generator sizing — why most people get it wrong

Sizing is where everyone makes their first expensive mistake. I've seen it go both directions and neither one is fun. Buy a generator that's too big and you overpay by thousands — for the unit, the installation, the concrete pad, the gas line, all of it scaled up for capacity you'll never touch. Buy one that's too small and it overloads and shuts down the first time your AC compressor and well pump kick on at the same time. Which is exactly when you need it most.

The frustrating part is that getting it right isn't complicated. It's just that nobody — not the manufacturer, not the salesperson, not the contractor — has any incentive to help you size it correctly. The manufacturer wants you to buy the bigger unit. The salesperson gets a bigger commission on the bigger unit. The contractor charges more to install the bigger unit. See the pattern.

So I wrote everything I know about sizing into a set of guides that assume you're starting from zero. No electrical background required. Just a willingness to walk around your house with a notepad and read some nameplates.

the basics: running watts vs starting watts

Every electrical device in your house has two numbers that matter for sizing. The first is running watts — the continuous power it draws while operating. A refrigerator runs at about 150 watts. A space heater pulls 1,500. A window AC unit might draw 1,200.

The second number is starting watts, sometimes called surge watts. This is the burst of extra power a motor needs for the first one to three seconds when it kicks on. Not every appliance has a significant starting surge — a light bulb draws the same wattage whether it's been on for a second or an hour. But anything with a motor in it — your refrigerator compressor, your well pump, your AC unit, your sump pump — needs a spike of power at startup that can be two to three times its running wattage.

A well pump that runs at 1,000 watts might need 2,500 watts to start. A central AC compressor running at 3,500 watts can surge to 7,000 or more. A sump pump drawing 800 watts at steady state might demand 2,000 at kickon.

Your generator doesn't need to handle every starting surge simultaneously. It needs to handle your total running watts plus the single highest starting surge that could happen at any given moment. This is where load management comes in — staggering when things turn on so you don't stack surges on top of each other. Some generators have built-in load management. Others, you handle it yourself with a transfer switch that prioritizes circuits.

load management: how to run more on less

Load management is the single most underrated concept in generator sizing. The idea is simple: you don't run everything at once, so don't size for everything at once.

A smart transfer switch can shed non-essential loads when demand spikes. Your AC compressor kicks on, so the transfer switch temporarily drops power to your electric dryer or water heater. The compressor finishes its startup surge, demand drops, and the switch restores the other circuits. All of this happens automatically, in seconds, and you never notice.

This is how a well-configured 16kW generator can do the work of a 22kW unit. You're not giving up comfort or capacity — you're just being intelligent about when things draw power. This is exactly what I did at my place, and I've never had an overload event in three years.

If you're going the portable generator route, you don't get automatic load management. You are the load manager. That means knowing not to flip on the microwave while the well pump is cycling, and being deliberate about which circuits you're feeding. It works fine — it just requires you to pay attention.

the sizing mistakes I see over and over

sizing to the panel, not the load

This is the big one. A 200-amp panel has a theoretical capacity of about 48,000 watts. No house ever draws that much. Not even close. Your real-world peak demand — the worst-case moment when the AC, the dryer, the oven, and the well pump are all running — is usually 30 to 50 percent of panel capacity. Sizing your generator to your panel is like buying a pickup truck because your grocery bags are heavy. It works, but you've massively overpaid for the capability.

Generators sized to the panel cost more up front, cost more to install, burn more fuel, and require more maintenance. The only people who benefit are the people selling them to you.

ignoring the well pump

If you're on well water, your well pump is probably the single most demanding load in your house — not because of its running watts, but because of its starting surge. A deep well pump can surge to 3,000-4,000 watts for a few seconds every time it kicks on. If you sized your generator without accounting for this, you're going to have a bad time when someone flushes a toilet during an outage.

I see this constantly with people moving from city water to well water. They've never thought about their water supply as an electrical load before. It's the first thing you should add to your sizing calculation if you have a well.

forgetting the AC compressor surge

Central air conditioning is the other load that kills undersized generators. A 3-ton AC unit might run at 3,500 watts but surge to 7,000 watts or more at startup. That's a 3,500-watt swing that lasts about two seconds, and if your generator doesn't have that headroom, it shuts down.

Some people solve this with a hard start kit on the AC compressor, which reduces the starting surge by 30-50%. It costs about $100 and it's one of the best investments you can make if you're trying to keep your generator size (and cost) down. I have one on my unit and it made a noticeable difference.

forgetting about 240V loads

Electric ranges, electric dryers, electric water heaters, and heat pumps run on 240 volts. If you have any of these and you want them operational during an outage, they draw significantly more than your typical 120V loads. An electric range can pull 8,000-10,000 watts. An electric water heater draws 4,500. A heat pump might run at 5,000 and surge higher. If you're all-electric, sizing gets serious fast.

This is one reason I always ask people what fuel their major appliances run on. A house with gas heat, gas range, and gas water heater is dramatically cheaper to back up than an all-electric house. If you're building new or renovating, that's worth thinking about.

all sizing and planning guides

• what size generator do I need? The complete walkthrough. How to audit your loads, calculate your minimum, and pick the right capacity with room to spare.
• generator sizing calculator Interactive tool. Plug in your appliances and it tells you what size generator you need. Takes about two minutes.
• how many watts to run a house Typical wattage ranges by house size and lifestyle. What the real numbers look like, not the theoretical maximums.
• transfer switch guide Manual vs automatic, whole-panel vs subpanel, and why your transfer switch choice affects your generator sizing.
• generator installation guide What to expect from installation. Permits, placement, fuel lines, electrical work, and how to not get overcharged.
• portable vs standby generator When a portable makes more sense than a permanent standby unit, and when it doesn't. The honest tradeoffs.
• generator for apartment vs house Backup power options if you rent, live in a condo, or can't install a permanent generator. Yes, you still have options.

where to go from here

If you're still in the research phase, start with what size generator do I need — it walks through the entire process. If you already have a rough idea of your wattage needs and want to see specific units, head to the generators hub where I review and compare actual equipment. And if you already know you want whole-home coverage, I wrote a detailed breakdown of the best whole-home generators on the market right now.