Power bank charge counts depend on usable watt-hours, your device battery size, and efficiency losses.
Wondering how many phone refills you’ll get from a portable battery? The answer comes from a little math and a few real-life factors like cable quality, voltage steps, and wear.
How Many Phone Recharges From A Power Bank — Easy Formula
Start with energy, not just milliamp-hours. Battery makers rate capacity in milliamp-hours (mAh), but energy is measured in watt-hours (Wh). The two are linked by the well-known formula: Wh = mAh × V ÷ 1000. Most single-cell banks use a nominal cell voltage near 3.7V, while your phone charges at higher negotiated voltages over USB-C. That conversion introduces losses, so only part of the stored energy reaches the device.
Rule of thumb: Estimated recharges ≈ (power bank Wh × efficiency) ÷ device Wh. A conservative efficiency for modern USB-C banks is 70–85% in everyday use. For a quick mental check, use 0.75 as the multiplier.
Quick Reference Table: Common Bank Sizes And Phone Refills
This table uses 75% efficiency and a 15.2Wh phone battery (about 4100mAh at 3.7V). Treat it as a baseline; your device and cables may vary.
| Bank Size (mAh / Wh) | Usable Wh (×0.75) | Approx Refills (4100mAh phone) |
|---|---|---|
| 5,000 mAh / ~18.5 Wh | ~13.9 Wh | ~0.9× |
| 10,000 mAh / ~37 Wh | ~27.8 Wh | ~1.8× |
| 20,000 mAh / ~74 Wh | ~55.5 Wh | ~3.6× |
| 26,800 mAh / ~99 Wh | ~74.3 Wh | ~4.9× |
| 30,000 mAh / ~111 Wh | ~83.3 Wh | ~5.5× |
Why Energy Beats Plain mAh
A milliamp-hour without voltage doesn’t tell you the whole story. Two batteries with the same mAh but different voltages hold different energy. Power banks store energy in 3.6–3.7V cells, then step it up to the negotiated USB-C level. Your phone then steps it down again, and each step sheds a little heat.
That’s why Wh is the clean yardstick for comparing banks, and why “10,000mAh” on the box does not translate line-for-line into two and a half phone refills. You only get the Wh that survives conversion and cable losses.
The Three Things That Decide Your Real Refill Count
1) Device Battery Size
Find your device’s battery energy. Phones sit near 11–19Wh, small tablets often 20–30Wh, large tablets 30–45Wh, earbuds cases 1–3Wh, and watches even less. If the maker lists only mAh, convert to Wh with the same formula. Keep this number handy; you’ll use it in every estimate.
2) Efficiency From Port To Cell
Losses happen at three spots: boost converter inside the bank, cable resistance, and the phone’s charge circuit. USB Power Delivery raises voltage to move power with lower current, which helps cables waste less, but the voltage steps still shed heat. Real-world round-trip efficiency usually lands between 70% and 85% for mainstream gear and decent cables.
3) Discharge Rate And Temperature
High-wattage fast charging isn’t “free.” Pushing 25W or 45W can shave a few percentage points off efficiency compared with a gentle 10–15W flow. Cold weather also trims usable capacity. If you’re counting on every last refill, keep the bank close to room temperature and use a solid, short cable.
Step-By-Step: Turn Specs Into A Solid Estimate
- Convert the bank’s rating to Wh. If the label gives only mAh, multiply by 3.7V and divide by 1000 (see the mAh-to-Wh formula).
- Pick an efficiency. Use 0.75 for day-to-day planning, 0.85 for best-case with great cables and modest speeds, 0.70 for harsh conditions.
- Find your device’s Wh. Example: a 5000mAh phone battery at 3.85V is ~19.3Wh.
- Calculate recharges. Refills ≈ (bank Wh × efficiency) ÷ device Wh.
Worked Examples You Can Trust
Example A: 10,000mAh bank → mid-size phone (4100mAh). Bank Wh ≈ 37.0. Usable at 75% ≈ 27.8Wh. Phone ≈ 15.2Wh. Recharges ≈ 27.8 ÷ 15.2 ≈ 1.8×.
Example B: 20,000mAh bank → large phone (5000mAh). Bank Wh ≈ 74.0. Usable at 75% ≈ 55.5Wh. Phone ≈ 19.3Wh. Recharges ≈ 55.5 ÷ 19.3 ≈ 2.9×.
Example C: 26,800mAh bank → small tablet (28Wh). Bank Wh ≈ 99.2. Usable at 75% ≈ 74.4Wh. Recharges ≈ 74.4 ÷ 28 ≈ 2.7×.
Example D: 5,000mAh bank → earbuds case (600mAh at 3.7V ≈ 2.2Wh). Bank Wh ≈ 18.5. Usable ≈ 13.9Wh. Recharges ≈ 13.9 ÷ 2.2 ≈ 6.3×.
How USB-C Charging Affects The Math
USB-C Power Delivery negotiates a voltage and current level before power flows. At higher voltages (9V, 15V, 20V), current drops for the same wattage, which reduces cable loss. Banks and phones step voltages up and down internally, and those conversions add a bit of heat that you never see. That’s the reason two setups with the same capacities can land at different refill counts.
Use a certified USB-C cable, avoid daisy-chain adapters, and stop fast charging once you hit the percent you need.
Travel Note: Airline Limits For Spare Batteries
Airlines restrict portable batteries by energy (see IATA passenger lithium battery rules). Units up to 100Wh generally fly in carry-on bags without special approval; 100–160Wh often require permission; bigger units are usually not allowed. Pack them in the cabin, tape off any exposed contacts, and avoid damaged gear.
Picking The Right Size For Your Use
Daily commute: A slim 5000–10,000mAh bank gives one to two phone refills without weighing you down.
Weekend trip: A 20,000mAh class unit covers two to three phone refills or one full tablet top-up.
Cold weather: Capacity drops in the cold. Start a size higher than you think you need and keep the pack warm.
Estimate Builder: Match Your Bank To Your Device
Pick the row that matches your device energy, then read across for refills at three common bank sizes. The math assumes 75% efficiency.
| Device Energy (Wh) | 10,000mAh Bank | 20,000mAh Bank |
|---|---|---|
| 12 Wh (small phone) | ~2.1× | ~4.6× |
| 15 Wh (mid phone) | ~1.9× | ~3.7× |
| 19 Wh (big phone) | ~1.5× | ~2.9× |
| 28 Wh (small tablet) | ~1.0× | ~2.0× |
| 35 Wh (large tablet) | ~0.8× | ~1.6× |
| 2.2 Wh (earbuds case) | ~11.8× | ~23.6× |
How To Read A Spec Label Without Guesswork
Cell Voltage And Rated mAh
The printed mAh is quoted at the cell’s native voltage, typically around 3.7V. When a brand lists “10,000mAh 37Wh,” it’s already showing you the converted energy. If only mAh appears, do the conversion yourself so every bank can be compared on an apples-to-apples basis.
Ports, Power Profiles, And Efficiency
Multiple ports share the same internal energy pool. If two phones pull power at once, the total conversion loss usually rises a little. Fast-charge logos tell you the top wattage, not the runtime. A 45W logo looks good, but it’s the Wh that earns you refills.
Cycle Life And Aging
Lithium cells fade with time and use. A two-year-old pack may deliver 10–20% less than day one. If you rely on a bank for long trips, plan a buffer or replace it before it becomes a headache.
Make Your Bank Go Further
- Charge the bank fully, then top it off the night before you travel.
- Use short, thick USB-C cables to cut resistance.
- Avoid charging and using the phone hard at the same time; screen-on drains steal energy from the refill.
- Fast-charge to the percentage you need, then switch to a slower port for better efficiency.
- Keep packs and phones warm in winter and shaded in summer.
Bring It All Together
If you remember one workflow, make it this: convert to Wh, multiply by a realistic efficiency, divide by your device’s Wh, and plan with a small buffer. This gives you a dependable answer before you step out the door or head for the airport.
Sources and further reading used for formulas and travel rules: lithium battery passenger guidance from IATA and the standard Wh↔mAh conversion used by battery makers.