Running watts keep things on, starting watts get motors going. The wattage math, an appliance table, and the three sizes that actually fit a home.
Setup Type
Quick answer: Most homes fall into three sizes. To keep the essentials running on a portable, the fridge, furnace fan, lights, and one motor load, you want 5,000 to 7,500 running watts. A standby covering the essential circuits runs 10 to 14 kW. A whole-house standby that runs everything runs 18 to 26 kW. The number that trips people up is not the running watts, it is the starting surge a motor pulls for a second or two, which can be 2 to 4 times its running draw.
Best for
Buyers trying to match a generator to what they actually need to keep on, before they get talked up or down a size.
Wrong fit
Buyers who already know their number and just want the brand shortlist or the installed price.
Tradeoff
Undersize it and it trips at 2am when the furnace and the well pump start together. Oversize it and you pay for watts and fuel you never use.
Most homes need one of three sizes, and the right one depends on what you refuse to lose when the power goes out. To keep the essentials on with a portable, the fridge, the furnace fan, the lights, and one motor like a well or sump pump, you want 5,000 to 7,500 running watts. A standby that covers the essential circuits runs 10 to 14 kW. A whole-house standby that runs everything, nothing switched off, runs 18 to 26 kW.
The number that trips people up is not the running watts. It is the starting surge, the jolt a motor pulls for a second or two when it kicks on. Size for running watts alone and the generator trips at 2am when the furnace and the well pump start together. This guide does the wattage math the spec sheet skips, gives you the appliance numbers, then hands you the three sizes and who each one fits.
If you are still choosing between a permanent standby and a portable you run yourself, read standby vs portable first. The lane changes the size answer.
Quick Answer: The Three Sizes That Fit a Home
What you want to keep on
Size that fits
Lane
Fridge, furnace fan, lights, one motor (well or sump pump)
5,000-7,500 running watts
Portable
The essential circuits, hands-off
10-14 kW
Standby
The whole house, nothing switched off
18-26 kW
Standby
Pick the row that matches what you actually need running, not the biggest number a salesperson can talk you into. Most homeowners who think they need whole-house discover that a well-chosen essentials list covers everything that matters for a fraction of the price.
Running Watts vs Starting Watts
Every appliance has two numbers, and only one of them is printed on the front.
Running watts, sometimes called rated watts, is what a device draws to keep going once it is on. A fridge holding temperature, a furnace fan pushing air, a string of lights. Add up everything you want on at the same time and that is your running total.
Starting watts, also called surge or peak watts, is the extra jolt some devices need for the first second or two. Anything with a motor pulls far more current to get the shaft spinning from a dead stop than it needs to keep it spinning. That spike is brief, but if the generator cannot deliver it, the motor stalls and the breaker trips.
Your generator has to cover your full running total plus the single largest surge on top of it. Not every surge at once, because your motors do not all start in the same instant, but the biggest one has to fit.
Why motors need a running start
A motor at rest is close to a dead short for a moment. To break free and spin up, it draws roughly 2 to 4 times its running watts, and the hardest starters spike higher. A well pump or an air conditioning compressor can pull 3 to 5 times its running draw for that first moment (est., it varies by model and how the motor is loaded).
That is why a 1,000 watt well pump can need 3,000 to 5,000 watts to start, and why a fridge that sips 150 watts all day can ask for over 1,000 at the flick of its compressor. Resistive and electronic loads, lights, a TV, a laptop charger, a microwave, draw close to their rated watts with little or no surge. The surge math is really a motor problem. Count your motors and you have found your sizing problem.
The Appliance Wattage Table
Real ranges, because the exact number depends on the model, the age, and how hard the motor works. Treat these as planning figures, then check the label on your own units.
Appliance
Running watts
Starting surge watts
Refrigerator or freezer
100-800
1,000-2,200
Furnace fan (blower motor)
600-900
1,300-2,400
Sump pump (1/3 to 1/2 HP)
800-1,050
1,300-2,150
Well pump (1/2 to 1 HP)
550-2,000
2,000-6,000 (est.)
Window AC (10,000-12,000 BTU)
1,200-1,500
2,200-3,600
Central AC (2 to 3 ton)
3,500-5,000
very high, a soft start kit helps (est.)
LED lights (one room)
60-300
no surge
Two things jump out of that table. Central air is the giant, which is why it is usually the first thing left off a portable's list, or given a soft start kit to tame its inrush. And the well pump has the widest surge range, which is why well owners so often undersize and then wonder why the generator groans every few minutes.
The Three Sizing Answers
Essentials-only portable: 5,000 to 7,500 watts
This is the sweet spot for most homes that lose power a few times a year. It covers the fridge, the furnace fan, the lights, a few outlets, and one motor load like a well or sump pump, with headroom for that motor's surge. A 7,500 watt unit gives you room to run the well pump and the furnace without choreographing which one starts when.
What it will not do is run central air and everything else at once. That is the trade for spending $700 to $2,500 on the machine instead of five figures on a standby. If a portable fits your outage pattern, start with the best portable generators for home backup.
Essential-circuits standby: 10 to 14 kW
A permanent standby wired to a transfer switch that feeds your important circuits: heat, water, fridge, some lights and outlets, maybe one AC zone. It starts itself within seconds of an outage and runs on your gas line, so nobody is pouring fuel at midnight. Ten to 14 kW covers the essentials for most homes without paying to back up the pool pump and every closet light.
Whole-house standby: 18 to 26 kW
This runs the house the way you live in it, central air included, nothing switched off. It is the right size if you have multiple AC tons, a well, and you refuse to think about load at all. It is also the most expensive to buy, install, and fuel. Most people who ask for it actually want the 14 to 20 kW middle, sized to their real simultaneous load rather than the sum of every nameplate in the house. A whole-house standby lands in the $12,000 to $18,000 installed range, because the install costs about as much as the machine.
Why Oversizing Wastes Money, and Undersizing Trips at 2am
Undersizing is the loud failure. The generator runs fine until the furnace and the well pump surge together, the voltage sags, and the breaker drops your power in the middle of the night. You paid for a generator and still got woken up.
Oversizing is the quiet failure. A generator loafing at a small fraction of its capacity costs more up front, costs more to install, and burns more fuel than the job needs, and running an engine far below its rated load for long stretches is not good for it either. Bigger is not safer once your real surge fits with headroom. The goal is your true simultaneous load, plus the largest single surge, plus about 20 to 25 percent breathing room. Not the sum of every nameplate in the house.
Getting this wrong is one of the most common and most expensive generator buying regrets, in both directions.
Run Your Own Numbers
You do not have to do this on a napkin. Our sizing calculator asks what you want to keep running, then returns a target wattage for a portable or a kW range for a standby, plus a rough fuel burn. Start there, confirm the surge on your biggest motor, and you will walk into any dealer or hardware aisle knowing your number before they tell you theirs.
Frequently Asked Questions
What size generator do I need to run my whole house?
For a true whole-house standby that runs central air and everything else with nothing switched off, plan on 18 to 26 kW, installed in the $12,000 to $18,000 range. Most homes are covered for less. A 10 to 14 kW standby on your essential circuits, or a 5,000 to 7,500 watt portable on the fridge, heat, lights, and one pump, keeps the important things on for a fraction of the cost.
What is the difference between running watts and starting watts?
Running watts keep a device going once it is on. Starting watts are the extra jolt a motor needs for a second or two to spin up from a stop. Your generator has to cover your full running total plus the single largest starting surge on top of it. Lights and electronics use their rated watts with almost no surge. Motors are where the surge lives.
Why does my well pump need so many more watts to start?
A motor at a dead stop draws far more current to break free than it does to keep spinning. A well pump commonly needs 3 to 5 times its running watts for that first moment (est.), so a pump that runs on 1,000 watts can spike to 3,000 to 5,000 to start. If your generator cannot deliver that spike, the pump stalls and the breaker trips.
Will a 7,500 watt generator run my house?
It will run the essentials of most homes: fridge, furnace fan, lights, outlets, and one motor like a well or sump pump, with headroom for the surge. It will not run central air plus everything else at the same time. If that trade fits how often you lose power, a 7,500 watt portable is the best value in backup power for a lot of buyers.
Can a generator run my central air conditioner?
Yes, if it is sized for the surge. Central air is the biggest single load in most homes and pulls a large inrush to start the compressor. A 2 to 3 ton system usually wants a large portable or a standby, or a soft start kit that cuts the inrush so a smaller unit can handle it. Check your compressor's LRA rating and size for that, not the running watts.
Is it bad to buy a generator that is too big?
Past the point where your real surge fits with headroom, bigger just costs more to buy, install, and fuel. A generator running far below its capacity wastes gas, and running an engine that light for long stretches is not good for it. Size for your true simultaneous load, plus the largest surge, plus about 20 to 25 percent. Not the sum of every nameplate in the house.
How do I add up the watts I need?
List everything you want on at once. Add their running watts for your base total. Then find the single appliance with the largest starting surge and add that one surge on top. That sum is your minimum. The sizing calculator does this for you and returns a portable wattage or a standby kW range.
Methodology
These guides are built from manufacturer documentation, public specifications, primary research where health claims matter, and repeated buyer questions that show up in real ownership and installation decisions.
Manufacturer responses can clarify pricing bands, warranty terms, support footprint, or common mistakes. They do not move a page up the shortlist on their own.
