What Size Generator Do I Need?
Pick what you want running when the power goes, and this gives you the size of unit that covers it. Sized the way an electrician sizes it, which is often smaller than a showroom will tell you.
How Generator Sizing Works
Running Watts, and the One Surge That Matters
Every load has a running figure, which is what it draws once it is going. Anything with a motor also has a starting figure, which is what it pulls in the first second, and that can be three times the running number. A well pump that runs at a thousand watts can ask for three thousand to get moving.
The generator has to survive the worst single moment. That moment is the largest one of those starts happening while everything else is already running. So the size you need is the running total plus the biggest single surge, and not the running total plus every surge added together. Two motors starting in the same instant is rare enough that sizing for it buys capacity you will never use, and it is the most common reason a quoted unit is bigger than it needs to be.
What Size Generator for a 200 Amp or 400 Amp Service?
This is the way the question is usually asked and it is worth answering directly: your service size is the wrong number to size a generator from. A 200 amp service is the maximum the utility supply could deliver, not what the house uses. Sizing a generator to match it means buying a unit for a demand the house has never once made.
The arithmetic makes the gap obvious. A 200 amp service at 240 volts is 48,000 watts of theoretical capacity. A typical New Hampshire house backing up heat, a well pump, a refrigerator, a freezer and lighting needs somewhere in the region of 8,000 to 12,000 watts. Sizing from the service would mean a unit four or five times larger than the job, at several times the cost, running at a fraction of its capacity for its whole life.
A 400 amp service makes it starker rather than different. Those are usually large houses, or houses with a workshop or an outbuilding on the same service, and the honest answer is the same: what matters is what you want running when the power is out, not what the meter could theoretically pass.
Where service size genuinely matters is the transfer equipment rather than the generator. A transfer switch is rated for the service it carries, not for the generator behind it, so a 200 amp service takes a 200 amp switch even with a modest unit feeding it. That is the one place the service rating is the number that counts, and it catches people out because it is the opposite of how the generator itself is sized.
Worked examples by service size have their own pages, generator size for a 200 amp service, generator size for a 400 amp service, and standby generator sizing for 100 to 300 amp service, each with the circuits added up the way this calculator does it.
How to Calculate What Size Generator You Need
Four steps, and the calculator above does all of them. List the circuits you actually want running in an outage rather than everything in the house. Add up their running watts. Find the single largest starting surge among them, which on most houses is a well pump or a furnace blower. Add that one surge to the running total.
That figure is your minimum. Round it up to a size that is manufactured, leave a little headroom so the unit is not running at its ceiling, and that is the honest answer. Doing it by hand takes about ten minutes with a list of nameplate ratings; the calculator does it from the circuits you tick.
The step people skip is the first one. It is tempting to list everything, and everything is what produces the oversized quotes. A house that decides it can live without the range, the dryer and the air conditioning for two days frequently halves the unit it needs, and a critical loads panel is what makes that decision permanent rather than something to remember during a storm.
Why the Answer Is Rounded Up
A generator run permanently at its ceiling is a generator that wears out early and stalls when something unexpected comes on. There is margin in the number above for that reason, and the result is then rounded to a size that is actually manufactured rather than to whatever the arithmetic produced.
If your total lands just over a unit boundary, that is worth a conversation rather than an automatic move to the next size up. Sometimes the answer is load management, which lets a smaller unit carry a bigger house by shedding the heavy things for a few minutes at a time.
Generator Questions, Answered
Why Is This Smaller Than the Number the Generator Dealer Gave Me?
Usually because the dealer sized to whole-house coverage and this sizes to what you picked. It can also be surge maths. The correct method is running watts plus the largest single starting surge, because two motors almost never start in the same instant. Adding every surge together is the common shortcut and it can oversize a unit by several thousand watts, which is real money for capacity you will never use.
What Is the Difference Between Running Watts and Starting Watts?
Anything with a motor draws far more in the first second than it does once it is turning. A well pump might run at 1,000 watts and pull 3,000 to get going. A generator has to survive that spike without stalling, so the unit is sized for the biggest one, not the average.
Should I Get a Portable or a Standby Generator?
A portable is cheaper, gets wheeled out and started by hand, and needs fuel carried to it during the outage. A standby sits on a pad, runs off gas or propane, and starts itself when the power goes. If outages here are long and you are away sometimes, a standby is the one that actually gets used. If they are short and rare, a portable with a correctly installed inlet is often enough.
Can I Run the Whole House or Just Some of It?
Both are possible and they cost differently. A critical-loads panel feeds a chosen set of circuits, which is cheaper and lets a smaller unit do the job. Whole-house means the generator carries everything, which needs a bigger unit and usually load management so it can shed the heavy things when it needs to.
What Size Generator Do I Need for a 200 Amp Service?
Service size is the wrong number to size from. A 200 amp service is what the utility could deliver, not what the house uses, and a generator matched to it would be four or five times larger than the job. Size from the circuits you actually want running instead. The one place the service rating does matter is the transfer switch, which is rated for the service rather than for the generator.
Does a 400 Amp Service Need a Bigger Generator Than a 200 Amp One?
Not by itself. A 400 amp service usually means a larger house, or a workshop or outbuilding on the same supply, and those loads may well need a larger unit. But the service rating is not the reason. What you want running during an outage is, and that is a different list on every property.
Does It Need a Transfer Switch?
Yes, always, and it is not optional. Backfeeding a house without one puts power onto the utility line and can kill the crew working to restore it. A transfer switch physically prevents the two sources from ever being connected at once, and it is what the inspector checks first.