Question 1

What size generator do I need for my house?

Accepted Answer

It depends on what you want to power. For essential circuits only (refrigerator, lights, sump pump, furnace fan), a 7–10 kW generator is typically sufficient for an average 1,500–2,000 sqft home. For whole-house coverage including central AC, you'll likely need 20–30 kW on natural gas or 16–24 kW on gasoline. Use the calculator above to get a precise estimate based on your specific appliances, fuel type, and altitude.

Question 2

How do I calculate what size generator I need?

Accepted Answer

Add up the running watts of all appliances you want to power simultaneously. Then identify the largest motor-driven appliance and add its extra surge watts (starting watts minus running watts) — this motor is assumed to start last while everything else runs. Multiply by 1.25 for a 25% surge margin, then divide by 1,000 to get kW. Finally, adjust for fuel type (propane delivers ~90% the power of gasoline, natural gas ~80%) and altitude (3% loss per 1,000 ft). The calculator above does all of this automatically. Tip: an appliance labeled '700W running / 2,200W starting' uses the extra 1,500W surge component in the calculation, not the full 2,200W, to avoid double-counting.

Question 3

How do I use a generator size calculator for home?

Accepted Answer

This free generator size calculator for home walks you through seven inputs: (1) select your use case — home backup, whole house, RV, or job site, (2) enter your home's square footage, (3) pick appliances from the categorized library and add any custom appliances, (4) set your surge margin (25% recommended), (5) choose your fuel type and altitude, (6) enter desired runtime and optional tank size, and (7) set your budget, noise, and transfer switch preferences. The result updates instantly with your recommended kW, kVA, cost estimate, fuel consumption, and runtime.

Question 4

What is the difference between running watts and starting watts?

Accepted Answer

Running watts (also called rated or continuous watts) is the power an appliance needs to operate normally — e.g., a refrigerator draws 700W while running. Starting watts is the total power drawn at the moment the motor or compressor kicks on — for that same refrigerator, about 2,200W total at startup. The extra surge component (2,200 − 700 = 1,500W) is what the generator must supply on top of the running load. Our calculator uses this extra surge when sizing, not the full starting watts, to avoid double-counting the appliance's running draw.

Question 5

How does altitude affect generator sizing?

Accepted Answer

Generators lose approximately 3% of their rated power for every 1,000 feet above sea level due to lower air density reducing combustion efficiency. At 5,000 feet (Denver), a 10 kW generator delivers about 8.5 kW. At 10,000 feet, it delivers about 7 kW. The calculator above automatically adjusts your recommended size for altitude — just enter your elevation in the fuel & altitude section.

Question 6

What size generator do I need for a 2,000 square foot home?

Accepted Answer

For essential circuits in a 2,000 sqft home (refrigerator, lights, sump pump, furnace fan, and a few outlets), a 10–13 kW generator is typical. For whole-house coverage including a 3-ton central AC, you'll likely need 20–30 kW. The exact size depends on your specific appliances, fuel type, and altitude — use the calculator above for a precise recommendation.

Question 7

How many watts does a refrigerator use on a generator?

Accepted Answer

A typical modern refrigerator uses 500–800 running watts and 1,500–2,200 starting watts. Older or larger units may draw more. Check your refrigerator's nameplate or use a watt meter for the most accurate measurement.

Question 8

What size generator do I need to run a well pump?

Accepted Answer

A 1/2 HP well pump typically needs 1,000 running watts and 2,000 starting watts. A 1 HP pump needs about 2,000 running watts and 4,000 starting watts. If the well pump is your largest motor load, your generator must be sized to handle its starting surge on top of all other running appliances.

Question 9

Should I choose gasoline, propane, or natural gas?

Accepted Answer

Gasoline is the most common and delivers full rated power, but has a short shelf life (6–12 months) and is hard to get during outages. Propane stores indefinitely and burns cleaner but delivers ~10% less power. Natural gas provides unlimited runtime from your utility line but delivers ~20% less power and may not be available during earthquakes. Diesel is most efficient for large generators (20 kW+) but has higher upfront cost. Choose based on fuel availability in your area and how long you expect outages to last.

Question 10

What size transfer switch do I need?

Accepted Answer

Transfer switch amperage should match your generator's output. A 10 kW generator at 240V produces about 42 amps, so a 50-amp transfer switch is appropriate. A 20 kW generator (83 amps) needs a 100-amp switch. For manual transfer switches, match the circuits you want to back up. The calculator above recommends a transfer switch rating based on your generator size.

Question 11

How long will a generator run on a tank of fuel?

Accepted Answer

A typical portable generator (7-gallon tank) running at 50% load consumes about 0.3–0.5 gallons per hour, giving 14–20 hours of runtime. A 500-gallon propane tank at 50% load on a 20 kW standby generator can last 4–7 days. Actual runtime varies by load — the calculator above estimates runtime based on your specific generator size and fuel type.

Question 12

What is the difference between kW and kVA?

Accepted Answer

kW (kilowatts) is real power — what your appliances actually consume. kVA (kilovolt-amperes) is apparent power — what the generator must produce. The ratio between them is the power factor, typically 0.8 for generators. So a 10 kW generator is rated at 12.5 kVA. When comparing generators, always check whether the rating is in kW or kVA — they are not the same.

Question 13

What size generator do I need for an RV?

Accepted Answer

For a typical RV with a 13,500 BTU AC, microwave, TV, and lights, a 3,000–4,000W portable generator is usually sufficient. If you run two AC units (15,000 BTU each), you'll want 5,500–7,000W. Inverter generators are preferred for RVs because they're quieter, lighter, and produce clean power safe for sensitive electronics. Select the 'RV & Camping' use case in the calculator above and add your specific appliances for an exact recommendation.

Question 14

What size generator do I need for home backup?

Accepted Answer

For basic home backup covering essential circuits — refrigerator, lights, sump pump, furnace fan, a few outlets, and electronics — a 7–10 kW portable generator is sufficient for most 1,500–2,000 sqft homes. If you want to run central AC, electric water heater, or an electric dryer during an outage, you'll need a 20–37.5 kW standby generator depending on your fuel type and altitude. Use the calculator above to get an exact recommendation for your home.

Question 15

How accurate is this generator size calculator?

Accepted Answer

This calculator uses industry-standard formulas: 5W/sqft for essential circuits (10W/sqft for whole-house), 3% altitude derating per 1,000 ft, fuel power factors matching manufacturer specs, and standard starting surge accounting with the largest motor starting last. The result is a planning estimate — actual generator requirements may vary by brand, installation specifics, and local code. Always consult a licensed electrician for final sizing and installation.

Appliance	Running W	Starting W
Refrigerator / Freezer	700	2,200
Freezer (chest)	500	1,500
Microwave	1,000	1,000
Dishwasher	1,300	1,800
Central AC (3 ton)	3,500	10,500
Window AC (10,000 BTU)	1,200	3,600
Furnace Fan (gas)	600	1,800
Electric Water Heater	4,500	4,500
Washing Machine	750	2,250
Electric Dryer	5,400	6,750
Sump Pump (1/2 HP)	1,050	3,150
Well Pump (1 HP)	2,000	4,000
TV (LED, 50")	100	100
LED Lighting (10 bulbs)	100	100
CPAP Machine	60	60

Fuel	Power Factor	Consumption	Cost/hr @ 10kW	Best For
Gasoline	100%	~0.08 gal/kWh	~$2.80	Portable, short outages
Propane	90%	~0.10 gal/kWh	~$3.00	Long shelf life, cleaner
Natural Gas	80%	N/A (utility)	~$1.50	Standby, unlimited supply
Diesel	100%	~0.06 gal/kWh	~$2.40	Large/commercial
Dual Fuel	95%	~0.09 gal/kWh	~$3.00	Flexibility: gas or propane

Home Size	Essential Circuits	Whole House (no AC)	Whole House (with AC)
500 – 1,000 sqft	3 – 5 kW	5 – 7 kW	10 – 13 kW
1,000 – 1,500 sqft	5 – 7 kW	7 – 10 kW	13 – 16 kW
1,500 – 2,000 sqft	7 – 10 kW	10 – 13 kW	16 – 20 kW
2,000 – 2,500 sqft	10 – 12 kW	12 – 16 kW	18 – 22 kW
2,500 – 3,500 sqft	12 – 15 kW	15 – 20 kW	22 – 30 kW
3,500 – 5,000 sqft	15 – 20 kW	20 – 25 kW	30 – 40 kW

Altitude	Power Loss	10 kW → Actual	20 kW → Actual
Sea level (0 ft)	0%	10.0 kW	20.0 kW
2,000 ft	6%	9.4 kW	18.8 kW
5,000 ft (Denver)	15%	8.5 kW	17.0 kW
7,000 ft	21%	7.9 kW	15.8 kW
10,000 ft	30%	7.0 kW	14.0 kW

Generator Size Calculator

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How to Calculate Generator Size

Appliance Wattage Reference

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Fuel Type Comparison

Generator Size Calculator for Home

Altitude Derating for Generators

Common Generator Sizing Mistakes

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