Power Station Capacity: Rated vs. Usable Watt-Hours

The watt-hour number printed on your power station is the energy sitting in the cells — not the energy that actually comes out the socket. By the time electricity travels through an inverter or a DC-DC converter to reach your device, a chunk is already gone. On an AC outlet that loss runs roughly 10–20%; on USB it’s smaller but still real. A “1000Wh” station running an AC load realistically delivers something closer to 800–900Wh in practice — and less in the cold or under surge conditions.

Then there’s the mAh problem, which is worse. Milliamp-hours are not energy. They’re charge, and they don’t tell you a thing without also knowing the voltage. Two banks can advertise the same mAh and carry meaningfully different amounts of energy, because they use different cell chemistries at different voltages. Marketers have exploited this gap for years. Understanding why it matters — and how to get from a label to a real planning number — is what this guide is for.

Wh and mAh: What the Units Actually Mean

A watt-hour is exactly what it sounds like: enough energy to supply one watt for one hour. That’s the useful unit for planning, because it connects directly to how long a device will run. Milliamp-hours are something different — they measure electrical charge, not energy. To convert between them you need the voltage:

Wh = (mAh × V) ÷ 1000

This is just physics, and it’s not in dispute. The problem isn’t the formula — it’s that mAh is routinely presented as though it were energy, which it isn’t.

Why the Same mAh Number Can Mean Very Different Things

Here’s where the mAh trap bites. The voltage in that formula depends on the cell chemistry, and different chemistries run at different voltages. Standard lithium-ion cells sit around 3.6–3.7V nominal. LiFePO4 (lithium iron phosphate) cells, which many newer power stations use because of their longer cycle life, sit around 3.2V nominal.

That gap is small enough to ignore-looking, but consider what it does to a 20,000mAh pack:

Same mAh, same physical size, different energy — a gap of roughly 13% before you’ve even turned the thing on. If you’re comparing two banks by their mAh ratings and one is LiFePO4, the comparison is meaningless. You’re not comparing the same thing.

It gets worse when manufacturers quote mAh at a converted output voltage rather than the cell voltage. A bank whose capacity is advertised at 5V output looks like it has far more mAh than its cells actually store. The headline number gets bigger; the energy doesn’t. This is the structural reason mAh is a bad comparison metric: it’s not a lie, exactly, it’s just a number that means nothing without context the spec sheet often omits.

The fix is always to look for the rated Wh printed on the unit and use that for comparisons. If a spec sheet shows only mAh, ask which voltage that’s calculated at before you make any decisions.

From Rated Wh to What Actually Comes Out

Even Wh isn’t the whole story, because the label says what’s stored in the cells, not what exits the output port. Getting that energy out requires conversion — through an inverter for AC, through a DC-DC boost or buck circuit for USB and 12V — and every conversion stage burns some of it.

The rough planning figures, drawn from vendor guidance rather than independent lab measurements, look like this:

• AC inverter path: roughly 80–90% efficient; plan on about 85% of rated Wh reaching your device
• USB / DC output path: roughly 85–95% efficient; plan on about 90%
• End-to-end phone charging (cable losses, heat, the phone’s own charge circuit): closer to 80%, sometimes below

These are planning rules of thumb, not measured constants — the single source behind these figures is a vendor blog, and no independent tester data exists in this research. Use the lower end of each range when conditions are unfavorable: cold weather, surge-heavy loads, or a very small device that keeps the inverter running at low efficiency just to power a 10W lamp.

That last point is worth dwelling on. An inverter draws power just to stay awake. If you’re running a tiny load through a large station’s AC outlet, the inverter’s idle consumption can eat a meaningful fraction of your available energy before anything useful happens. For small loads, the DC path — USB or 12V direct — wastes far less.

Calculating Real Runtime

The formula most spec sheets show — battery Wh divided by device watts — ignores the conversion loss. The better planning formula is:

Runtime = (Battery Wh × efficiency factor) ÷ Device W

For AC loads, use 0.85 as the efficiency factor unless you have reason to use the lower end. For USB, use 0.90. Both are rough, but they’re better than the idealized math.

What the formula still can’t tell you is how surge affects real-world use. Motor appliances — fridges, pumps, microwaves — draw far more than their running wattage for a moment at startup. That spike can trip an inverter’s output limit even when average wattage looks comfortable. A mini fridge rated at 40–100W running may surge to several times that on compressor startup. The 20% output buffer you’ll see recommended for sizing is a general safety margin; it does not absorb a surge that is two to five times running wattage. If you’re running motor-driven appliances, check the inverter’s peak (surge) output rating against the worst-case startup draw, not just the running watts.

What Common Devices Actually Draw

Running wattages vary enormously — and the range matters for sizing decisions:

These are running wattages — typical values for planning, not measurements. Use your device’s actual label or charger rating wherever you can; the laptop range alone spans 30 to 200W, which is almost useless as a planning number without knowing the specific machine. Add roughly 20% to your summed running watts when sizing a station‘s continuous output — then check the inverter’s surge rating separately if any of your devices have motors or compressors.

Where the Wh Conversion Becomes Non-Optional: Airline Rules

Airline lithium battery rules are written in watt-hours, not milliamp-hours. That makes the mAh-to-Wh conversion a practical necessity, not just a nerdy footnote. The general thresholds:

• Under 100Wh: generally permitted in carry-on without special approval
• 100–160Wh: typically requires airline approval before flying
• Above 160Wh: typically prohibited entirely on passenger aircraft
• All power banks: carry-on only, never checked baggage

These thresholds are standard, but policies vary by carrier and route — confirm with your airline before flying.

The 26,800mAh power bank size you see everywhere isn’t an accident. At 3.7V, that computes to roughly 99Wh — deliberately engineered to land just under the 100Wh line. But at 3.2V (LiFePO4), the same mAh rating is only about 86Wh. This is why travelers can’t rely on mAh to know which side of the threshold they’re on. Always check the Wh figure printed on the unit itself, and use that number when you talk to your carrier. If your bank doesn’t have a Wh rating printed on it, do the conversion — with the right voltage for your chemistry — before you pack it.

The One Number Worth Trusting

mAh tells you charge; Wh tells you energy. When you’re planning runtime, sizing a station, or clearing security at the airport, only Wh is the right unit — and even that is a ceiling, not a promise. For AC loads, plan on roughly 85% of the label; for USB, roughly 90%; for motor appliances, check the surge rating separately before you assume any of it will work. The label is a starting point. The math is the plan.