How to Size a Portable Power Station: Wh and Watts

Two numbers: capacity in Wh, output in W

Every power station spec sheet lists a capacity in watt hours (Wh) and an output in watts (W). The Department of Energy uses the same split for all energy storage: energy capacity is the total amount that can be stored, usually in kilowatt hours, and power capacity is how much can be released at a given time, usually in kilowatts. The kW vs kWh guide explains the difference between the two units, so here we only use it.

The two numbers answer different questions:

• Capacity (Wh): how long? It is the fuel tank. It decides how many hours or days your devices run before you have to recharge.
• Output (W): how much at once? It is the size of the pipe. It decides which devices can run together. If the total draw goes over the rating, the station shuts the outlet off.

The ratio between them tells you how fast a station can drain itself. The National Laboratory of the Rockies (formerly NREL) uses a reference home battery of 5 kW of power with 12.5 kWh of energy, which it describes as a 2.5 hour system: at full output it is empty in 2.5 hours. A portable station works the same way at a smaller scale. Size the two numbers separately, in the order below.

Step 1: list your devices and their watts

Write down every device you want to run, how many watts it draws, and how long it runs per day. Be honest about the hours. The list is the whole calculation; everything after it is arithmetic.

Where to find the watts:

• The nameplate. Virginia Cooperative Extension points to the rating stamped on the back or bottom of the appliance. Many devices have settings, so the actual draw depends on the setting in use.
• Amps and volts. If the label gives only amps, multiply by the voltage. The volts, amps and watts guide walks through reading a label.
• The EnergyGuide label for fridges and freezers. According to the Department of Energy's Energy Saver guide, the label lists the estimated yearly electricity consumption. Divide that kWh figure by 365 and multiply by 1,000 to get Wh per day. This beats nameplate watts for anything that cycles on and off.
• Battery devices in Wh. A phone, a camera or a power bank is easier to count by its battery size in Wh than by its charger. The mAh vs Wh guide converts the mAh on the box.

For the examples on this page we use nameplate figures that the Virginia extension publication lists: a laptop at 50 W, a window fan at 55 to 250 W, a microwave at 750 to 1,100 W, a coffee maker at 900 to 1,200 W, and a single electric blanket at 60 W. The LED bulb is the 9 W bulb on the Lighting Facts label example in the Energy Saver guide. The phone is a 4,400 mAh battery at 3.7 V, which is 16.3 Wh. Where we had to pick one value inside a range, or invent a figure such as the fridge's label value, it is marked as an example input. Replace all of them with the numbers from your own devices.

Step 2: watts × hours = Wh per day

For each device, multiply watts by hours of use per day. That gives watt hours per day. The extension formula divides by 1,000 to get kWh; for a power station, keep it in Wh, because that is the unit on the spec sheet. A microwave that runs for 10 minutes counts as 10 ÷ 60 of an hour.

Here is a day without grid power at home. The fridge uses an example label value of 400 kWh a year, the fan an example 100 W from inside the extension's range, and the microwave the top of its range.

The fridge is about half of the day. Anything that runs around the clock adds up this way, and it is why the hours column matters as much as the watts column. The electricity cost calculator does the same watts × hours sum for one device at a time if you want to check a line.

Next, multiply by the number of days you want to cover without recharging. For a camping weekend that is two days. For an outage, it is the length you want to ride out, not the length you hope for.

Step 3: allow for conversion losses

The battery stores direct current. Your wall outlets deliver alternating current, so a power station runs its AC outlets through an inverter, and USB ports through converters of their own. The Department of Energy puts it plainly: storage is never 100% efficient, and some energy is always lost in converting it and getting it back. So the battery has to hold more than your devices use.

Use the efficiency from your spec sheet if it lists one. If it does not, these are the only published reference points we could verify, and neither is a portable power station:

• NLR's PVWatts solar calculator uses a default nominal DC-to-AC efficiency of 96% for grid-tied solar inverters. It also models efficiency on a curve, so it changes with the load.
• The lab's Annual Technology Baseline uses 85% as a representative round-trip efficiency for a residential battery system: energy out divided by energy in, so it covers charging and discharging together.

We use an example input of 90% for the worked numbers. The table shows how much the choice matters for the 2,152 Wh outage day, with the usable share from step 4 held at 90%:

The spread between the first and last row is 472 Wh. When you do not know the figure, size with the lower one; a station that is a little too big costs money once, a station that is too small fails on the night you need it.

Step 4: rated capacity versus usable capacity

The Wh on the box is the rated capacity. Not all of it may be available. PVEducation, the photovoltaics textbook by Christiana Honsberg and Stuart Bowden, defines the depth of discharge as the fraction of a battery's capacity that can be used, specified by the manufacturer. Many batteries cannot be fully emptied without damage, so the maker sets a limit; with a 25% depth of discharge, only a quarter of the capacity reaches the load.

Check whether the power station's spec sheet gives a usable capacity or a depth of discharge. If it gives neither, ask the maker before you buy. We use an example input of 90% usable.

The same textbook adds two effects that cut into capacity in practice. Discharging fast gets less energy out of a battery than discharging slowly, and capacity is lower at low temperature. A station that runs a microwave in a cold garage will deliver less than one that runs a fan at room temperature. If either applies, leave extra margin.

For the outage day at 90% efficiency and 90% usable:

If that is more than you want to carry, the list is where to cut. Dropping the fan from the outage day saves 741 Wh of rated capacity per day; running the fridge from the station only part of the day, and keeping the door shut, saves more. To convert the result to kWh or joules, use the energy converter.

Step 5: check the output watts and the motor start

Capacity is only half the job. Now add up the watts of everything that could be running at the same moment, not over the day. In the outage example, the worst moment is someone heating food while the fridge compressor, the fan, the lights and the laptop are all on:

• Microwave 1,100 W + fridge 150 W running (an example input you would measure with a plug-in meter) + fan 100 W + lights 18 W + laptop 50 W = 1,418 W.

Then allow for motors. Virginia Cooperative Extension notes that a motor draws about three times more current when it starts than when it runs smoothly. A fridge compressor, a fan or a pump is a motor. If the fridge kicks in while everything else is on, the draw for that moment is roughly 1,418 W + 300 W extra = 1,718 W, using the running watts as a stand-in for current at the same voltage.

Read the station's spec sheet for two output ratings: the continuous rating, which must cover the sum of running loads, and the surge or peak rating, if listed, which must cover the start. In this example the station needs at least 1,418 W continuous and room for about 1,718 W for a moment. Without the microwave the sum falls to 318 W, which is the cheapest fix: run heating devices one at a time.

Anything that heats, such as a microwave, a toaster, a coffee maker, a hair dryer or a portable heater, carries a high nameplate rating in the extension's list. These are short-burst devices with high output and modest Wh, so they decide the output rating even when they barely dent the capacity. If you need to compare a rating given in horsepower or BTU per hour, the power converter turns it into watts.

Three setups compared

Here are three common uses run through steps 2 to 5, all with the same example inputs of 90% efficiency and 90% usable. The camping list is lights, two phones, a laptop for 2 hours and a single electric blanket for 8 hours. The van day is a laptop for 6 hours, lights, phones, a fan for 8 hours and a 1,000 W coffee maker for 12 minutes. The outage is the table from step 2, for two days.

The camping weekend needs a fair amount of capacity, most of it for the blanket, but very little output. The van day is the opposite story at the output end: one coffee maker sets the watt rating. The outage needs both. Your list will look different, and the method is the same.

Step 6: plan the recharge

For anything longer than a day, the recharge matters as much as the capacity. A station that you can refill each day only has to hold one day.

From the wall or a car

The spec sheet lists a charging input in watts for each port. The fastest possible refill is the energy you took out divided by that input:

The outage day draws 2,391 Wh from the battery (2,152 Wh ÷ 90%). At an example input of 500 W, that takes at least 4.8 hours. Real charging takes longer, because charging has losses too; the maker's stated charge time is the better guide. What a full charge costs from the wall is a job for the electricity cost calculator: it is the Wh divided by 1,000, times your rate, plus a little for losses.

From solar panels

Solar is where most sizing plans go wrong, because a panel's watt rating is what it produces in full sun under test conditions, not what it gives you on an average day. NLR's PVWatts calculator defines a panel's DC rating at standard test conditions of 1 kW of sunlight per square meter. Daily sunlight is published in kWh per square meter per day, so a rough daily estimate is:

For losses we use PVWatts' default of 14%, which it builds from categories such as soiling, shading, wiring and connections for a typical installed system. More dirt or shade on your panel means a higher figure. PVWatts also models the effect of panel temperature separately, which this rough formula leaves out, so treat the result as an estimate, not a promise. The sun values below are example inputs; to get the real ones, open PVWatts, enter your location, and read the monthly solar irradiance column for the months you care about.

The camping weekend draws about 761 Wh a day from the battery. A 200 W panel at an example 4 kWh/m² per day gives about 688 Wh, which is 90% of one day's draw; on a 2 kWh/m² day it is 45%. PVWatts warns that monthly sunlight can vary from the long-term average by ±30%, and annual sunlight by ±10%, so a cloudy week can bring far less than the table. Plan the solar side for your worst likely month, not the average, and keep the station's capacity for the days the panel falls short.

Also check the station's solar input rating in the spec sheet: it caps how many watts the station takes in from panels, whatever the panels are rated at.

Safety while you use and charge it

A power station is a large lithium-ion battery. New York State's consumer safety guide for lithium-ion batteries gives practical rules that apply here:

• Buy from reputable makers and look for a safety mark from a nationally recognized testing laboratory. Check the Consumer Product Safety Commission's recall database before you buy, and again now and then.
• Charge, store and use it as the manufacturer instructs, with the charger that came with it or one the maker approves.
• Do not leave it charging unattended or overnight while you sleep, and do not charge it below 32°F (0°C) or above 105°F.
• Store it cool and dry, out of direct sun, and check the case for cracks or damage.
• If it gets extremely hot, swells, hisses, smells or smokes, stop using it, unplug it, leave the area and call 911. Do not try to put out a lithium-ion fire yourself.

Mistakes that make a station too small

• Sizing from watts only. A 1,000 W rating says nothing about how long it lasts. Do both checks.
• Forgetting the fridge runs all day. Use the EnergyGuide label's yearly kWh, not a guess at hours.
• Taking the rated Wh at face value. Divide by efficiency and by the usable share.
• Ignoring motor starts. Leave output headroom for compressors, fans and pumps.
• Counting on the panel's full rating every day. Use your location's monthly sunlight from PVWatts and plan for the weak months.
• Adding a day you did not plan for. Outages and bad weather do not end on schedule. If you can, size for one more day than you expect, or plan a way to recharge.

Frequently asked questions

Sources

• Grisso, Walker and Ignosh, Estimating Appliance and Home Electronic Energy Use, Virginia Cooperative Extension publication 2901-9014, 2020 (wattage × hours ÷ 1,000 = daily kWh; nameplate wattages such as laptop 50 W, window fan 55 to 250 W, microwave 750 to 1,100 W, coffee maker 900 to 1,200 W, single electric blanket 60 W; amps × volts; refrigerators cycle; a starting motor draws about three times its running current)
• US Department of Energy, Energy Saver Guide 2022 (EnergyGuide label shows estimated yearly electricity consumption; Lighting Facts label example of a 450 lumen bulb using 9 watts)
• US Department of Energy, Solar Integration: Solar Energy and Storage Basics (energy capacity in kWh versus power capacity in kW; storage is never 100% efficient)
• National Laboratory of the Rockies (formerly NREL), Annual Technology Baseline 2025: Residential Battery Storage (5 kW / 12.5 kWh reference system, 2.5 hours; 85% representative round-trip efficiency)
• NLR PVWatts Calculator help: Inverter Efficiency (default nominal DC-to-AC efficiency 96%)
• NLR PVWatts Calculator help: System Losses (default 14%) and DC System Size (rated at standard test conditions, 1 kW/m²)
• NLR PVWatts Calculator help: Cautions for Interpreting the Results (solar radiation may vary from the long-term average by ±30% for monthly values and ±10% for annual values)
• NLR PVWatts Calculator
• Honsberg and Bowden, PVEducation: Battery Charging and Discharging Parameters (depth of discharge is the fraction of capacity that can be used, specified by the manufacturer)
• Honsberg and Bowden, PVEducation: Battery Capacity (capacity is lower at fast discharge and at low temperature)
• New York State Division of Homeland Security and Emergency Services, Lithium-Ion Battery: A Consumer Safety Guide (certification marks, CPSC recall check, manufacturer charger, unattended charging, charge between 32°F and 105°F, warning signs)
• US Consumer Product Safety Commission, Recalls

Written by Tobias. Updated October 21, 2026.