What size generator do I need for my home?

The generator size you need depends on which appliances you want to power. A typical home running essentials (refrigerator, lights, a window AC, and sump pump) needs a 5,000-8,000 watt generator. A whole-house standby generator for a medium-sized home typically requires 12,000-20,000 watts. Use our calculator above to get a precise recommendation.

What is the difference between running watts and surge (starting) watts?

Running watts is the continuous power an appliance needs while operating normally. Surge watts is the brief spike of power required to start motor-driven appliances like refrigerators, AC units, and pumps. This surge typically lasts only a second or two but can be 2-4x the running watts. Your generator must handle both requirements.

How do I calculate what size generator I need?

To calculate generator size: (1) List every appliance you need to power. (2) Find each appliance's running watts. (3) For motor-driven appliances, multiply running watts by the surge factor (typically 2-4x). (4) Add all running watts together. (5) Identify the appliance with the highest surge watts. (6) Your generator must exceed both totals, with a 20-25% safety margin. Our free calculator does all of this automatically.

Can I run a whole house on a portable generator?

You can power a partial house with a large portable generator (7,500-12,000 watts), but running everything including central AC, electric dryer, and electric water heater simultaneously requires 15,000+ watts -- typically the territory of standby/whole-house generators. Most people prioritize: refrigerator, lights, sump pump, a few outlets, and a window AC unit on a 5,000-8,000 watt portable generator.

What generator size do I need for a refrigerator and freezer?

A standard refrigerator runs on 100-400 watts but has a startup surge of 300-1,200 watts. A standalone chest freezer uses 100-400 watts with a similar surge. To run both safely, a 2,000-watt generator is the minimum, but a 3,000-watt generator gives comfortable headroom.

What is a safe load percentage for a generator?

Generator manufacturers recommend running at 70-80% of rated capacity for optimal performance, fuel efficiency, and longevity. Running a generator at 90-100% capacity continuously can cause overheating and shorten engine life. Our calculator automatically applies an 80% load factor.

How much does it cost to run a generator per day?

A 5,000-watt gasoline generator running for 8 hours at 50% load uses approximately 2-3 gallons of fuel, costing $7-$12 at current prices. Natural gas standby generators are the most economical at $5-$8/day for equivalent coverage.

How long can a generator run on a tank of gas?

Runtime depends on generator size, load percentage, and tank capacity. A typical 5,000-watt generator with a 5-gallon tank running at 50% load lasts 8-11 hours. At 100% load, that drops to 4-6 hours.

What size generator do I need for a sump pump?

A typical 1/2 HP sump pump runs at 800-1,050 watts with a starting surge of 2,400-3,150 watts. A 3,000-watt generator can safely handle a sump pump, though a 3,500-watt or larger gives better headroom.

What size generator do I need for central air conditioning?

A 3-ton (36,000 BTU) central AC unit draws 3,000-4,000 running watts with a starting surge of 10,500-14,000 watts. You need a generator rated for at least 14,000 watts peak. This is why whole-house standby generators are typically 12-20 kW.

What is the difference between standby and portable generators?

Portable generators are movable, typically 1,000-12,000 watts, run on gasoline or propane, and start manually. Standby generators are permanently installed, automatically start when power goes out, run on natural gas or propane, and are sized for whole-home coverage (7-20+ kW).

Do I need a transfer switch for a generator?

Yes, a transfer switch is legally required and critically important for safety. Without a transfer switch, backfeeding electricity from your generator into utility lines can electrocute utility workers. A manual transfer switch costs $200-$600 installed; an automatic transfer switch costs $500-$1,500 installed.

Free Generator Size Calculator

What Size
Generator
Do I Need?

Name: Generator Size Calculator
Author: Generator Size Calculator

Select your appliances, get exact running watts, surge watts, fuel costs, and runtime — instantly. No guesswork, no signup.

⚡40+ Appliances🔋4 Fuel Types📊6 Calculator Tools🆓100% Free

Calculate My Generator Size View Wattage Chart

Real-time calculations

Recommended Size

5,000 W

for home essentials — adjust above

5,000W

Typical Home Essentials

14 kW

Whole-House Standby

80%

Recommended Max Load

2–4×

Motor Startup Surge

Always size for surge watts — motors draw 2–4× running watts at startup. This is the #1 mistake buyers make.

Quick Answer

For home essential backup (refrigerator, sump pump, lights, furnace blower), a 5,000-7,500 watt generator is sufficient. For whole-home coverage including central air conditioning, plan for 10,000-20,000 watts depending on home size and AC tonnage.

Essentials Only

5-7.5 kW

With Window AC

7.5-10 kW

With Central AC

12-16 kW

Whole Home

14-22 kW

Published January 1, 2025Updated June 17, 2026Reviewed by Generator Size Calculator Editorial Team

Key Takeaways

1Always size for surge watts, not just running watts -- motor appliances spike 2-4x at startup.
2Apply the 80% rule: your continuous load should not exceed 80% of generator rated capacity.
3A 5,000W portable handles most home essentials; central AC requires 14,000W+ standby.
4A transfer switch is legally required to connect a generator to your home wiring.
5Fuel type matters: propane stores indefinitely; natural gas standby generators never run dry.

Select Your Appliances

Choose everything you need to power

❄️

Window AC (10,000 BTU)

1200W running · 3600W surge

❄️

Portable AC

1100W running · 2750W surge

🏠

Central AC (3 Ton)

3500W running · 12250W surge

🏠

Central AC (4 Ton)

5000W running · 17500W surge

♨️

Heat Pump

4700W running · 16450W surge

🔥

Space Heater

1500W running · 1500W surge

🌬️

Furnace / Blower

600W running · 1800W surge

💨

Ceiling Fan

60W running · 90W surge

💧

Dehumidifier

785W running · 1963W surge

Generator Settings

Configure your power requirements

Fuel Type

Phase

Voltage

Hours of Use Per Day: 8h

1h12h24h

Fuel Price (per gallon) — optional

⚡

Select Your Appliances

Choose the appliances you need to power and we'll calculate the exact generator size you need.

⚡

40+

Appliance Presets

📊

Generator Categories

⛽

Fuel Types Covered

🆓

100%

Free to Use

Generator Sizing at a Glance

Visual references to help you understand wattage requirements — before you use the calculator above.

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 Watts vs. Surge Watts by Appliance

Surge (startup) watts are the critical sizing constraint for motors. Your generator must handle both bars.

Running vs. Surge Watts

Running wattsSurge watts

Generator Size by Home Square Footage

Left bar = essentials only (no central AC). Right bar = full coverage including central air.

Generator Size by Home Size

Essentials onlyFull coverage (with AC)

Quick Estimates by Home Size

For a precise number, use the calculator above.

< 800 sq ft

Small Apartment

4-5 kW

6 typical appliances

800–1,200 sq ft

Small Home

5-7 kW

8 typical appliances

1,200–2,000 sq ft

Medium Home

8-10 kW

10 typical appliances

2,000–3,500 sq ft

Large Home

14-18 kW

12 typical appliances

Need a More Specific Calculator?

Choose the right tool for your situation — each covers a distinct use case with dedicated content and detailed guidance.

How the Generator Size Calculator Works

Select each appliance you need to power during an outage, set the quantity, and the calculator instantly totals your running watts and surge watts — the two numbers every generator purchase decision must account for. Running watts is what your appliances draw continuously. Surge watts is the startup spike that motor-driven appliances (refrigerators, pumps, air conditioners) demand for a second or two when they first switch on. Your generator must handle both.

The recommendation applies the industry-standard 80% load rule: your continuous load should not exceed 80% of the generator's rated capacity. This preserves engine life, improves fuel efficiency, and leaves headroom for unexpected loads.

For a full walkthrough with square-footage tables, appliance-by-appliance breakdowns, and installation cost estimates, visit our home generator sizing calculator guide. For standby generator sizing, see the whole house generator sizing calculator. For emergency backup planning, the backup generator calculator covers critical-load prioritization. The generator size chart lists running and surge watts for every common appliance.

Free Generator Calculator

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Use the calculator above to get a precise recommendation tailored to your specific appliances and usage.

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Frequently Asked Questions

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Home Generator Size Calculator

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

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Whole House Generator Calculator

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

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Backup Generator Calculator

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

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Portable Generator Calculator

Find the right portable generator for camping, RV, tailgating, or job sites.

What SizeGeneratorDo I Need?

Key Takeaways

Select Your Appliances

Generator Settings

Select Your Appliances

Generator Sizing at a Glance

Generator Size Categories

Daily Fuel Cost — 5 kW Generator

Running Watts vs. Surge Watts by Appliance

Generator Size by Home Square Footage

Quick Estimates by Home Size

Need a More Specific Calculator?

How the Generator Size Calculator Works

Get Your Free Generator Size Recommendation

Frequently Asked Questions

Related Generator Tools

Home Generator Size Calculator

Whole House Generator Calculator

Backup Generator Calculator

Portable Generator Calculator

What Size
Generator
Do I Need?