THE DEFINITIVE GENERATOR GUIDE · SIZING
Generator sizing. Start with the loads.
The generator that is too small overloads and shuts down. The one that is too large wastes fuel and money. How to calculate the right size from your actual loads, not from a marketing label on a box.
The sizing methodTHE PROBLEM
Why generator sizing is harder than it looks
A generator rated at 7,500 watts does not mean you can run 7,500 watts of equipment. That is the number on the sticker. The number that matters is the one your electrical panel demands at the worst possible moment: everything running, plus the largest motor starting at the same time.
Three facts make sizing harder than simple addition. First, motors surge. A refrigerator compressor that draws 150 watts while running may need 500 watts for the first second it starts. A well pump that draws 1,000 watts running may need 2,500 watts to start. Second, these surges stack. If the well pump starts while the refrigerator compressor is already cycling, the generator must handle both the pump's surge and the refrigerator's running load simultaneously. Third, generators lose capacity at altitude, in high heat, and on propane or natural gas compared to their gasoline rating.
The result: a household that adds up 4,000 running watts and buys a 5,000-watt generator can still overload it. The math is not wrong. The math is incomplete. This guide walks through the complete calculation.
THE TWO NUMBERS
Running watts and starting watts. Both matter.
Every appliance has two power demands. Running watts (also called rated watts or continuous watts) are the steady draw during normal operation. Starting watts (also called surge watts or peak watts) are the brief spike when a motor-driven appliance first turns on.
Resistive loads like light bulbs, space heaters, toasters, and coffee makers draw the same power from the moment they switch on. They have no startup surge. Their starting watts equal their running watts.
Motor-driven loads are different. A compressor, pump, fan motor, or power tool must overcome inertia to get the motor spinning. That initial effort draws 2 to 3 times the running wattage for a fraction of a second to about one second. Some large motors surge even higher. The generator must deliver that spike or the motor will not start, the overload protection will trip, or the generator will shut down.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator (standard, 18 to 22 cu ft) | 100 to 200 | 400 to 600 |
| Chest freezer (15 cu ft) | 50 to 100 | 300 to 500 |
| Sump pump (1/3 HP) | 800 | 1,300 to 2,000 |
| Well pump (1/2 HP, 240V) | 1,000 | 2,000 to 3,000 |
| Well pump (1 HP, 240V) | 1,500 to 2,000 | 3,000 to 4,500 |
| Furnace blower (1/2 HP) | 500 to 800 | 1,000 to 1,500 |
| Central air conditioner (3-ton) | 3,000 to 3,500 | 6,000 to 9,000 |
| Window air conditioner (10,000 BTU) | 1,200 | 1,800 to 3,600 |
| Microwave (1,000W) | 1,000 | 1,000 |
| Electric water heater | 4,500 | 4,500 |
| LED lights (10 bulbs) | 100 | 100 |
| Phone charger | 5 to 25 | 5 to 25 |
| Router and modem | 20 to 50 | 20 to 50 |
| Circular saw (7-1/4 in) | 1,400 | 2,300 to 3,500 |
Figures are general ranges based on DOE appliance data and manufacturer specifications. Your specific equipment may differ. The measuring your power use guide explains how to read the nameplate or measure actual draw with a watt meter.
THE METHOD
Five steps to the right number
List every load you want to power
Walk through the house and write down every appliance, device, and system you want running during an outage. Be specific: not "kitchen" but "refrigerator, microwave, overhead light, coffee maker." Include the furnace blower if you heat with gas or oil. Include the well pump if you are on well water. Include the sump pump if you have one.
Do not list everything in the house. List what you actually need. The difference between "everything the house has" and "everything the household needs during an outage" is often $5,000 to $10,000 in generator cost.
Find the running watts for each
Check the nameplate on the appliance (usually on the back or bottom). It lists watts directly, or amps and volts from which you calculate watts (amps x volts = watts). For 120-volt appliances, multiply the listed amps by 120. For 240-volt equipment, multiply by 240.
If the nameplate lists only amps, that is usually the maximum running draw. For motor-driven loads, the nameplate amps may represent the locked-rotor or maximum draw, which is closer to starting than running. When in doubt, measure with a watt meter (a Kill-A-Watt or similar plug-in meter costs $25 to $40 and measures the actual running watts of anything that plugs into a standard outlet).
Identify the starting watts for motor loads
For every item on your list that has a motor (compressor, pump, blower, fan, power tool), find its starting watts. The manufacturer's specification sheet or the generator sizing guide from the equipment maker usually lists this. If not, a safe estimate for most single-phase motors is 2 to 3 times the running watts. Some older or larger motors can surge higher.
Resistive loads (lights, heaters, toasters, coffee makers, chargers) do not surge. Their starting watts equal their running watts. Skip this step for those.
Add the running total, then add the largest surge
Add the running watts of everything that will operate simultaneously. This is your continuous demand. Then find the motor load with the single largest starting-watt surge. Add that surge to your running total. This is your peak demand.
You add only the largest surge, not all surges, because motors do not all start at the exact same moment. In practice, the worst case is everything running steadily while the biggest motor kicks on. That is the number the generator must handle.
Add a 20 to 25 percent margin
A generator running at 100% of its rated capacity is working at the absolute limit. It is louder, burns more fuel, wears faster, and has no headroom for an unexpected load. A 20 to 25% margin means the generator operates at 75 to 80% of capacity under normal demand, which is the sweet spot for fuel efficiency, noise, and engine longevity.
If you will run on propane or natural gas instead of gasoline, the margin matters even more. Propane typically produces 85 to 90% of the gasoline rating. Natural gas typically produces 80 to 85%. Size for the fuel you will actually use, not the gasoline-rated peak number on the specification sheet.
THE STACKING PROBLEM
When two motors start at the same time
The sizing method above accounts for the largest single motor surge. In practice, there are moments when two motors can demand surge power simultaneously. A refrigerator compressor cycles on and off automatically. A sump pump activates when the water level rises. A furnace blower starts when the thermostat calls for heat. None of these are under your control during an outage.
If the refrigerator compressor kicks on at the same instant the sump pump starts, both startup surges hit the generator at once. The odds of this exact overlap are low, but during a storm where the sump pump cycles frequently and the refrigerator is working harder because the kitchen is warmer than usual, the probability increases.
There are three ways to handle this:
Oversize slightly
The 20 to 25% margin recommended in Step 5 usually covers occasional double surges for smaller motors like refrigerators and freezers. If you have multiple large motors (well pump plus sump pump, or well pump plus air handler), consider sizing for two simultaneous surges rather than one.
Stagger manually
With a portable generator and extension cords or an interlock, you control what is connected. You can start the well pump first, let it reach running speed, then plug in the refrigerator. This avoids stacking entirely. It works but requires you to be present and attentive.
Use load management
On a standby generator with a managed-load transfer switch, load-shedding modules temporarily disconnect lower-priority circuits when the generator approaches its limit. The module reconnects them automatically when demand drops. This is the automated solution to the stacking problem and is standard on well-designed standby installations.
COVERAGE DECISION
Essential circuits vs. whole house
The single biggest factor in generator sizing is not which brand you buy. It is which loads you choose to include. The difference between "keep the critical systems running" and "run the house like nothing happened" is enormous in capacity, cost, and fuel consumption.
Here is what the numbers look like for a typical 2,000-square-foot home with a well pump, gas furnace, and central air conditioning:
| Approach | Running watts | Peak with surge | Generator size |
|---|---|---|---|
| Essential circuits only | 2,500 to 3,500 | 5,000 to 6,500 | 7,500 to 9,000 W |
| Essential + central AC | 5,500 to 7,000 | 11,000 to 16,000 | 14 to 20 kW |
| Whole house (all loads) | 10,000 to 15,000 | 18,000 to 25,000+ | 22 to 30+ kW |
Essential circuits typically include: refrigerator, freezer, well pump, sump pump, furnace blower, 10 to 15 LED lights, phone and laptop chargers, router, and a few convenience outlets. Total running watts for these loads often falls between 2,500 and 3,500 watts. A good dual-fuel portable or inverter generator in the 7,500 to 9,000-watt range handles this comfortably with margin.
Adding central air conditioning typically adds 3,000 to 3,500 running watts and 6,000 to 9,000 starting watts. That single load roughly doubles the required generator capacity. This is why central AC is the most common dividing line between a portable generator and a standby system.
Adding an electric water heater (4,500 watts), electric range (8,000 to 12,000 watts), and electric clothes dryer (4,000 to 5,500 watts) pushes the total into territory that only a large standby generator can serve. These are the loads most essential-circuit plans deliberately exclude.
WORKED EXAMPLES
Four households, four different answers
Each example follows the five-step method. Your numbers will differ, but the process is the same.
Apartment renter
No well, no sump, city water
Loads: refrigerator (150W run / 500W start), 5 LED lights (50W), phone and laptop chargers (50W), router (30W), window fan (100W), coffee maker (900W, not simultaneous with microwave), microwave (1,000W).
Running total (everything except microwave and coffee maker at once): 380W. Add microwave for peak cooking: 1,380W. Largest surge: refrigerator at 500W. Peak: 1,880W.
Sized for: 2,200 to 3,500-watt inverter generator. Or a 1,500Wh battery power station for a two-day outage without a generator at all.
Suburban home, city water
Gas furnace, no well, has sump pump
Loads: refrigerator (150W / 500W), freezer (80W / 400W), furnace blower (700W / 1,300W), sump pump (800W / 1,800W), 10 LED lights (100W), router (30W), phone chargers (50W).
Running total: 1,910W. Largest surge: sump pump at 1,800W. Peak demand: 3,710W. With 25% margin: 4,640W.
Sized for: 5,000 to 7,500-watt dual-fuel portable or inverter generator. A 120-volt model works because there are no 240-volt loads.
Rural home with well
Well pump (1/2 HP, 240V), gas furnace, no central AC
Loads: refrigerator (150W / 500W), freezer (80W / 400W), well pump (1,000W / 2,500W), furnace blower (700W / 1,300W), 10 LED lights (100W), router (30W), chargers (50W).
Running total: 2,110W. Largest surge: well pump at 2,500W. Peak demand: 4,610W. With 25% margin: 5,760W.
Sized for: 7,500-watt 120/240-volt dual-fuel portable or open-frame inverter generator. Must be 120/240V split-phase because the well pump is 240 volts. A 120-volt-only generator cannot run this pump at any wattage.
Large home, wants AC
Well pump, central AC (3-ton), gas furnace, two refrigeration units
Loads: refrigerator (150W / 500W), freezer (80W / 400W), well pump (1,000W / 2,500W), central AC (3,500W / 7,000W), furnace blower (700W / 1,300W), 15 LED lights (150W), router (30W), chargers (50W).
Running total: 5,660W. Largest surge: central AC at 7,000W. Peak demand: 12,660W. With 25% margin: 15,825W.
Sized for: 16 to 22 kW standby generator, or a load-managed standby that sheds the AC compressor when the well pump starts. This is the point where a standby system typically makes more sense than a portable.
COMMON MISTAKES
Six sizing mistakes that cost people money
Sizing from peak watts instead of running watts
Generators are marketed with their peak or starting-watt rating in large type. A "9,500-watt generator" may have a running rating of only 7,500 watts. Size from the running (rated, continuous) number. The peak number tells you what the generator can briefly surge to, not what it can sustain.
Ignoring the voltage question
A 9,000-watt 120-volt generator cannot power a 240-volt well pump. Watts and volts are independent specifications. If any load on your list is 240 volts, you need a generator with 120/240-volt split-phase output. The portable generator and inverter generator guides cover this in detail.
Using the gasoline rating while planning to run propane
A dual-fuel generator rated at 7,500 running watts on gasoline may produce only 6,750 watts on propane and 6,000 watts on natural gas. If your preparedness plan relies on stored propane, size from the propane rating. The gasoline number on the spec sheet is irrelevant to your actual fuel plan.
Forgetting altitude derating
Engines produce less power at higher elevations because the air is thinner. A common rule of thumb is 3 to 3.5% power loss per 1,000 feet above sea level. A generator rated at 7,500 watts at sea level may produce only about 6,700 watts at 3,500 feet. If you live at elevation, account for this in your sizing. Some manufacturers publish altitude derating tables.
Buying the biggest generator "just in case"
A generator that is dramatically oversized for the load it carries wastes fuel, costs more to buy and maintain, is heavier and louder than necessary, and on some engine types can develop wet stacking or carbon buildup from prolonged light loading. Size for your actual loads plus the recommended margin. Bigger is not automatically better.
Assuming "whole house" means what you think it means
"Whole-house generator" is marketing, not an engineering standard. A 14 kW air-cooled standby sold as a whole-house solution cannot run a 3-ton central AC, an electric water heater, and all other loads simultaneously. The term means the generator connects at the service entrance, not that it can power everything at once. Sizing still starts with the loads.
REDUCING SURGE DEMAND
Ways to make a smaller generator work harder
If the peak demand from your load calculation lands just above what a generator can handle, there are several ways to reduce the surge requirement without buying a larger unit:
Soft-start devices
An aftermarket soft-start module (such as a MicroAir EasyStart or similar) installed on an air conditioner compressor or large motor ramps the startup current gradually instead of hitting the generator with the full surge at once. A soft-start can reduce the starting wattage of an air conditioner by 60 to 75%. This is the most common way to run an RV air conditioner on a smaller inverter generator, and it works on residential window units and some mini-split systems as well.
Load sequencing
If you are managing loads manually with extension cords, start the largest motor first before connecting other loads. Let the well pump reach running speed, then plug in the refrigerator, then the freezer. Each motor's surge is absorbed one at a time instead of stacking.
Load management modules
On standby systems, load-management modules automatically shed lower-priority circuits when the generator approaches capacity and reconnect them when demand drops. This lets a 16 kW generator serve loads that would otherwise require a 22 kW unit by ensuring the heaviest loads never run simultaneously.
Eliminate loads instead of powering them
An electric water heater draws 4,500 watts continuously. Turning it off during an outage (or switching it to a lower element) removes that load entirely. A gas water heater, a propane-on-demand heater, or simply going without hot water for two days eliminates a load that would otherwise require a much larger generator. The cheapest watt is the one you do not need to generate.
WHICH TYPE FITS YOUR NUMBER
From the size to the generator type
Once you know the wattage you need, the sizing number points you toward the right category of generator:
| Your number | Generator type | Typical cost range |
|---|---|---|
| Under 2,000 W | Battery power station or small inverter | $200 to $800 |
| 2,000 to 4,500 W | Enclosed inverter generator | $400 to $1,500 |
| 4,500 to 7,500 W | Portable conventional or open-frame inverter | $500 to $2,000 |
| 7,500 to 12,000 W | Large portable or heavy-duty split-phase inverter | $1,000 to $3,000 |
| 12,000 W and above | Standby generator (air-cooled or liquid-cooled) | $7,000 to $30,000+ installed |
These are general guidelines, not hard boundaries. A well-managed 7,500-watt portable with a transfer switch handles many homes that might initially calculate into the 10,000+ range after load-management adjustments. The type guides above explain what each category can and cannot do.
The layered approach
Many well-prepared households do not pick one generator to do everything. They layer: a battery power station for overnight quiet loads and indoor-safe communications, a small inverter generator for efficient medium-duty use, and a larger portable or standby system for the heavy loads. Each piece handles what it does best. The layers complement each other rather than forcing one machine to cover the full spectrum from a phone charger to a well pump.
Sizing does not change the safety rules
A correctly sized generator still produces carbon monoxide. Every fuel-burning generator operates outdoors only, at least 20 feet from any door, window, or vent, with exhaust directed away from the building. Install CO alarms on every level of the home. A bigger generator does not make indoor operation safer. No generator is safe indoors.
Full generator safety guideCONTINUE READING
More from the generator guide.
Generator Guide Hub
Compare all three generator types and find the right starting point for your household.
Power Needs Calculator
Run the actual numbers for your equipment with DOE-sourced running and starting watt figures.
Measuring Your Power Use
How to read nameplates, use a watt meter, and find the real numbers instead of guessing.
Ready to compare specific models? The generator buying guide covers current picks across all three types, sized for different household needs.