Power bank boxes are written to impress, not to inform. Capacity is printed in milliamp-hours (mAh), your phone and laptop actually use energy measured in watt-hours (Wh), and the conversion between the two hides a loss that nobody puts on the packaging. This guide gives you the one formula you need, the real-world discount to apply, and worked examples for a weekend trip, a long-haul flight and a week of camping.
The only formula you need
Watt-hours = (mAh × 3.7) ÷ 1000.
The 3.7 is the nominal voltage of the lithium-ion cells inside almost every consumer power bank. Because every bank uses the same cell chemistry, this gives you an honest baseline for comparing any two products, whatever the box says.
- 10,000 mAh ≈ 37 Wh
- 20,000 mAh ≈ 74 Wh
- 26,800 mAh ≈ 99 Wh (the largest size most airlines allow without approval)
Some manufacturers now print the Wh figure directly on the label. When they do, trust it over the mAh number, because it already accounts for the cell voltage.
Then subtract the conversion loss
The cells inside store energy at about 3.7 V, but USB outputs run at 5 V, 9 V, 15 V or 20 V. Stepping the voltage up turns part of the stored energy into heat, and the cable and your device’s own charging circuit lose a little more. In practice, plan on getting 60–70% of the rated capacity into your device.
So a 20,000 mAh (74 Wh) bank realistically delivers about 45–50 Wh. Cheap no-name banks can be worse, and some overstate their capacity entirely. If a “30,000 mAh” bank is suspiciously light and cheap, assume it is closer to 15,000 mAh until proven otherwise.
What your devices actually use
Look for the battery capacity in your device’s specifications (often listed in Wh for laptops, and in mAh for phones, which you can convert with the formula above). Typical figures:
| Device | Battery (approx.) | Energy for one full charge from empty* |
|---|---|---|
| Smartphone | 12–19 Wh | 15–22 Wh |
| Wireless earbuds + case | 1–2 Wh | 2 Wh |
| Tablet | 25–40 Wh | 30–45 Wh |
| Ultrabook laptop | 50–60 Wh | 60–70 Wh |
| Larger laptop | 70–100 Wh | 80–115 Wh |
| GoPro-style action camera battery | 6 Wh | 7 Wh |
| Headlamp (USB rechargeable) | 3–6 Wh | 4–7 Wh |
*Includes charging losses inside the device itself.
This is why a pocket bank sold as “charges laptops” often delivers less than one full laptop charge: 45–50 usable Wh against a laptop that needs 60–70 Wh.
A three-step sizing method
- List what needs charging and how often. For example: phone once a day, earbuds every two days, headlamp once.
- Add up the Wh for the whole time you are away from a wall socket. Use the right-hand column of the table above.
- Divide by 0.65 to cover conversion loss, then round up to the next common size. That number is the rated Wh you need on the box.
Worked examples
Weekend city break (2 days, sockets at night)
You only need to get through each day. Phone use while navigating and taking photos might drain one full charge by late afternoon: about 20 Wh. Divide by 0.65 and you get roughly 31 Wh, so a 10,000 mAh (37 Wh) bank is plenty and keeps weight down to around 200 g.
Long-haul flight with a layover (24 hours door to door)
Phone twice (40 Wh) plus a tablet once (40 Wh) is 80 Wh, divided by 0.65 is about 123 Wh. That is above the 100 Wh airline threshold, so the sensible answer is one 20,000 mAh bank plus in-seat or airport power, rather than one oversized bank. Remember power banks must go in carry-on luggage, never in checked bags.
Four nights camping, no electricity
Phone kept in airplane mode most of the day (half a charge per day: 4 × 10 = 40 Wh), headlamp twice (14 Wh), earbuds once (2 Wh). Total 56 Wh, divided by 0.65 is about 86 Wh. A 26,800 mAh bank covers it, or a 20,000 mAh bank plus a small folding solar panel for top-ups. For longer trips or bigger loads like a CPAP or a cooler, step up to a portable power station instead; we compare those in our portable power station rankings.
Charging speed matters too
Capacity tells you how much; output watts tell you how fast. A phone happily charges at 18–25 W, but a laptop usually needs at least 45 W over USB-C Power Delivery (PD), and many need 60 W or more. A bank with a 20 W port will keep a laptop alive only very slowly, and may not charge it at all while it is in use. Check three things on the specification sheet:
- USB-C PD output in watts (45 W minimum for most laptops).
- Total output when several ports are used at once. Many banks share one budget, so two devices charge more slowly than one.
- Recharge input. A 20,000 mAh bank with only 18 W input takes five hours or more to refill. With 30–65 W input it can be full in under two hours, which matters on a short layover.
Flying with power banks
Most airlines follow the same pattern: banks up to 100 Wh are allowed in cabin baggage without approval, banks between 100 and 160 Wh need airline approval (usually a limit of two), and anything above 160 Wh is not allowed. Several airlines now also ban using or charging power banks during the flight, so check your carrier’s current rules before you go. A label that clearly shows the Wh figure saves arguments at security.
Common mistakes
- Buying on mAh alone. Compare Wh, output watts and weight together.
- Forgetting the cold. Lithium cells lose a noticeable part of their usable capacity near freezing. On winter trips keep the bank inside your jacket or sleeping bag.
- Storing it empty. For long storage, keep the bank around half charged and top it up every few months.
- Ignoring weight. A 26,800 mAh bank weighs about 500 g. For day hikes a 10,000 mAh model is usually the better trade-off.
Quick reference
| Trip | Suggested size |
|---|---|
| Daily carry, phone only | 5,000–10,000 mAh |
| Weekend away, phone + earbuds | 10,000 mAh |
| Travel day with tablet or laptop | 20,000 mAh with 45 W+ USB-C PD |
| 3–5 days off-grid, phone + lights | 20,000–26,800 mAh, or add a small solar panel |
| Longer off-grid, bigger devices | Portable power station |
FAQ
Why does my 10,000 mAh bank only charge my phone twice?
Because 10,000 mAh is about 37 Wh before losses, roughly 24 Wh after them, and a modern phone uses 15–20 Wh per full charge. Two charges is exactly what the maths predicts.
Is a higher mAh number always better?
No. Beyond what you will use between sockets, extra capacity is just extra weight and cost. Buy for your realistic trip, not the biggest number.
Can I recharge a power bank with solar?
Yes, with a folding USB panel, but expect slow results in anything other than direct sun. Our portable solar panel comparison explains what output to expect.
Ready to buy? See our best power banks under $50, scored with the method described on our how we review page.