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 generator size?

Accepted Answer

Add up the running watts of all appliances you want to power simultaneously, then add the surge watts (extra power beyond running) of the largest motor-driven appliance — since it starts last. Multiply by 1.25 for a 25% surge margin. Divide by 1,000 to get kW. Then adjust for fuel type (propane = 90% power, natural gas = 80%) and altitude (3% loss per 1,000 ft). The calculator above handles all of this automatically. Note: an appliance labeled '700W running / 2,200W starting' draws 700W continuously and 2,200W total at startup — the extra surge component is 1,500W.

Question 3

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 4

How does altitude affect generator performance?

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. Always factor in altitude when sizing — especially in mountain states.

Question 5

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 6

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 7

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 8

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 9

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 10

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 11

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 12

Does a generator lose power at high altitude?

Accepted Answer

Yes. Generators lose approximately 3% of their rated power for every 1,000 feet above sea level due to reduced air density. This affects all internal combustion engines — gasoline, propane, natural gas, and diesel. The calculator above automatically adjusts your recommended size for altitude.

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

Power Specifications

How to Calculate Generator Size

Appliance Wattage Reference

Worked Examples

Fuel Type Comparison

What Size Generator Do I Need for My House?

Altitude Derating for Generators

Common Generator Sizing Mistakes

Frequently Asked Questions

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