Size a home backup generator by entering essential loads. Calculate the total kW needed and find the right generator size including a safety margin.
Choosing the right size home generator means adding up the wattage of every essential load you want to power during an outage and applying a safety margin. An undersized generator will overload, trip, or fail when too many loads start simultaneously. An oversized generator wastes fuel and money.
The key challenge is accounting for starting watts (surge/inrush current) from motor-driven loads like air conditioners, well pumps, and refrigerators. A refrigerator may run at 200W but requires 1,200W to start its compressor. The generator must handle these startup surges even briefly. Typical home generators range from 7.5 kW (essential circuits only) to 22+ kW (whole-house).
This calculator lets you input individual essential loads to calculate the total running and starting watts needed. It applies a 25% safety margin and recommends a generator size. Use it to make an informed purchase decision and ensure your critical loads are protected during power outages.
Buying the wrong generator size is an expensive mistake. This calculator tallies your essential loads with proper starting watt allowances and recommends the right generator size, ensuring reliable backup power. Having accurate metrics readily available streamlines utility bill analysis, budget forecasting, and investment planning for energy efficiency projects and renewable energy installations.
kW Needed = Σ(Essential Loads) × 1.25
Result: 10.3 kW recommended
Running loads: 6,000W. Largest starting surge: 2,200W. Peak demand: 8,200W. With 25% safety margin: 8,200 × 1.25 = 10,250W = 10.3 kW. A 12 kW generator would be the recommended standard size.
Refrigerator: 150–400W running, 1,200W starting. Furnace blower: 300–800W running, 1,500W starting. Well pump (1/2 HP): 750W running, 1,500W starting. Sump pump: 800W running, 1,300W starting. Window AC: 1,200W running, 3,600W starting. Central AC (3-ton): 3,500W running, 6,000W starting. Lights (10 LED): 100W. Microwave: 1,000–1,500W.
Portable inverter generators (2–7 kW): Quiet, fuel-efficient, clean power for electronics, but limited capacity. Portable conventional (3‒12 kW): Affordable, higher output, but noisier. Standby natural gas/propane (10–22+ kW): Automatic, unlimited fuel, whole-house capability, highest cost.
A load management system (smart transfer switch) can reduce the generator size needed by 25–40%. It staggers motor starts so they don't surge simultaneously and can shed non-essential loads when the generator is near capacity.
For essential circuits (fridge, furnace, lights, well pump): 7.5–12 kW. For whole-house including AC: 16–22 kW. A large home with multiple AC units, electric range, and EV charger may need 30+ kW. Calculate your specific loads for accuracy.
Running watts is the continuous power a load draws during normal operation. Starting watts is the brief surge (1–3 seconds) when a motor starts. Starting watts can be 3–7 times running watts for compressors and pumps.
Yes, but it requires a large generator. A 3-ton AC unit draws about 3,500W running with a 6,000W starting surge. With other loads, you'd need at least a 14–16 kW generator for AC plus essentials.
Portable generators (3‒12 kW, $500–$2,000) are cheaper but require manual setup. Standby generators (10–22+ kW, $3,000–$15,000 installed) start automatically within seconds of an outage and are permanently wired.
At 50% load, a typical generator consumes: gasoline: 0.5–1.5 gal/hr, propane: 1–3 gal/hr, natural gas: 100–300 ft³/hr, diesel: 0.4–1.0 gal/hr. Fuel consumption scales roughly with load.
Yes. A transfer switch disconnects your home from the grid before connecting the generator, preventing back-feeding that endangers utility workers. It's required by the NEC and local codes. Options include manual and automatic transfer switches.