How-to guide

How Big a Power Station Do I Need? A Watt-Hour Sizing Guide

By Jeremy Kelly · Specs last checked November 7, 2026 · Research-based, not hands-on tested

A step-by-step way to size a portable power station for an outage: watts vs. watt-hours, surge, inverter losses, and four worked examples with all the arithmetic shown.

“How big a power station do I need?” is the question most people get wrong in one of two directions. Some buy a small unit that can’t start the fridge. Others pay for 3kWh when a 300Wh unit would have run their router and laptop all day. The fix is about ten minutes of arithmetic, and this guide walks you through it.

We haven’t measured your appliances, and we haven’t tested the power stations mentioned here. Example wattages are clearly labeled and come from published sources where possible: Virginia Cooperative Extension’s appliance table (developed with the U.S. Department of Energy’s Building Science Community of Practice) and manufacturer documents. Your own devices will differ, so measure them if you can.

Two numbers: watts and watt-hours

Every power station has two headline specs, and they answer different questions:

  • Output (watts, W): Can it run this? The rated AC output is the most power the station can deliver at once. If your microwave needs 1,100W and the station is rated for 600W, it won’t run, however full the battery is.
  • Capacity (watt-hours, Wh): For how long? This is how much energy is stored. A 1,024Wh battery could, in theory, supply 1,024 watts for one hour, or 100 watts for about ten.

There’s a third number that trips people up: surge or peak watts. Motors and compressors (fridges, pumps, some fans) draw a short spike when they start. Virginia Cooperative Extension notes that a motor’s starting current is about three times its running current. If the station’s surge rating can’t cover that spike, it may shut off.

The sizing formula

  1. List what you want to run during an outage.
  2. Find each device’s watts. Check the label or nameplate, or multiply amps × volts (most U.S. outlets are 120V). Better yet, measure with a plug-in energy meter.
  3. Estimate hours of use for each device during the outage.
  4. Multiply watts × hours to get watt-hours for each device, then add them up. This is the same formula Virginia Cooperative Extension gives for daily energy use (watts × hours ÷ 1,000 = kWh).
  5. Divide by 0.85 to allow for conversion losses. EcoFlow and BLUETTI use this 0.85 factor in runtime formulas on some of their listings. Treat it as a planning assumption, not a guarantee.
  6. Check the largest single load (and its surge) against the station’s rated output and surge rating.

If you’ll use AC outlets for small loads all day, also budget for the inverter’s own draw. Jackery lists 18W standby for its Explorer 1000 v2. BLUETTI’s AC180 formula assumes about 15W of self-consumption.

Example wattages (labeled, with sources)

Device Example watts Source Notes
Wi-Fi router 3 W EcoFlow (RIVER 3 runtime example) Many routers differ; measure yours
Laptop 50 W Virginia Cooperative Extension Charging draw varies
CPAP, humidifier off, 10 cm H₂O, on DC 11.2 W ResMed Battery Guide (AirSense 10, 0.93A at 12V) See our CPAP guide
Ceiling fan 65–175 W Virginia Cooperative Extension We use 75W in examples
Window fan 55–250 W Virginia Cooperative Extension
Flat-screen TV 120 W Virginia Cooperative Extension Older table; newer sets often use less
Microwave 750–1,100 W Virginia Cooperative Extension Short bursts, high output needed
Coffee maker 900–1,200 W Virginia Cooperative Extension
Portable heater 750–1,500 W Virginia Cooperative Extension Drains batteries fast
Refrigerator Measure it — Use a 24-hour meter reading (kWh/day)

These are nameplate-style examples, not measurements of your devices. A refrigerator’s nameplate tells you little about daily energy, because the compressor cycles on and off. That’s why we use a measured daily figure in the examples below.

Worked example 1: work-from-home day (8 hours)

Device Watts Hours Watt-hours
Router (example) 3 W 8 24 Wh
Laptop (example) 50 W 4 of the 8 hours 200 Wh
Total 224 Wh

224 Wh ÷ 0.85 = about 264 Wh.

If you run both from AC outlets and the inverter uses about 15W the whole time, add 15 W × 8 h = 120 Wh. (224 + 120) ÷ 0.85 = about 405 Wh.

What that means: a 250–300Wh station is borderline on AC but comfortable on DC/USB-C. Charge the laptop over USB-C, and run the router from 12V or USB-C if you’ve confirmed the right adapter. Otherwise step up to around 500Wh.

Worked example 2: overnight with a CPAP and a fan (8 hours)

Device Watts Hours Watt-hours
CPAP, humidifier off, DC (ResMed figure) 11.2 W 8 89.3 Wh
Ceiling fan (example, 75 W) 75 W 8 600 Wh
Total 689.3 Wh

689.3 Wh ÷ 0.85 = about 811 Wh.

What that means: the fan uses almost seven times more energy than the CPAP. A 1kWh-class station covers this night with some margin. Without the fan, a 250–300Wh station would cover the CPAP alone (89.3 ÷ 0.85 ≈ 105 Wh).

Worked example 3: a full day of essentials, including the fridge (24 hours)

Device Watts Hours Watt-hours
Refrigerator (illustrative meter reading) — 24 1,200 Wh
Router (example) 3 W 24 72 Wh
Laptop (example) 50 W 4 200 Wh
CPAP (ResMed figure, DC) 11.2 W 8 89.3 Wh
Total 1,561.3 Wh

1,561.3 Wh ÷ 0.85 = about 1,837 Wh.

The 1,200 Wh fridge figure is an illustrative meter reading (1.2 kWh per day), not a measurement of any particular fridge. Yours may be much lower or higher.

What that means: a 1kWh station won’t get through 24 hours of this load on its own. Your choices are a ~2kWh station (or a 1kWh station with an extra battery), recharging during the day from solar or a car, or cutting loads. Running the fridge on a schedule and keeping the doors shut are the big levers. Also check output: the fridge’s startup surge has to fit within the station’s surge rating. Our refrigerator guide covers that in detail.

Worked example 4: the microwave problem

A 1,100W microwave (the top of Virginia Cooperative Extension’s range) running for 5 minutes uses:

1,100 W × (5 ÷ 60) h = about 92 Wh.

That’s a small amount of energy, but the station needs at least 1,100W of continuous AC output to run it at all. A 300W station can’t do it at any battery level. Some stations offer a “boost” mode (EcoFlow X-Boost, BLUETTI Power Lifting) that lowers voltage to run resistive loads above the rated output. Manufacturers say these modes suit heating devices, not everything. Microwaves aren’t simple resistive loads, so check the manufacturer’s guidance.

Size classes at a glance

Class Typical capacity Good fit for Examples (manufacturer specs)
Compact ~250–300 Wh Router, phones, laptop, CPAP on DC EcoFlow RIVER 3 (245Wh, 300W); Anker SOLIX C300 DC (288Wh, DC only)
Mid ~1 kWh Fridge for part of a day, CPAP for many nights, fans Anker SOLIX C1000 Gen 2 (1,024Wh, 2,000W); EcoFlow DELTA 3 Plus (1,024Wh, 1,800W)
Large ~4 kWh Multi-day essentials, well pumps, 240V loads EcoFlow DELTA Pro 3 (4,096Wh, 4,000W, 120V/240V)

Signs you’re oversizing

  • Your loads are all USB-C or small DC devices
  • Outages in your area are usually short
  • You’ll need to carry it around

Signs you’re undersizing

  • Any single load is near the rated output
  • You want the fridge and overnight loads
  • You can’t recharge during the outage

Picks by size

Compact: the EcoFlow RIVER 3 is a 245Wh, 7.8 lb unit with 300W AC (600W X-Boost). It suits worked example 1 or a CPAP on DC.

Check EcoFlow RIVER 3 on AmazonAffiliate link

Mid: the Anker SOLIX C1000 Gen 2 lists 1,024Wh and 2,000W AC (3,000W peak). It covers worked example 2 and can run many fridges, subject to the surge check.

Check Anker SOLIX C1000 Gen 2 on AmazonAffiliate link

Mid, expandable: the EcoFlow DELTA 3 Plus lists 1,024Wh. EcoFlow says it can add extra batteries for up to 5kWh total, which is a path to worked example 3 without buying a large unit up front.

Check EcoFlow DELTA 3 Plus on AmazonAffiliate link

Large: the EcoFlow DELTA Pro 3 lists 4,096Wh, 4,000W continuous (8,000W surge) and both 120V and 240V outputs.

Check EcoFlow DELTA Pro 3 on AmazonAffiliate link

Bottom line

Add up watts × hours for what you actually need, divide by 0.85, and check that your largest load and its surge fit the station’s output. For most people, that lands at about 300Wh for network and CPAP needs, about 1kWh when a fridge or fans are involved, and about 4kWh for multi-day or 240V needs. Then test your setup at home before the next storm.

Sources