- Capacity is in mAh, but real charges are fewer than the maths suggests — expect to actually deliver roughly 60–70% of the rated capacity after conversion losses.
- Match the output (watts/PD) to your device. A phone wants ~20–30 W; a laptop needs 60–100 W USB-C Power Delivery.
- Mind the rules and the weight. Airlines cap what you can carry on, and big banks get heavy fast.
A power bank (portable charger) is a battery you carry to top up phones, earbuds, tablets and even laptops. The category is full of inflated capacity claims and confusing wattage labels, so a little understanding saves you from a brick that is too weak, too heavy, or banned from your flight. This guide explains what the numbers really mean — and you can run your own figures in our free battery life calculator.
Step 1: Capacity, and why you get fewer charges than you expect
Capacity is rated in milliamp-hours (mAh), and bigger means more stored energy. But the rated figure is measured at the battery's internal voltage (3.6–3.7 V), while your phone charges at 5 V or more — converting between them wastes energy as heat. In practice a power bank delivers only about 60–70% of its rated mAh to your device. So a "10,000 mAh" bank realistically provides around 6,000–7,000 mAh of usable charge.
To estimate charges: take roughly 65% of the bank's rated capacity and divide by your phone's battery size. A 10,000 mAh bank (~6,500 mAh usable) against a 4,000 mAh phone gives roughly 1.5 full charges. Our battery life calculator does this for you.
Step 2: Output is what makes it useful
Capacity stores energy; output decides how fast and what you can charge. This is where many buyers go wrong:
- Phones: look for USB-C Power Delivery (PD) around 20–30 W for fast top-ups. Most phones cannot use much more than that.
- Tablets: 30–45 W PD charges them at a sensible rate.
- Laptops: you need a high-wattage USB-C PD bank — 60 W for ultrabooks, 100 W for larger laptops. A 5 W bank simply will not charge a laptop meaningfully.
- Ports & total power: check both per-port wattage and the bank's total output if you charge several devices at once — total is often shared, not summed.
Match the output to your most demanding device. A huge-capacity bank with weak output is the classic mismatch — lots of stored energy you can only dribble out slowly.
Step 3: Size, weight and how you will use it
Capacity and weight trade off directly — lithium cells have a fixed energy density, so more mAh always means more grams. Buy for the job, not the biggest number:
- Pocket top-up (5,000–10,000 mAh): light, slips in a pocket or bag; one full phone charge plus a bit. The everyday default.
- Day-out / multi-device (10,000–20,000 mAh): a couple of phone charges or a tablet top-up; still bag-friendly.
- Laptop / weekend / off-grid (20,000–27,000 mAh, 60–100 W): charges a laptop and multiple devices; noticeably heavy.
Step 4: Airline rules and safety
Power banks contain lithium-ion cells, so flights regulate them. Most authorities require power banks in carry-on only (never checked baggage) and limit capacity by watt-hours (Wh): typically up to 100 Wh freely, 100–160 Wh with airline approval, and above 160 Wh not allowed (FAA PackSafe). Wh ≈ (mAh ÷ 1000) × voltage (3.7 V), so a 27,000 mAh bank is roughly 100 Wh — right at the common limit. For safety generally, buy from reputable brands, avoid cheap unbranded cells, and stop using any bank that swells or runs very hot.
Capacities compared
| Rated capacity | ~Usable | Rough phone charges* | Best for |
|---|---|---|---|
| 5,000 mAh | ~3,250 mAh | ~0.7–1 | Slim emergency top-up, earbuds |
| 10,000 mAh | ~6,500 mAh | ~1.5 | Everyday carry — the default |
| 20,000 mAh | ~13,000 mAh | ~3 | Long days, two devices, tablets |
| 26,800 mAh (~99 Wh) | ~17,000 mAh | ~4 | Laptops, travel — near airline cap |
*Assuming a ~4,000 mAh phone and ~65% real-world efficiency. Your numbers vary by phone — check the calculator.
Claims to distrust
Treat eye-catching mAh figures as best-case before losses — assume ~65% reaches your device. Be wary of banks that boast huge capacity but list weak output, vague "fast charge" wording without a stated PD wattage, and unbranded cells with no safety certification. The right buy is a reputable bank whose output matches your devices, in a capacity sized to your day rather than the shelf's biggest number.
See specific picks in the best power banks roundup, and pair charging with the best wireless earbuds or a USB hub. Crunch your own numbers with the battery life calculator.