PickMyWatt

Backup Generator Sizing Calculator

Plan your home backup power system. Select the appliances you want to run during an outage, and get an NEC-compliant generator size recommendation with fuel cost estimates.

⚡
Power Outage
Keep your home running during storms, grid failures, and rolling blackouts with reliable backup power.
❄️
Winter Storms
Power your heating system, water heater, and essential circuits when temperatures drop.
🌊
Hurricane Season
Run sump pumps, fridge, freezer, and lights for days during extended outage events.
🏥
Medical Equipment
Keep CPAP machines, oxygen concentrators, and other medical devices running 24/7.
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NEC 80% Rule: Per NEC 210.20(A), continuous loads (running 3+ hours) should not exceed 80% of rated capacity. Your 10,000W generator safely handles 8,000W 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
Generator Size
10,000W
watts
Load Status36%
NEC Load: 2,888WUsable: 8,000W
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/Day12h
Diversity?75%
Avg Load?
8%
Rate: 0.075 gal/kW/hr. Sources: Generac, Honda, US EIA
Per Hour
$0
Per Day
$2
Per Month
$65
Calculation breakdown
Base Load: 950W ÷ 10,000W = 10%
Avg Load: 10% × 75% = 8%
Fuel/hr: 10.0 kW × 0.075 × 8% = 0.06 gal/hr
Cost/hr: 0.06 × $3.00 = $0.18/hr
Cost/day: $0.18 × 12h = $2/day
Cost/month: $2 × 30 = $65/mo

PickMyWatt.com

Backup Generator Sizing Calculator

Your Generator
10,000
watts
Load: 36%
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
$0.18
Per Day
$2
Per Month
$65
gasoline

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)

How to Size a Backup Generator for Your Home

A backup generator keeps your home powered during outages caused by storms, grid failures, and natural disasters. The right size depends on which appliances you want to run. Our calculator uses NEC Article 220 to give you an accurate, code-compliant recommendation.

Backup Generator Size by Scenario

ScenarioGenerator SizeWhat You Can Run
Bare Minimum3,000-4,000WFridge, lights, phone charging, WiFi router
Essential Comfort5,000-7,500WAbove + window AC or space heater, sump pump, TV
Extended Outage10,000-12,500WAbove + water heater, well pump, washing machine
Near Whole House15,000-22,000WMost circuits including central AC and dryer

Portable vs Standby: Which Backup Generator Is Right for You?

Portable backup generators are affordable ($400-$3,000) and flexible — you can use them for home backup, camping, or job sites. They require manual setup during an outage: wheel it out, start it up, and connect via extension cords or a transfer switch.

Standby backup generators ($3,000-$15,000+ installed) are permanently installed outside your home with an automatic transfer switch (ATS). When the power goes out, the ATS detects it and starts the generator in 10-30 seconds — even if you are not home. They run on natural gas or propane, so no refueling is needed.

Fuel Planning for Extended Outages

During extended outages, fuel planning is critical. Use our calculator above to estimate your hourly fuel consumption and plan accordingly:

Fuel TypeProsConsStorage Life
GasolineWidely available, cheapStale in 3-6 months, flammable3-6 months
PropaneLong shelf life, clean burningNeeds tank, lower energy densityIndefinite
DieselMost fuel efficient, long engine lifeExpensive, louder12-24 months
Natural GasUnlimited supply, no storageDependent on utility infrastructureN/A (piped)

Transfer Switch: Required for Home Backup

Per NEC Article 702, a transfer switch is required when connecting a generator to your home electrical panel. It prevents dangerous backfeed into the utility grid, which can electrocute line workers. Options include manual transfer switches ($300-$800 installed) and automatic transfer switches ($1,000-$3,000 installed, included with standby generators).

Frequently Asked Questions

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References & Sources