The Complete Guide to Sizing a Home Standby Generator

The right home standby generator must carry your planned running load and start the equipment that can turn on while that load is active. The answer depends on your appliances, fuel, and outage priorities. A house-size estimate can help begin the conversation, but it cannot establish the final generator rating.

First, describe what a normal outage day should look like

Write a short description before collecting wattages: “Keep the refrigerator, well, lighting, internet, and downstairs HVAC available. Water heating can pause, and vehicle charging can wait.” That gives the installer something specific to design and price.

Decide whether you want selected circuits backed up or the entire service connected through transfer equipment. A whole-house transfer arrangement makes circuits accessible to generator power. It does not promise that every connected appliance can operate at full output simultaneously.

Build the load list from actual equipment

Photograph accessible labels rather than guessing from a breaker handle. Mark missing values as unknown. The appliance-nameplate guide explains which ratings help and which describe circuit protection.

Ask for two checks: sustained demand and motor starting

The running-load check asks whether the generator can carry the equipment expected to operate together. The starting check asks what happens when a compressor or pump starts against that background load. Passing one does not automatically mean passing the other.

Do not add an appliance’s full starting demand to a total that already includes that appliance running. Use consistent definitions and account for overlapping starts. The running and starting watts explanation includes a worked example.

For larger motors or sensitive controls, the installer may need starting current, apparent power, and acceptable voltage or frequency drop. Those inputs help determine whether equipment will start reliably, beyond a simple sum of household watts.

Confirm the rating available at your property

Compare exact models on the fuel you will use. As checked on September 15, 2026, Generac lists its model 7260 at 22 kW on propane and 21 kW on natural gas. That example applies to the named model, not every generator advertised as 22 kW.

The proposal should also address fuel delivery, applicable elevation or temperature adjustments, and transfer equipment. Extra generator capacity cannot compensate for an inadequate fuel supply.

Make every load-management assumption visible

If the proposed size depends on pausing the water heater or delaying a second air conditioner, put that in writing. Identify the controlled equipment and the method used. “We probably will not use those together” is a household preference; it is not an installed control.

Reserve capacity should reflect a named future load, such as a planned heat-pump replacement. Ask for a comparison with and without that addition rather than automatically adding an arbitrary percentage.

What a useful sizing proposal should include

• The backed-up loads and the important exclusions.
• The generator model and fuel-specific output used in the calculation.
• The major starting event and how overlapping loads are handled.
• Managed appliances, expected interruptions, and restart behavior.
• The installation assumptions and a commissioning check under representative loads.

Begin with the Brite Generators calculator, then bring your equipment photos and outage priorities to Brite Generators. Generac’s professional sizing tool also calls for fuel, voltage, motor-starting information, and managed loads, showing why those details belong in the final assessment.