How to Size a Whole-Home Standby Generator

How to size a whole-home standby generator for a Puget Sound outage, why bigger is not better, and the hidden PSE gas-meter upsizing that decides feasibility. Size to your largest starting loads, not your square footage. Estimate the loads yourself with the steps below, then call (206) 260-1981 for the load assessment.

What Size Standby Generator Do I Need

Sizing starts with the loads you cannot lose during a multi-day windstorm outage, not with the floor area of the house. Most single-family homes on natural gas heat land in the 10 to 14 kW essentials tier, which carries the furnace blower or heat-pump air handler, the well or sump pump, refrigeration, and key circuits. Homes that run a heat pump for heat usually move up to the 16 to 22 kW whole-home tier, because the compressor inrush is what drives the number. Large estate homes with multiple heat-pump zones, a pool, and an EV charger can exceed what an air-cooled unit will hold and move into 24 kW-plus liquid-cooled units.

The NEC 220 Load Calculation, In Plain Terms

The right unit is the one that starts your largest motor load while carrying everything else, and that is a real NEC Article 220 calculation, not a chart lookup. Managed transfer switches let a smaller unit shed and rotate large loads so it carries more than its raw kilowatts suggest. Guessing off a square-footage rule of thumb is exactly how homeowners overspend by thousands on a generator that is bigger than the house needs.

Natural Gas vs Propane, and the Hidden PSE Gas-Meter Question

On natural gas, the question that quietly decides feasibility is whether your Puget Sound Energy gas meter can supply the added demand of a whole-home generator on top of the furnace, water heater, and range. Many existing meters need to be upsized by PSE before a large unit will run correctly, and that coordination is part of the job. Propane sidesteps the meter question but adds a tank. We work through both before recommending a model.

What Happens If You Size It Wrong

Oversizing wastes money up front and makes the unit short-cycle, which is hard on the engine. Undersizing leaves you cold when the compressor will not start. We run the load math first so the recommendation is honest either way. When you are ready, see our generator installation in Bothell and generator installation in Seattle pages for the local install process.