Size from a load list, not from square footage

The most common way people end up with the wrong standby generator is starting from the size of the house. Two 2,400 square foot homes can have wildly different backup requirements depending on whether the heat is gas or electric, whether there is a well, and whether anyone works from home.

Sizing starts with a decision, not a measurement: what do you actually want running when the power is out?

Three tiers of backup

Essential circuits

Furnace or boiler controls, refrigerator, well pump, sump pump, some lighting, a few outlets, internet. This is the tier that keeps the house habitable and prevents damage. It typically lands somewhere in the 10–14kW range.

Essential plus comfort

Add central air or a heat pump. Air conditioning is a large starting load, and it is usually the single item that pushes a project up a tier. Commonly 18–22kW.

Whole house

Everything, including an electric range, electric water heating, and a hot tub. Frequently 22–26kW or more.

There is no virtue in the largest tier. An oversized generator costs more to buy, more to install, and runs inefficiently at light load.

The starting-load problem

Running watts are not the constraint. Motors — well pumps, sump pumps, air conditioning compressors, refrigeration — draw several times their running current for the first fraction of a second as they start.

A generator that comfortably runs your loads can still stall trying to start them. This is why a load list is not simply an addition problem, and why soft-start devices on air conditioning are sometimes a cheaper answer than the next generator size up.

Transfer switch: whole-home or essential-circuits

The transfer switch is what disconnects you from the utility and connects you to the generator. It is not optional and it is not a detail — backfeeding a utility line endangers the crews working to restore it.

• Essential-circuits switch: a subpanel holding the circuits you chose. Cheaper, and it enforces your sizing decision physically.
• Whole-home switch with load management: transfers everything, then sheds selected large loads automatically when demand approaches the generator’s capacity. More flexible, and it is what makes a smaller generator behave like a larger one.

Fuel

Natural gas from the utility is the low-friction option: no tank, no delivery, effectively unlimited runtime. Propane requires a tank, and the tank size sets your runtime — worth calculating at your actual expected load rather than at the unit’s maximum.

Diesel is common in commercial standby and rare residentially.

What people underestimate

Placement. Units have minimum clearance requirements from windows, doors and vents, both for exhaust and for service access. That constraint sometimes decides where the unit can go before anything else does.

Noise. Standby generators run a weekly self-test, typically for several minutes. Where the unit sits relative to your bedroom and your neighbour’s is worth thinking about before the pad is poured.

Annual service. It needs one. Skipping it is the most common reason a standby generator fails on the one night it was bought for.

The sequence that avoids rework

Build the load list, decide the tier, confirm the fuel supply is adequate, then size the unit — and only then choose a transfer switch. Doing it in the other order is how people end up with a correctly-sized generator behind a transfer switch that cannot deliver what they wanted.

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Summary

Sizing a standby generator starts with a decision, not a measurement: what do you want running when the power is out? Two homes of the same square footage can have very different requirements depending on gas or electric heat, a well, and who works from home. Essential circuits typically land in the 10–14kW range, essential plus central air or a heat pump commonly 18–22kW, and whole-house frequently 22–26kW or more. Motor starting loads, not running watts, are the real constraint. The transfer switch is mandatory, and fuel choice sets your runtime. Placement clearances, weekly self-test noise and annual service are the three things people underestimate.

Frequently Asked Questions

Can I size a generator from my home’s square footage?

No — it is the most common way people end up with the wrong unit. Two 2,400 square foot homes can have wildly different backup requirements depending on whether the heat is gas or electric, whether there is a well, and whether anyone works from home. Start from a load list instead.

What size standby generator do I need?

It depends which tier you want. Essential circuits — furnace or boiler controls, refrigerator, well pump, sump pump, some lighting and outlets, internet — typically land somewhere in the 10–14kW range. Adding central air or a heat pump commonly moves it to 18–22kW. Whole house, including an electric range, electric water heating and a hot tub, is frequently 22–26kW or more.

Why do motors matter more than total wattage?

Motors such as well pumps, sump pumps and air conditioning compressors draw several times their running current for the first fraction of a second as they start. A generator that comfortably runs your loads can still stall trying to start them, which is why a load list is not simply an addition problem.

Do I really need a transfer switch?

Yes. The transfer switch disconnects you from the utility and connects you to the generator. It is not optional and it is not a detail — backfeeding a utility line endangers the crews working to restore power.

Essential-circuits or whole-home transfer switch?

An essential-circuits switch feeds a subpanel holding the circuits you chose. It is cheaper and it enforces your sizing decision physically. A whole-home switch with load management transfers everything and then sheds selected large loads automatically as demand approaches capacity, which is what makes a smaller generator behave like a larger one.

Is a bigger generator always better?

No. There is no virtue in the largest tier: an oversized generator costs more to buy, more to install, and runs inefficiently at light load. Soft-start devices on air conditioning are sometimes a cheaper answer than the next size up.

What do people most often underestimate?

Three things. Placement, because units have minimum clearances from windows, doors and vents for both exhaust and service access. Noise, because standby units run a weekly self-test for several minutes. And annual service — skipping it is the most common reason a standby generator fails on the one night it was bought for.