How is a standby generator sized?
A standby generator is sized by a load calculation. The electrical code is specific about this. Under the 2023 National Electrical Code, which Massachusetts uses, a generator on an automatic transfer switch has to carry everything the switch connects to it, unless a load management system limits the load to what the generator can supply.
So the question is what the switch will connect and whether something manages it. On the site visit we look at the panel, the nameplates on the large equipment and how you use the house, then size the generator to that.
What drives the size up?
Large motors matter most, because they draw several times their running power for a moment when they start. Central air conditioning, heat pumps, well pumps and sump pumps are the usual ones. Electric ranges, dryers, water heaters and EV chargers add a lot of steady load. Lights, electronics and a gas furnace's blower add very little.
| Size | What Kohler says it covers |
|---|---|
| 10 to 14 kW | The basics: furnace fan, refrigerator, sump pump, lights and well pump |
| 20 kW | Most major appliances |
| 26 kW | The entire home |
| 48 kW | Everything in a home with 200 amp service |
Kohler's own descriptions, October 2026. Your load calculation decides.
How does load management change the size?
Load management lets a smaller generator do a bigger job. A controller watches the generator and switches large loads on and off so they never all start together. Generac's 200 amp service-rated transfer switch manages up to four air conditioners with no extra hardware, and its Smart Management Modules handle more. Kohler offers PowerBoost to start large loads without dropping the rest of the house, and a load shed kit.
Many whole-house air-cooled systems work this way, and it is the main way to keep the cost down. See what a standby generator costs.
Whole house or selected circuits?
A whole-house transfer switch moves the entire panel onto the generator, so everything works in an outage and nobody has to remember which outlets are live. A switch for selected circuits backs up only the ones you choose, such as heat, the refrigerator, the well pump and a few lights, and it pairs with a smaller generator.
For very large homes that want everything on at once, a liquid-cooled generator is the answer. If parts of the house cannot tolerate even the seconds it takes a generator to start, look at a RoseWater system in front of it.
