Watt-Hours Explained (2026): How to Size a Power Station Without Overbuying
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Every portable power station is sold on one headline number, and almost nobody explains it. This is watt hours explained in plain terms — what the number means, why it is not the same as watts, how it relates to the amp-hours printed on RV batteries, and how to turn it into an honest answer to the only question that matters: how long will this thing actually run my stuff?
The 30-second version
A watt is how fast something uses energy. A watt-hour is how much energy is stored. Runtime in hours equals watt-hours divided by watts. A 1,000 Wh battery running a 100 W load lasts about 10 hours on paper — call it 8.5 in reality after conversion losses.
Watts versus watt-hours
This is the confusion that costs people money, and a water analogy fixes it permanently.
Watts are flow rate — litres per minute coming out of the tap. A 1,500 W kettle pulls hard. A 5 W LED bulb trickles. This tells you nothing about how long anything lasts.
Watt-hours are tank size — how many litres the tank holds. A 2,048 Wh battery holds twice what a 1,024 Wh battery holds, regardless of what you connect.
You need both numbers for different reasons. Watt-hours decide how long. Watts decide whether it works at all — plug a 1,800 W appliance into a unit rated 1,000 W continuous and it trips instantly no matter how large the battery is.
The runtime formula
Runtime in hours = capacity in watt-hours ÷ load in watts
Then subtract about 15% for real-world losses. Inverters are roughly 85–90% efficient converting DC battery power to AC, compressor startups waste energy, and cold weather reduces available capacity. Anyone quoting you the raw arithmetic is quoting a best case you will not see.
| Appliance | Draw | Runtime on 1,000 Wh |
|---|---|---|
| LED bulb | 10 W | ~85 hours |
| CPAP, humidifier off | 30 W | ~28 hours |
| Laptop | 50 W | ~17 hours |
| Refrigerator, cycling | 150 W avg | ~5.5 hours |
| Microwave | 1,000 W | ~50 minutes |
| Space heater | 1,500 W | ~34 minutes |
That last row is the one that surprises people. Resistive heating devours batteries. No portable power station is a practical heating solution.
Amp-hours versus watt-hours
RV and marine batteries are sold in amp-hours, power stations in watt-hours, and they are not interchangeable without knowing voltage.
Watt-hours = amp-hours × volts
A 100 Ah battery at 12V holds 1,200 Wh. The same 100 Ah at 24V holds 2,400 Wh. This is why comparing a 200 Ah battery bank against a 2,000 Wh power station is meaningless until you convert — 200 Ah at 12V is 2,400 Wh, so the bank is actually larger.
It is also why higher-voltage systems win at scale — the relationship between voltage, current and cable cost is covered in watts vs volts vs amps.
Rated capacity versus usable capacity
Lead-acid batteries should only be discharged to about 50% before damage, so a 100 Ah lead-acid battery realistically gives you 600 Wh, not 1,200 Wh. LiFePO4 can be run to 80–90% depth of discharge routinely, so a 100 Ah LiFePO4 battery genuinely delivers around 1,000–1,100 Wh.
Portable power stations quote usable capacity already, which makes them easier to compare. This usable-capacity gap is a large part of why LiFePO4 replaced lead-acid despite the higher sticker price.
Worked example: an overnight outage
Fridge at 150 W average, router at 15 W, three LED lamps at 30 W, phone charging at 10 W. Total 205 W continuous. Over 8 hours that is 1,640 Wh. Add 15% for losses and you need about 1,900 Wh.
So a 1 kWh unit like the Anker SOLIX C1000 Gen 2 covers roughly half the night, while a 2 kWh unit like the Bluetti AC200L covers the full night with margin. That is the entire decision, and it took two lines of arithmetic.
Apply the same method to camping in our complete portable power stations guide, or to fridge-specific backup in how to run a refrigerator on backup power.
If you would rather not estimate, the ENERGY STAR product database publishes annual kilowatt-hour consumption for most major appliance categories. Divide that annual figure by 8,760 to get the true average draw in watts, which is far more accurate for sizing than the peak number printed on the appliance label.
Frequently asked questions
Is a higher watt-hour number always better?
No. More watt-hours means more weight, more cost, and more battery sitting idle. The right capacity is your real daily load plus about 30% reserve. Buying past that is money spent on a number rather than an outcome.
Why does my unit not deliver its rated watt-hours?
Inverter conversion costs 10–15%, cold weather reduces available capacity, and high-draw loads are less efficient than gentle ones. Expect roughly 85% of the rating in practice, and less in freezing conditions.
How do I find an appliance wattage?
Check the label on the appliance or its power brick. For anything that cycles, like a fridge, the label shows peak draw rather than average — use the yellow Energy Guide annual kWh figure divided by 8,760 to get true average watts, or measure with a plug-in meter.
What is surge wattage and why does it matter?
Anything with a motor or compressor draws several times its running wattage for a second or two at startup. A fridge running at 150 W can surge past 1,200 W. If your unit cannot supply that spike it shuts down, regardless of capacity.
Last updated: July 18, 2026. Efficiency figures are typical values for LiFePO4 units with pure sine inverters; individual results vary with temperature and load profile.
