Home Generator Sizing Calculator

About the Home Generator Sizing Calculator

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.

When This Page Helps

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.

How to Use the Inputs

1. List every essential appliance you want to power during an outage.
2. Enter the total running watts of those loads.
3. Enter the highest starting watts among your motor-driven loads.
4. The calculator adds running + largest starting surge for total demand.
5. A 25% safety margin is applied for the recommended size.
6. Select the nearest standard generator size above the recommendation.

Example Calculation

Tips & Best Practices

• Always account for the largest motor starting surge on top of running loads.
• Refrigerators, AC units, and well pumps have the highest starting surges.
• A transfer switch prevents back-feeding power to the grid (required by code).
• Natural gas generators have unlimited fuel during outages if gas lines are intact.
• Propane generators need a properly sized tank (500+ gallons for extended outages).
• Exercise your generator monthly to ensure it starts reliably in emergencies.
• Consider a load management system to stagger motor starts and reduce generator size needed.

Common Home Loads

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.

Generator Types Compared

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.

Sizing for Load Management

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.

Frequently Asked Questions

• What size generator do I need for a house?

  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.

• What is the difference between running and starting watts?

  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.

• Can I run my central air conditioner on a generator?

  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.

• Should I get a portable or standby generator?

  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.

• How much fuel does a home generator use?

  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.

• Do I need a transfer switch?

  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.