Fact-checked by Marcus Hayes, Master Electrician (License #CA-EL-89241), 15 years generator installation experience.
Sizing a generator is the single most important decision in the buying process. Too small and it will
trip its breaker or stall when your refrigerator compressor kicks on. Too large and you are wasting money
on fuel and capacity you will never use. This guide walks you through the exact calculation, step by step.
Key Takeaways
Generator size is measured in running watts (continuous power) and starting watts (surge power for motor startup).
Motor-driven appliances like refrigerators and AC units need 2 to 3 times their running watts for 2 to 3 seconds at startup.
A typical 2,000 sq ft house needs 7,500 to 10,000 running watts for essential backup; whole-house coverage needs 20kW to 22kW.
Always add a 20% safety margin to your calculated total to handle future additions and prevent overloading.
Use our generator size calculator to add up your appliance watts automatically and get a recommended size.
How to calculate generator size
To calculate generator size, list every appliance you want to power, find each one's running watts and
starting watts, add all running watts together, then add the largest single starting-watt surge on top.
The total plus a 20% safety margin is your minimum generator size. For example, if your appliances total
6,000 running watts and the largest surge is 2,200W, you need at least 8,200W — round up to a 9,000W generator.
The calculation has three steps. First, list every appliance you want to power during an outage. Second,
find the running watts and starting watts for each one — check the appliance nameplate, the owner's manual,
or use our appliance wattage chart
which lists 50+ common household appliances. Third, add all the running watts together, then add the single
largest starting-watt surge (because only one motor starts at a time in most households).
The formula is: Total running watts + largest starting watt surge + 20% safety margin = minimum generator size.
Generator sizing by home size
For a 2,000 sq ft house, you need 7,500 to 10,000 running watts for essential backup. For a 3,000 sq ft
house, you need 10,000 to 14,000 watts. Whole-house coverage including central air conditioning requires
a 20kW to 22kW standby generator regardless of square footage, because the AC starting load — not the
house size — is the determining factor.
Home Size
Essential Backup
Whole-House (with AC)
Recommended Type
Under 1,500 sq ft
5,000 – 7,500W
14kW – 16kW
Portable or small standby
1,500 – 2,000 sq ft
7,500 – 10,000W
18kW – 20kW
Large portable or standby
2,000 – 3,000 sq ft
10,000 – 14,000W
20kW – 22kW
Standby generator
3,000+ sq ft
12,000 – 15,000W
22kW – 26kW+
Whole-house standby
These ranges assume a typical US home with gas heating, electric refrigerator, and standard lighting.
Homes with electric heat, electric water heaters, or multiple AC zones will need more capacity.
What size generator to run a refrigerator?
A standard refrigerator needs 800 to 1,200 running watts and 2,200 starting watts. A 2,200W generator
is the minimum to run a refrigerator alone. A 3,000W generator gives you comfortable headroom to also
power lights and a few small appliances alongside the refrigerator.
Refrigerator compressors are inductive loads — they need a surge of 2 to 3 times their running wattage
for the first 2 to 3 seconds of operation. A typical modern refrigerator draws 800W running but needs
2,200W to start. If your generator cannot deliver that surge, the refrigerator will stall the generator
or trip its breaker every time the compressor cycles on.
Wattage requirements for common appliances
The most power-hungry household appliances are: central air conditioning (3,500W running, 10,500W
starting), electric water heater (4,500W running), well pump (1,000W running, 3,000W starting), and
electric range (2,000W per burner). Lights, TVs, and phone chargers draw negligible power by comparison.
For a complete list of 50+ appliances with running and starting watts, see our
appliance wattage chart.
For an automated calculation, use our
generator size calculator
which adds up your appliance loads and recommends a generator size with the 20% safety margin applied.
Running watts vs starting watts
Running watts are the continuous power an appliance needs to operate. Starting watts (surge watts) are
the extra burst of power needed for 2 to 3 seconds to start motor-driven appliances. Your generator's
starting watt rating must exceed the combined running watts plus the single largest starting surge.
Every generator has two ratings: running watts (also called rated watts) and starting watts (also called
surge or peak watts). The running watt rating is what the generator can deliver continuously. The starting
watt rating is what it can deliver for a few seconds to handle motor startup surges. When sizing, your
generator's starting watt rating must exceed the sum of all running watts plus the largest single
starting-watt surge from any one appliance.
Frequently Asked Questions
What size generator do I need for a 2000 sq ft house?
A 2,000 sq ft house typically needs a 7,500 to 10,000 watt generator to run essential loads (refrigerator, furnace blower, well pump, lights, and outlets). If you want to run central air conditioning, you will need 12,000 to 15,000 starting watts. Use our generator size calculator for a precise figure based on your specific appliances.
What size generator do I need for a 3000 sq ft house?
A 3,000 sq ft house usually requires a 10,000 to 14,000 watt generator for essential backup, or a 20kW to 22kW standby generator for whole-house coverage including central air. The square footage alone does not determine sizing — the appliances you want to run do. Count your running watts and starting watts first.
Will a 7500 watt generator run a house?
A 7,500 watt generator can run most essential household loads: refrigerator (800W running), furnace blower (600W), lights (300W), TV (200W), and several outlets. It cannot run central air conditioning (which needs 4,000+ starting watts) or an electric water heater (4,500W). For whole-house coverage including AC, you need at least 12,000W.
What is the difference between running watts and starting watts?
Running watts are the continuous power an appliance needs to operate. Starting watts (also called surge watts) are the extra burst of power needed for 2 to 3 seconds to start motor-driven appliances like refrigerators, air conditioners, and well pumps. Starting watts are typically 2 to 3 times the running watts. Your generator must handle the combined starting watts of all appliances that start simultaneously.
Can I run central air conditioning on a portable generator?
A typical 3-ton central AC unit needs 3,500 running watts and 10,500 starting watts. You need a generator with at least 12,000 starting watts to handle the AC alone. A 22kW standby generator can run most central AC units along with other household loads. Portable generators below 10,000W generally cannot start central AC.
How do I calculate what size generator I need?
List every appliance you want to power, find its running watts and starting watts (check the nameplate or our appliance wattage chart), add all running watts together, then add the highest single starting watt surge on top. The total is your minimum generator size. Add a 20% safety margin for future additions and to avoid overloading.