave (1000W)1,200WNon-Cont
Washing Machine500/1,200WNon-Cont

Fuel Cost Estimate

gal
Price
$$/gal
Hours/Day24h
Diversity?70%
Avg Load?
3%
Rate: 0.1 gal/kW/hr. Sources: Generac, Honda, US EIA
Per Hour
$0
Per Day
$5
Per Month
$143
Calculation breakdown
Base Load: 950W ÷ 22,000W = 4%
Avg Load: 4% × 70% = 3%
Fuel/hr: 22.0 kW × 0.1 × 3% = 0.07 gal/hr
Cost/hr: 0.07 × $3.00 = $0.20/hr
Cost/day: $0.20 × 24h = $5/day
Cost/month: $5 × 30 = $143/mo

PickMyWatt.com

Whole House Generator Size Calculator

Your Generator
22,000
watts
Load: 16%
Continuous
950W
Non-Cont
1,700W
Starting Surge
1,200W

Selected Appliances

Gas Furnace BlowerMAX SURGE800/2,000W
Refrigerator150/600W
Microwave (1000W)1,200W
Washing Machine500/1,200W
Fuel Cost Estimate
Per Hour
$0.20
Per Day
$5
Per Month
$143
propane

Calculated with PickMyWatt.com

NEC Article 220 Compliant

Calculation Standard: Based on NEC Article 220 load calculation principles. Continuous loads (>3 hours) sized at 125% per NEC 210.20(A). Non-continuous loads use only the largest expected simultaneous load.
Data Sources: Energy Star, DOE, Manufacturer specs (Generac, Honda, Champion, Kohler)

What Size Whole House Generator Do I Need?

A whole house generator (also called a standby generator) is permanently installed outside your home and automatically powers your entire house during an outage. Unlike portable generators, whole house generators connect directly to your electrical panel through an automatic transfer switch (ATS) and run on natural gas or propane — no refueling required.

Whole House Generator Size Guide

Home SizeGenerator SizeTypical AppliancesInstalled Cost
Small (under 1,500 sq ft)10,000-14,000WEssentials + small AC, gas heat$5,000-$12,000
Medium (1,500-2,500 sq ft)16,000-20,000WMost circuits, central AC, electric water heater$8,000-$18,000
Large (2,500-4,000 sq ft)20,000-24,000WFull house, central AC, electric dryer, well pump$12,000-$22,000
Extra Large (4,000+ sq ft)24,000-48,000WEverything including multiple ACs, pool, EV charger$18,000-$40,000+

Use our calculator above to select every appliance in your home for an exact NEC-compliant sizing recommendation.

How a Whole House Generator Works

  1. Utility power fails — the automatic transfer switch (ATS) detects the outage
  2. Generator starts automatically — typically within 10-30 seconds
  3. ATS transfers load — your home switches from utility to generator power
  4. Full power restored — all connected circuits receive power
  5. Utility returns — ATS detects stable utility power and transfers back
  6. Cool-down cycle — generator runs for a few minutes to cool down, then shuts off

Natural Gas vs Propane: Which Fuel Is Better?

FactorNatural GasPropane
SupplyPiped from utility — unlimitedOn-site tank — limited by tank size
Cost per kWhGenerally cheaperSlightly more expensive
Energy DensityLower (1,030 BTU/cu ft)Higher (2,516 BTU/gal)
IndependenceDependent on gas utilityFully independent — works if gas lines are damaged
InstallationNeeds gas line to generator locationNeeds tank (100-1,000 gal) + line

Sizing with NEC Article 220

Our calculator follows NEC Article 220 load calculation methodology, the same standard used by licensed electricians:

  • Continuous loads (3+ hours: fridge, HVAC, water heater, lights) sized at 125%
  • Non-continuous loads (dryer, dishwasher, microwave) — only the largest is added
  • Motor starting surges accounted for (AC compressor, well pump, etc.)
  • Final recommendation rounded up with 20% headroom per NEC 210.20(A)

Select all appliances in your home above for an accurate recommendation. For a 2,000 sq ft home with central AC, electric water heater, and standard appliances, you will typically need a 20,000-22,000W whole house generator.

Frequently Asked Questions

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References & Sources