Generator Sizing Calculator
Free generator tool
Generator Sizing Calculator
Enter your appliances and see the exact generator size you need — including motor startup surge, fuel consumption, and runtime estimates.
Quick add appliances
Your loads
Generator sizing accounts for the largest single motor surge occurring while all other loads are running. Actual fuel consumption varies by load, altitude, temperature, and generator model.
Frequently Asked Questions
What size generator do I need for my house?
For essentials only (fridge, lights, router, phone charging), you need a 3 to 5 kW generator. For comfort backup adding fans, TV, and a laptop, you need 5 to 7.5 kW. For a full house including air conditioning and water pump, you need 10 to 20 kW. The key factor is motor startup surge — an AC compressor draws 3 to 4 times its running watts for 2 to 3 seconds at startup. The generator must handle all running loads plus the largest single motor surge simultaneously. Enter your appliances in the calculator above for an exact recommendation.
What size generator do I need for a 2,000 sq ft house?
A 2,000 sq ft house typically needs a 7.5 to 10 kW generator for essential circuits or a 15 to 22 kW standby generator for whole-house coverage including central air conditioning. The house size alone does not determine the generator size — the appliances you need to power do. A 2,000 sq ft home with a gas furnace, gas stove, and gas water heater needs far less generator capacity than the same home with all-electric appliances. List your actual loads in the calculator above for precise sizing.
What size generator to run a refrigerator and freezer?
A 2,000 to 3,000 watt generator runs both a refrigerator and a freezer comfortably. A standard fridge draws 150W running with a 600W startup surge. A chest freezer draws 100W running with a 500W surge. Combined running: 250W. Worst-case surge (both starting at the same time, which is unlikely): 1,100W. A 2 kW generator handles this with margin. If you also want lights, a router, and phone charging (adding another 100W), a 3 kW generator covers everything.
What size generator do I need to run a window AC?
A 5,000 BTU window AC needs a 2,000 to 3,000 watt generator (draws 500W running, surges to 1,500W). An 8,000 BTU unit needs 3,000 to 4,000 watts (draws 800W, surges to 2,400W). A 12,000 BTU unit needs 4,000 to 5,000 watts (draws 1,200W, surges to 3,600W). A 15,000 BTU unit needs 5,000 to 6,500 watts. Always size for the surge, not the running watts — the generator must handle the compressor startup while everything else keeps running.
What size generator do I need for central air conditioning?
Central AC requires a large generator because of the compressor surge. A 2-ton (24,000 BTU) central AC draws 2,400W running and surges to 7,200W. A 3-ton (36,000 BTU) unit draws 3,500W and surges to 10,500W. A 4-ton (48,000 BTU) draws 4,800W and surges to 14,400W. When you add other household loads (2,000 to 3,000W), a home with a 3-ton central AC needs a minimum 15 to 20 kW generator. This is why standby generators (which connect to natural gas) are the standard for whole-house backup with central AC.
What is the difference between running watts and starting watts?
Running watts (also called rated watts) is the continuous power an appliance draws during normal operation. Starting watts (also called surge or peak watts) is the brief spike of power needed when an electric motor starts — typically 2 to 4 times the running watts, lasting 1 to 3 seconds. A generator must be sized for the scenario where all loads are running AND the largest motor starts simultaneously. A generator rated “5000W running / 6500W surge” can power 5,000W continuously and handle one motor startup up to 6,500W.
How long will a generator run on a tank of gas?
Runtime depends on the generator size, tank capacity, and load percentage. At 50% load, a typical 5 kW portable generator with a 6.5-gallon tank runs 10 to 12 hours. A 3.5 kW generator with a 4-gallon tank runs 9 to 11 hours. At full load, runtime drops by 30 to 40%. Diesel generators are 25 to 35% more fuel-efficient than gasoline. Propane generators are less efficient per gallon but the fuel stores indefinitely. The calculator above estimates runtime based on your specific load and fuel type.
How much does it cost to run a generator per hour?
At 50% load with gasoline at $3.50/gallon: a 3 kW generator costs about $1.40/hour, a 5 kW costs about $2.10/hour, a 7.5 kW costs about $3.15/hour, and a 10 kW costs about $4.20/hour. Diesel is 25 to 35% cheaper per hour. Propane costs about the same per hour as gasoline despite the higher consumption rate because propane costs less per gallon ($2.75 vs $3.50). Over a full day (8 hours), a 5 kW gasoline generator costs roughly $17 to run.
Portable generator vs standby generator: which is better?
Portable generators ($400 to $2,000) are manual-start, run on gasoline or propane, and power selected circuits via extension cords or a transfer switch. Standby generators ($3,000 to $6,000 installed) start automatically within 10 seconds of a power outage, run on natural gas or propane (unlimited fuel supply), and power the entire house through an automatic transfer switch. Choose portable if outages are rare (a few times per year) and you only need essentials. Choose standby if you have medical equipment, frequent outages, a sump pump, or want hands-free whole-house coverage.
Is a generator better than a solar battery backup?
Each has advantages. A generator provides unlimited runtime (as long as you have fuel), handles any load size, and costs less upfront. A solar battery backup is silent, has zero fuel cost, produces no emissions, and starts instantly. The main downside of batteries is limited runtime — a 10 kWh battery runs essentials for 8 to 12 hours versus a generator that runs as long as you refuel it. For most homeowners, the best approach is solar panels with a battery for daily use and short outages, plus a generator for extended multi-day events. Compare both options in our Solar vs Generator Calculator.