What size generator do I need?

Sizing a standby generator comes down to adding up what you want to keep running, allowing for the momentary surge when motors start, and rounding up to a standard size. The sizing calculator does this from a list of common appliances. This guide explains the math behind it so you know what the number means.

What you’re adding up

A generator has two ratings that matter for sizing:

• Running watts keep an appliance going once it’s on. A refrigerator draws roughly 600 to 800 W while the compressor runs, a furnace blower about 500 to 800 W, and lights and chargers a few watts each.
• Starting watts (surge) is the extra hit a motor needs for the first second or two. An air conditioner, well pump, or fridge compressor can draw two to three times its running watts for that brief start.

Generators are sold by running watts (22 kW, 24 kW, and so on). The number you need is the sum of your running loads, plus the single largest motor’s surge, because two big motors rarely start in the same instant.

The four steps

1. List your essential loads. What do you want running during an outage? For most homes that’s the fridge, furnace blower, some lights, the well pump or sump if you have one, and the air conditioner if you want it.
2. Sum the running watts. Check the data plate on each appliance, or use the typical values the calculator carries for common equipment.
3. Add the single largest motor’s surge. Take the biggest motor on your list and add its starting watts on top of the running total. You don’t add every motor’s surge, because they don’t all start together.
4. Add 20% headroom and round up. A generator that runs flat out wears faster and has nothing left for a load you forgot. Headroom plus rounding to the next standard size (10, 14, 18, 22, 24, 26 kW and up) gives you the answer.

That’s the number behind the calculator’s suggestion.

What size covers what

Roughly, and depending on your home:

• 10 to 14 kW runs the essentials. Fridge, furnace blower, sump, lights, and a small window AC, but not central air.
• 18 to 22 kW is the most common whole-house band. It covers central air in a smaller home plus the essentials, which is why 22 kW is the most-installed size.
• 24 kW and up handles central air and most of the panel in a larger home. Very large houses or big properties may need 26 to 48 kW.

These are starting points. Two houses with the same square footage can need different sizes because of how their loads stack. Run the calculator on your actual list before you compare models.

Where people go wrong

• Sizing off square footage alone. A 2,000 sq ft house with gas heat and no central air needs a smaller generator than the same house with electric heat and a 4-ton AC. Loads drive the number, not floor area.
• Forgetting the well pump or septic lift pump. These are short, high-surge motors that can catch you out. Check their starting watts.
• Counting every surge. Add the single largest motor surge, not all of them.
• Buying exactly to the load. No headroom means no margin for the appliance you add next year.

Verify before you buy

This method gives you a defensible starting point, not the final answer. Real sizing depends on your actual appliances, your panel, and local code. A licensed electrician confirms the load calculation, the transfer switch pairing, and the permit. See what a whole-house generator costs installed once you know the size, and compare the ranked picks at that band.