How do I calculate generator size?

To calculate generator size: (1) List every appliance you need to power. (2) Find each appliance's running watts. (3) For motors (AC, pump, compressor), multiply running watts by 2–4× for surge watts. (4) Add all running watts. (5) Find the single highest surge value. (6) Your generator must handle both totals with a 20–25% safety margin. Enter your appliances above and the calculator does all of this automatically.

What is the difference between running watts and surge watts?

Running watts (rated watts) is the continuous power a device draws while operating. Surge watts (starting watts) is the brief spike required to start motor-driven appliances — refrigerators, air conditioners, pumps. Surge typically lasts under 1 second but can be 2–4× the running watts. Your generator must handle the surge, not just the running load. This is the most common generator-sizing mistake.

What size generator do I need to run my house?

To run a house on generator power: essential circuits only (refrigerator, sump pump, lights, furnace, devices) = 5,000–7,500W. Add window AC = 7,500–10,000W. Whole home with 2-ton central AC = 12,000–15,000W. The most common choice for US homeowners is a 5,500–8,500W portable for emergency backup, or a Generac Guardian 14kW standby for automatic whole-home protection.

What is the 80% rule for generators?

The 80% rule states your continuous electrical load should not exceed 80% of your generator's rated capacity. Running at 90–100% load causes overheating, reduces engine life, and increases fuel consumption. When sizing, divide your total running watts by 0.80 to get the minimum generator size. Our calculator applies this rule automatically.

What size generator for a 200 amp service?

A home with 200 amp service needs a 14–18 kW standby generator for whole-home coverage. For essential backup only (no central AC): a 7,500–10,000W portable is sufficient. Your 200A panel can theoretically handle 48,000W, but you never use all circuits simultaneously — generators are sized to your actual running loads.

Can a portable generator run central air conditioning?

A standard portable generator cannot run most central AC systems reliably. A 2-ton central AC surges to 7,800W at startup — plus all your other running loads, you need 12,000W+ generator capacity. A soft-start kit ($200–$400) reduces the AC surge by 70%, potentially allowing a 7,500W generator to handle a 1.5–2 ton system.

What size generator is needed for a refrigerator and sump pump?

To run a refrigerator (150W running, 600W surge) and a sump pump (800W running, 2,400W surge) simultaneously, you need a generator that can handle 950W running and 3,000W peak surge. A 3,500W generator safely covers both. A 5,500W generator gives you comfortable headroom to also add lights, devices, and a furnace blower.

What is the best size generator for emergency backup?

For emergency home backup, the most practical sizes are: 5,500W for essentials (fridge, pump, furnace, lights); 8,500W to add a window AC unit; 12,000W+ to run central air. The Generac Guardian 14kW or Kohler 14RESAL is the most popular automatic standby choice for medium homes. For portables, the Champion 9000W dual-fuel is a best-seller for emergency backup.

How long will a 5,000 watt generator run on a tank of gas?

A 5,000W generator running at 50% load (2,500W — typical home essentials) consumes approximately 0.45–0.55 gallons per hour. A standard 4-gallon tank lasts 7–9 hours. A 5-gallon tank lasts 9–11 hours. At 75% load, runtime drops to 6–8 hours on 5 gallons. Use our runtime calculator for exact estimates based on your load and tank size.

Is a transfer switch required for a generator?

Yes — connecting a generator to your home's wiring without a transfer switch (a 'suicide cord') is illegal under the National Electrical Code, dangerous to utility workers through backfeeding, and will destroy your generator when utility power returns. A manual transfer switch costs $400–$1,200 installed. An automatic transfer switch costs $2,000–$5,000. Both are legal, safe, and code-compliant.

What is the difference between a portable and standby generator?

Portable generators are movable, manually started, run on stored fuel (gasoline/propane), and cost $600–$2,500. Standby generators are permanently installed, automatically start when power goes out, connect to natural gas or LP, and cost $6,500–$20,000 installed. Portable generators cover essential circuits; standby generators cover the whole home automatically. See our full portable vs standby comparison.

Free Generator Sizing Tool

Generator Size Calculator

Q: What size generator do I need?

The size generator you need depends on which appliances you want to power. For home essentials (refrigerator, sump pump, furnace blower, lights): a 5,000–7,500 watt generator. To also run a window AC: 7,500–10,000 watts. For whole-home coverage with central AC: 12,000–20,000 watts. Use the generator size calculator above for a precise answer based on your specific appliances.

Enter your appliances — get running watts, surge watts, your recommended generator size, fuel cost, and runtime in under 60 seconds. No guesswork, no signup.

⚡ 40+ Appliances📊 6 Calculators⛽ 4 Fuel Types🆓 100% Free

Calculate My Generator Size View Wattage Chart

5–22 kW

Residential Range

80%

Recommended Max Load

2–4×

Startup Surge Factor

<60 sec

To Your Result

Published September 1, 2025Updated June 17, 2026Reviewed by Daniel Hart

Loading calculator…

Editor’s Top Picks

Most Popular Generators for Every Budget

Matched to the most common sizing results our calculator produces. All available on Amazon with free returns.

Best Portable🏕️

Champion 9000W Dual Fuel

Most popular choice for home essentials + window AC

Power

9,000W / 11,250W surge

Fuel

Gas or Propane

Runtime

10.5 hrs @ 50%

Rating

4.5

2,847 verified reviews · 4.5/5.0 stars

Best for: Medium homes, essentials + window AC, dual-fuel flexibility

$949–$1,099Full review →

Check Price on Amazon

Best Mid-Size⚡

DuroMax XP12000EH

12,000W dual-fuel — runs central AC on most homes

Power

12,000W / 15,000W surge

Fuel

Gas or Propane

Runtime

8.8 hrs @ 50%

Rating

4.6

1,923 verified reviews · 4.6/5.0 stars

Best for: Larger homes, 2–2.5 ton central AC, full home coverage

$1,299–$1,499Full review →

Check Price on Amazon

Best Standby🏠

Generac Guardian 14kW

America's #1 automatic standby — starts itself in 10 seconds

Power

14,000W continuous

Fuel

Natural Gas / LP

Runtime

Unlimited (gas line)

Rating

4.7

3,142 verified reviews · 4.7/5.0 stars

Best for: Whole-home automatic protection, 1,500–2,500 sq ft with central AC

$4,499 + installationFull review →

Check Price on Amazon

ⓘ We may earn a commission from Amazon links at no extra cost to you. Prices change — click through for the current price. Recommendations are based on wattage performance, reliability ratings, and value for money, not commission rates.

How It Works

Your Generator Size in 3 Steps

📋STEP 01

Select Your Appliances

Choose every appliance you need to power during an outage from 40+ presets — refrigerators, AC units, pumps, and more. Set quantities for each.

⚡STEP 02

Get Your Wattage Totals

The calculator instantly computes total running watts and peak surge watts — the two numbers every generator purchase must account for.

✅STEP 03

Receive Your Recommendation

Get your exact recommended generator size with the 80% safety margin applied, plus fuel cost and runtime estimates for all 4 fuel types.

What Size Generator Do I Need?

The quick reference answer — before you scroll back to the calculator.

Quick Answer

For home essential backup (refrigerator, sump pump, furnace blower, lights): a 5,000–7,500 watt generator. To also run a window AC unit: 7,500–10,000 watts. For whole-home coverage with central air conditioning: 12,000–20,000 watts. Use the generator size calculator above for a precise answer based on your specific appliances — including startup surge watts, which are the critical sizing constraint.

5–7.5 kW

Essentials Only

7.5–10 kW

+ Window AC

12–16 kW

With Central AC

14–22 kW

Whole Home

Key Takeaways

Size for surge watts — motors draw 2–4× running watts at startup. This is the #1 sizing mistake.
Apply the 80% rule: your continuous load should not exceed 80% of rated generator capacity.
Central AC is the critical variable — a 2-ton system surges to 7,800W, requiring 12,000W+ capacity.
A transfer switch is legally required for any generator connected to your home wiring.
Portable generators cover essentials; standby generators cover the whole home automatically.

Generator Size by Home Coverage

Coverage Level	What’s Included	Generator Size
Bare Essentials	Fridge, lights, phone	2,000–3,500W
Critical Circuits	Essentials + sump pump + furnace	5,000–7,500W
Comfortable Backup	Critical + window AC + TV	7,500–10,000W
Partial Whole Home	Everything except central AC / dryer	10,000–12,500W
Whole Home	Full coverage with central AC	12,000–22,000W+

Running Watts vs. Surge Watts — Why It Matters

Every generator has two wattage ratings: running (rated) watts for continuous operation and surge (starting) watts for the brief spike when motors start. Air conditioners, sump pumps, well pumps, and refrigerators all surge 2–4× their running wattage for a fraction of a second at startup. Your generator must handle both — and most sizing mistakes happen when buyers only look at running watts.

The generator size calculator above handles both automatically. Enter your appliances and it applies the correct surge multiplier for each motor-driven device, adds the totals, and applies the 80% load rule before recommending a generator size.

The 80% Load Rule

Generator manufacturers recommend running at 70–80% of rated capacity for optimal efficiency and longevity. Running at 90–100% continuously causes overheating and shortens engine life. The rule: divide your total running watts by 0.80 to get your minimum generator size. A 4,000W continuous load requires at minimum a 5,000W generator.

Reference Charts

Generator Size at a Glance

Generator Size Categories

Typical wattage ranges and what each covers

Micro

1 kW

Phone charging, lights

Small

1–3 kW

Fridge + sump pump

Mid-Size

3–5 kW

Home essentials + window AC

Large

5–7.5 kW

Partial home + well pump

Standby

7.5–12 kW

Whole home + small AC

Heavy Duty

12–22 kW

Full home + central AC

Scale up to 22 kW for very large homes

Daily Fuel Cost — 5 kW Generator

50% load · 8 hrs/day · typical US prices

🔥Natural Gas

$5.20/day

Cheapest — unlimited supply

🔵Propane

$9.80/day

Stores indefinitely

⛽Gasoline

$14.00/day

Most widely available

🛢️Diesel

$16.80/day

Most fuel-efficient engine

Natural

$1,898

/ year

Propane

$3,577

/ year

Gasoline

$5,110

/ year

Diesel

$6,132

/ year

Annual cost if running 8 hrs/day every day (emergency planning scenario)

Running vs. Surge Watts — Key Appliances

Teal = running watts your generator sustains. Purple = startup surge your generator must also handle.

Running vs. Surge Watts

Running wattsSurge watts

Specific Scenarios

Generator Size For Every Situation

Detailed sizing guides — by appliance, by home size, and by special scenario.

1,000 Sq Ft House 1,500 Sq Ft House 2,000 Sq Ft House 2,500 Sq Ft House Central AC Sump Pump Well Pump Refrigerator Heat Pump RV / Camper 200 Amp Service Welder

Why Gen Size Calculator

The Most Accurate Free Generator Size Calculator

⚡

Surge-Accurate Results

Most calculators only add running watts. Ours applies exact surge multipliers per appliance type — the data that actually matters for generator selection.

📐

80% Load Rule Built In

Automatically applies the industry-standard 80% capacity rule to every recommendation — matching what licensed electricians calculate by hand.

⛽

4 Fuel Types Modelled

See fuel consumption and runtime estimates for gasoline, propane, diesel, and natural gas — so you can compare operating costs before buying.

🔋

Solar + Battery Sizing

Dedicated solar generator and battery bank calculator for off-grid and backup solar configurations, not just conventional generators.

✅

Transfer Switch Sizing

Calculates the correct transfer switch amperage for your selected loads — ensuring legal, code-compliant connection to your home panel.

📱

Mobile-First, No Signup

Works perfectly on any device, saves nothing about you, requires no account. Calculate, note your size, close the tab. That's it.

All Free Tools

Every Generator Calculator You Need

Related Generator Tools

🔌

Generator Size by Appliance

Exact generator size for 24 appliances and home sizes — refrigerator, sump pump, AC, well pump, and more.

🏠

Home Generator Size Calculator

Specifically sized for residential use. Get recommendations for your home's square footage.

🏡

Whole House Generator Calculator

Size a standby generator to power your entire home during an outage.

🔋

Backup Generator Calculator

Plan for emergency power with a backup generator sized for your critical loads.

Frequently Asked Questions

Generator Sizing Questions Answered

Every common generator sizing question, with direct numeric answers.

Free Tool

Ready to Find Your Generator Size?

The calculator is free, instant, and above — no signup, no sales pitch, just your accurate wattage recommendation.

Calculate Generator Size Now