BUILD YOUR ENERGY INDEPENDENCE · POWER SIZING
Pick what you need to keep running and this page returns the three numbers that decide whether a station or generator will carry the load. The figures come from DOE, ENERGY STAR, and Cooperative Extension, not from companies selling the equipment.
Before you start
Every figure on this page is a typical value for a class of appliance. Your refrigerator is not a class of appliance. It has a compressor of a particular size, a door seal in a particular condition, and a kitchen at a particular temperature.
Start here to get within range. Then put a meter on whatever matters most, because a plug-in monitor settles in a day what any estimate can only approximate.
Our guide to measuring your own power use covers the meter method, the utility smart-meter portal, and how to read what you find.
Where these numbers come from
Most sizing charts on the internet are published by companies that sell generators and battery stations. A larger published wattage sells a larger unit.
Every figure here traces to DOE guidance, FTC EnergyGuide methodology, ENERGY STAR program criteria, Cooperative Extension engineering publications, or an appliance nameplate. Retailer and generator-manufacturer figures were rejected as sources.
If nothing has appeared here, JavaScript is switched off or the page is still loading. The arithmetic is set out in full below, in the order the tool applies it, so a load list can be worked by hand in about five minutes with a pencil. Nothing on this page hides a step.
The arithmetic
One number cannot answer the sizing question. Three separate limits can each stop a unit from doing the job, and a fourth question can disqualify a device before any of them apply.
00
Some household equipment runs on 240 volts. No portable power station can supply it at any capacity, because a station cannot produce split-phase 240V. Watt-hours do not fix this, so it is settled before any arithmetic runs.
Check the breaker. Single-pole means 120V and the device stays in the load list. Double-pole means 240V and it moves to a separate list with a plain statement of what that rules out. Per NEC Article 430, breaker size tells you the voltage and nothing about running amperage.
01
How much energy the load list consumes in a day, measured in watt-hours. This is the number that decides whether a battery station lasts the night.
device Wh = watts × hours × duty cycle daily Wh = sum of every device required = daily Wh ÷ 0.85
The 0.85 covers inverter and conversion losses. It appears as a visible line item rather than folded in quietly, so you can check it or replace it with a figure you trust more.
02
Whether the unit can supply everything running at the same moment. A station with plenty of stored energy still fails here if too much runs at once.
simultaneous W = sum of watts running together required W = simultaneous W ÷ 0.8
That is the 80 percent rule. A unit rated for 1,000W continuous should not be planned at 1,000W of load.
03
Motors draw far more current for the moment they start than they do while running. This check is pass or fail, and it is the one that sends people back to the store.
peak surge W = (largest motor W × its surge factor) + watts of everything else running
The realistic worst case is the largest motor starting while the rest of the house is already running. Adding every device's surge together would describe an event no household performs, and it pushes readers toward equipment they do not need.
Where the numbers come from
The number printed on an appliance is the maximum it can draw with every feature firing at once, including defrost heaters and resistance elements.
Cooperative Extension publication BSE-301 lists a 16 cubic foot frost-free refrigerator at 725W[1]. Compressor draw on a modern unit runs 100 to 200W. Building a calculator on nameplate figures overstates a motor device roughly fourfold.
A refrigerator plugged in for 24 hours does not draw 150W for 24 hours. The compressor cycles, and DOE guidance puts its actual run time at roughly 8 hours in every 24[2].
Multiplying watts by hours without a duty cycle overstates a refrigerator's daily energy by about three times, and that error compounds into a recommendation two tiers larger than needed.
The rule inverts for resistive loads
For a heater, kettle, toaster, or hot plate the nameplate figure is the running figure. There is no compressor cycling, no motor inefficiency, and no startup surge to account for. A 1,500W space heater draws 1,500W the entire time it is on.
This matters because it runs opposite to everything above. Readers who have just learned to distrust the sticker on a refrigerator will under-size for a space heater if they carry that lesson across. Resistive heat is the fastest way to empty a battery precisely because none of the usual discounts apply.
No published dataset of per-device household starting multipliers exists. Manufacturers report locked-rotor amps inconsistently, and that figure is a worst case rather than a realistic starting draw.
BSE-301's observation that a motor draws about three times its running current at startup is the only primary-source anchor available[3]. So surge is assigned by load type rather than guessed device by device. Centrifugal pumps are low-inertia loads that start more gently than the charts claim, and sealed compressors start harder than brush motors, so a single blanket multiplier would be less accurate than the evidence supports.
If your appliance nameplate prints a locked-rotor or LRA figure, that beats every estimate on this page. Use it.
| Load type | Surge |
|---|---|
| Sealed compressor | 3 to 5x |
| Centrifugal pump | 2 to 4x |
| Fan and blower | 2 to 3x |
| Universal or brush motor | 2 to 3x |
| Resistive and electronic | 1x |
Three figures in the device table ship contested: the sump pump, the utility transfer pump, and the shallow-well jet pump. Sources genuinely disagree on all three, so the ranges are stated wide and labelled rather than given false precision.
The limits
A calculator that only reports its answers teaches less than one that reports its boundaries.
A freezer in a hot garage runs far more of the hour than the same freezer in a conditioned basement. The figures here assume ordinary conditions.
A station rated at room temperature delivers less in an unheated garage in January. Store it where you live, not where you park.
Lithium iron phosphate cells are commonly rated for 2,000 to 3,500 cycles before falling to 80 percent of original capacity. Plan for the last year of service, not the first.
An engine generator produces less as elevation rises. The manual for your model states the figure. Battery stations are unaffected.
Water heaters, dryers, ranges, central air, EV charging, and deep-well submersible pumps sit outside this page. Read them through your utility smart-meter portal instead.
Do not size life-sustaining equipment from an estimate. Work from your own device specifications and see power outage planning for medical devices.
Work at the panel
Opening a panel, adding a circuit, or installing a transfer switch is licensed-electrician work. Reading a breaker to learn whether a circuit is single-pole or double-pole is not, and that is the only panel task this page asks of you.
Common questions
A full-size refrigerator draws roughly 100 to 200 running watts, but the compressor runs only about a third of the time, so a full day costs roughly 1 to 2 kWh. A station holding 1,000Wh of usable capacity will typically carry a refrigerator through most of a day. Surge is a separate question. A sealed compressor can pull three to five times its running watts for a moment at startup, so a unit that clears the capacity check can still fail on surge.
Most published sizing charts come from companies that sell generators and battery stations, and a larger published wattage sells a larger unit. Those charts also tend to quote nameplate wattage rather than what an appliance draws in normal operation. The figures here are running watts, derived from DOE guidance, FTC EnergyGuide methodology, ENERGY STAR criteria, Cooperative Extension publications, and appliance nameplates.
Only if the pump runs on 120V. Shallow and convertible jet pumps are commonly sold as convertible between 115V and 230V, so owning a shallow well does not tell you which you have. Check the breaker: single-pole means 120V, double-pole means 240V. A 240V pump cannot run from a portable power station at any capacity, because a station cannot produce split-phase 240V. Breaker size tells you the voltage and nothing about running amperage.
Resistive heat is the one case where the nameplate figure is also the running figure. A 1,500W heater draws 1,500W continuously, with no compressor cycling and no motor discount to apply. That is 1,500Wh in a single hour, which is more than many portable power stations hold in total.
No. Size life-sustaining equipment against the specifications for your own device, with a plan reviewed by your provider and equipment supplier. Oxygen concentrators in particular are poorly suited to extended battery backup, and the spread between a portable pulse-dose unit and a 10 litre home concentrator is roughly sixfold, which makes a typical figure actively misleading.
Where to go next
What the three numbers mean once you are reading a spec sheet.
Fuel, transfer switches, and the carbon monoxide rules that do not bend.
Storage across an outage, from power banks to a household bank.
How much you can put back into a battery in a day of sun.
The most-asked runtime question, worked through end to end.
Replace every estimate on this page with your own readings.
Continue reading
Field Notes
The average U.S. outage is 11 hours, and that number hides everything that decides how much capacity you need.
Preparedness
The household picture around the numbers: food, water, heat, medical equipment, and communications when the grid is down.
Case Study
What 328 days without power did to every household assumption about how long backup has to last.
Water
43 million American households lose water when the power goes out. Generator sizing, hand pump backup, and solar options for well owners.