How to Size a Standby Generator for Your Home

Generator sizing goes wrong in both directions. Undersized units stall when the air conditioner starts. Oversized ones cost more to buy, more to install, and run inefficiently at light load. The calculation is not complicated, but it has to be done.

First decide: whole house or essentials?

Whole house means the generator feeds the entire panel through an automatic transfer switch, and an outage is nearly invisible. It costs the most and needs the largest unit.

Essential circuits means a sub panel holding what has to keep running: refrigeration, well pump, furnace or heat pump controls, some lighting, networking, and medical equipment. Often half the generator for eighty percent of the benefit.

Why starting current, not running watts, sets the size

This is the part people miss. Motors draw several times their running current for the first fraction of a second. An air conditioner that runs at 3,500 watts may demand 15,000 for an instant on start. A well pump behaves the same way. Generators are rated for both continuous output and surge capacity, and it is the surge figure that determines whether your AC will actually start.

The practical consequence: sizing by adding up nameplate wattages will under-size the unit. A proper calculation identifies the largest motor loads and their starting characteristics.

A worked approach

1. List what must run. Be honest about it. Most people discover the list is shorter than they assumed.
2. Note the motor loads. AC compressor, heat pump, well pump, pool pump, septic pump. These dominate the sizing.
3. Add continuous loads. Refrigeration, lighting, electronics, furnace blower.
4. Decide what can be shed. Anything that can wait — EV charging, electric dryer, oven, pool — comes off the backed-up list.
5. Apply load management. Smart controllers stagger large loads so they never start simultaneously, which frequently lets a smaller generator cover more.

Air-cooled versus liquid-cooled

Air-cooled units cover most homes: less expensive, simpler, adequate for essential and many whole-house applications. Liquid-cooled units run cooler and quieter under sustained load and last longer in extended outages, at meaningfully higher cost. If you expect multi-day outages regularly, or you are covering an all-electric home, the liquid-cooled conversation is worth having.

Transfer switches

• Automatic, whole panel. Senses the outage, starts the generator, transfers, and transfers back. The premium option.
• Automatic, essential sub panel. Same automation, smaller scope, lower cost.
• Manual transfer switch or interlock kit. For an existing portable generator. You start it and throw the switch yourself. Dramatically cheaper and completely legitimate.

Whatever you choose, a transfer switch is mandatory. Backfeeding a generator through a dryer outlet energizes the utility conductors outside your house and can kill a line worker. It is also illegal.

Fuel, placement and noise

Natural gas means no refueling but depends on the gas supply staying up. Propane needs a tank sized for the runtime you want. Placement is governed by clearances from windows, doors, vents and property lines, plus local noise limits — and those rules vary by jurisdiction, so they need checking before the pad goes in, not after.