The short answer
To back up a full electrical service, you need a generator rated close to what that service can deliver. A 200-amp, 240V single-phase home service tops out around 48kW. A 400-amp, 480V three-phase service tops out around 266kW. An 800-amp, 480V service lands near 530kW, and a 2000-amp, 480V switchgear is about 1,330kW.
Here’s the thing though. Almost nobody actually pulls the full rating of their service. Your panel size tells you the ceiling. Your real load tells you the generator.
We get this question every day, from facility managers, contractors, and folks who just found out their building has no backup at all. As one of our sales managers put it, not everybody speaks power. So we’re going to walk through it the way our techs do on site: start with the amps, convert to kW, then figure out what you really need to carry.
One quick note on our side of things. Generator Source works on commercial and industrial units from 20kW up to 4MW+, all 60Hz. We’ve included home standby numbers below so you can see the whole range, but our wheelhouse starts where the bigger homes and small businesses leave off.
How to convert amps to generator kW
You need three numbers off your service: amps, voltage, and whether it’s single-phase or three-phase. That’s usually printed right on the main breaker or the switchgear nameplate.
For single-phase service (most homes and small shops):
kW = \frac{Volts \times Amps}{1000}For three-phase service (most commercial and industrial buildings):
kW = \frac{Volts \times Amps \times 1.732 \times 0.8}{1000}The 1.732 is the square root of three, which is just how three-phase power math works. The 0.8 is the power factor. Commercial and industrial generators are rated in kW at 0.8 power factor, so that’s the number you plug in to match a generator spec sheet.
If you want a shortcut you can do in your head, at 480V three-phase every 1kW of generator is good for roughly 1.5 amps. At 208V it’s closer to 3.5 amps per kW. So a 1,000kW unit at 480V puts out about 1,500 amps, and a 100kW unit at 208V is about 350 amps.
Home Standby and Small Buildings: 100 to 400 amps, 120/240V single-phase
A 200-amp home service needs about 48kW to cover every circuit at full capacity, but most homes that size run fine on 22kW to 26kW with load management. Here’s the full single-phase range.
| Service size | Max capacity at full service | Common real-world range |
|---|---|---|
| 100A, 240V | 24kW | 14kW to 20kW |
| 150A, 240V | 36kW | 20kW to 26kW |
| 200A, 240V | 48kW | 22kW to 30kW (up to 48kW for whole-house, no load shedding) |
| 400A, 240V (320A meter base) | 77kW to 96kW | 38kW to 60kW |
Why the gap between the max and the real-world column? Your 200-amp panel is sized so everything in the house could, in theory, run at once. In practice your dryer, oven, both AC units, the well pump and the EV charger almost never peak at the same moment. Load management modules shed the big stuff for a few seconds so a smaller generator can handle it.
Where it gets tricky is the 400-amp home or the small shop that’s really a business. Shop compressors, welders, and walk-in coolers change the math fast, and that’s usually the point where folks end up talking to us.
Light Commercial: 100 to 1200 amps, 120/208V three-phase
A 400-amp, 208V three-phase service maxes out around 115kW, which usually means a 125kW generator if you want the whole building covered. This is your typical retail strip, restaurant, small office, clinic, or church.
| Service size | kVA at full service | kW at 0.8 PF | Generator size to cover full service |
|---|---|---|---|
| 100A, 208V | 36kVA | 29kW | 30kW to 35kW |
| 200A, 208V | 72kVA | 58kW | 60kW |
| 400A, 208V | 144kVA | 115kW | 125kW |
| 600A, 208V | 216kVA | 173kW | 175kW to 200kW |
| 800A, 208V | 288kVA | 231kW | 250kW |
| 1200A, 208V | 432kVA | 346kW | 350kW to 400kW |
Restaurants are the ones that catch people off guard. Walk-in coolers, hood fans, and rooftop units all have motors, and motors pull a big spike of current the second they kick on. We’ll get to that in a minute, because it can bump you up a size even when your running load looks small.
Commercial and Industrial: 200 to 5000 amps, 277/480V three-phase
At 480V, a 2000-amp service maxes out around 1,330kW, so most sites that need full coverage land on a 1,500kW generator. Past roughly 3000 amps you’re usually looking at two or more units paralleled together.
| Service size | kVA at full service | kW at 0.8 PF | Generator size to cover full service |
|---|---|---|---|
| 200A, 480V | 166kVA | 133kW | 150kW |
| 400A, 480V | 333kVA | 266kW | 300kW |
| 600A, 480V | 499kVA | 399kW | 400kW to 450kW |
| 800A, 480V | 665kVA | 532kW | 600kW |
| 1200A, 480V | 998kVA | 798kW | 800kW to 1,000kW |
| 1600A, 480V | 1,330kVA | 1,064kW | 1,250kW |
| 2000A, 480V | 1,663kVA | 1,330kW | 1,500kW |
| 3000A, 480V | 2,494kVA | 1,995kW | 2,000kW single unit, or paralleled |
| 4000A, 480V | 3,326kVA | 2,660kW | 3,000kW, typically paralleled |
| 5000A, 480V | 4,157kVA | 3,326kW | 3,500kW to 4MW, paralleled |
This is where our guys spend most of their days. Hospitals, water treatment plants, cold storage, manufacturing, and data centers. These are engines the size of a small house, and at this level a sizing mistake isn’t a minor headache. It’s a six-figure one.
It’s also where lead times matter. New units at 1,500kW and up have been running a year or more out from the factory in a lot of cases, which is a big reason more of our customers are looking at quality used equipment or a rental to bridge the gap.
Why you usually shouldn’t size to your full service rating
Most buildings never come close to pulling their full service amps, so sizing to the panel rating alone often gets you a generator that’s too big. And too big is a real problem, not just wasted money.
The better starting point is your actual demand. Pull twelve months of utility bills and find your peak kW demand. The National Electrical Code already uses a year of demand data plus a 25% cushion to figure existing load on a service, and plenty of engineers size generators the same way. It’s a much more honest number than the breaker gives you. Your engineer and local inspector make the final call. If you don’t have that data, a load study does the same job.
What happens when it’s oversized
Diesel generators want to work. Run one at a light load for long stretches and it never gets hot enough to burn all its fuel. That’s called wet stacking, and you’ll see it as black, oily gunk dripping out of the exhaust.
Our CEO, Eddie, explains it like this: the engine isn’t producing enough heat to burn the fuel in the combustion chamber, so raw fuel comes out the stacks, and if the piston rings never seat you get oil blowing by too. Left alone long enough, that can end in a catastrophic engine failure.
What happens when it’s undersized
This one’s more obvious. Put more load on the generator than it can carry and you get overload shutdowns and tripped breakers, usually at the worst possible moment. It also happens slowly. A business adds a couple of air conditioners, a new production line, maybe some EV chargers, and the generator that fit five years ago doesn’t anymore.
The goal, in Eddie’s words, is to “find that sweet spot where it’s not undersized, it’s not oversized.” Leave room to grow, but not so much that the engine sits there loafing.
Four things that change the number
Amps get you in the ballpark. These are the details our techs and sales team look at before anybody signs off on a size.
Motors and inrush current
A motor can pull several times its running amps for a second or two when it starts. Chillers, pumps, compressors, elevators, and big rooftop units all do this. If several of them start at once when the generator picks up the building, that spike can drop your voltage or trip the unit. Sometimes the fix is a bigger generator. Sometimes it’s just staggering what comes on and when.
Altitude and heat
Generators lose output as the air gets thinner and hotter. Here in Brighton we’re sitting around a mile high, and a unit that makes its full rating at sea level might not make it here. Every manufacturer publishes a derate curve. A generator going to Vail, one going to Denver, and one going out on the eastern plains can be the exact same model and still need to be sized three different ways.
Life safety requirements
Hospitals and other life safety applications fall under NFPA 110. Most hospitals need what NFPA 110 calls a Type 10 system, meaning the generator has to pick up the emergency load within 10 seconds. That affects how you size for the block load the generator takes all at once, not just the steady load it runs at afterward.
Runtime and fuel
Size also drives fuel burn. A big diesel running hard can drink a few hundred gallons in a shift, so the tank and your refueling plan need to match the generator. During a long outage, fuel is often the first thing that runs short, not the generator.
FAQ
What size generator do I need for a 200-amp service? A 200-amp, 240V single-phase service needs about 48kW to cover the full service. Most homes get by on 22kW to 30kW with load management. A 200-amp, 208V three-phase commercial service needs about 60kW, and a 200-amp, 480V service needs about 150kW.
What size generator do I need for a 400-amp three-phase service? At 208V, a 400-amp service maxes out around 115kW, so a 125kW generator covers it. At 480V, the same 400 amps works out to about 266kW, so you’d look at a 300kW unit.
What size generator do I need for an 800-amp service? An 800-amp, 480V three-phase service is about 532kW at full capacity, so most sites land on a 600kW generator. At 208V, 800 amps is about 231kW, which points to a 250kW unit.
How many amps does a 1MW generator put out? A 1,000kW generator at 480V three-phase and 0.8 power factor puts out about 1,500 amps. At 208V it would be about 3,470 amps, which is why units that big are almost always 480V.
Do I have to size my generator to my full panel rating? Not always. Code lets you size to your calculated load instead of the breaker rating, and most buildings run well below their service rating. Twelve months of peak demand data plus a 25% margin is a solid starting point, but your engineer and local inspector have the final say.
Can a generator be too big? Yes. A diesel generator that runs lightly loaded for long periods can wet stack, which fouls the engine with unburned fuel and oil and can lead to serious engine damage.
Does altitude affect generator size? Yes. Generators lose output at higher elevations and higher temperatures. In places like Denver and the Colorado mountains, check the manufacturer’s derate curve before you pick a size.
Get it sized right the first time
The charts above will get you close. Getting it exactly right means somebody looking at your load, your motors, your elevation, and where your business is headed in the next five years.
That’s what we do every day out of Brighton, Jacksonville, Pensacola, and Austin. Run your numbers through our generator sizing tool for a quick estimate, or call the branch closest to you and one of our team will talk it through with you. If you need power before a new unit can get there, we’ve got rental units from 20kW to 2,000kW ready to go.