How many phone charges a power bank gives, and how to estimate it

A power bank gives a phone roughly as many full charges as its stored energy allows, after losses. The method is short: convert the bank to watt-hours, multiply by an efficiency, then divide by the energy in one full phone battery. The answer is a count, and it is only as good as the efficiency and the phone figure you put in. The worked figures below use a 10,000 mAh bank and three efficiency scenarios, then replace the assumptions with measured values.

The method in one line

Full charges are the bank’s delivered energy divided by the phone’s battery energy:

full charges ≈ (bank Wh × efficiency) ÷ phone battery Wh
phone battery Wh = phone mAh × phone volts ÷ 1000

The bank’s Wh comes from milliamp-hours and cell voltage, as set out in the mAh to Wh conversion. The efficiency covers everything between the bank’s cell and the phone’s battery: the bank’s voltage conversion, the cable, and the phone’s charging circuit. The phone battery energy is the value a phone’s specification gives, converted in the same way.

The efficiency is the least certain number in the chain. The power budget tool defaults to 80% and calls it an assumed end-to-end figure. Real efficiency depends on the bank, the cable, the charge rate and the phone, so the figures below use a range rather than one claimed value.

What a phone battery holds

Phone specifications often give the battery in milliamp-hours, and the voltage of a lithium-ion phone cell is not always printed. The example below uses one hypothetical battery, not a claim about any particular model:

4,500 mAh × 3.85 V ÷ 1000 = 17.325 Wh, rounded to 17.3 Wh

Replace that with your phone’s figures. If the specification gives only mAh, use the voltage the manufacturer states, and if it gives neither, a watt-hour value from the manufacturer is better than a guess. The divisor drives the count, so a phone battery 20% larger (1 ÷ 1.2 = 0.83) gives about 17% fewer charges from the same bank.

How many full charges a 10,000 mAh bank gives

A 10,000 mAh bank at a 3.7 V cell stores 10 × 3.7 = 37.0 Wh. The table applies the two efficiency scenarios to that figure and divides by the 17.3 Wh example phone battery.

The spread is the useful part. Between the most and least favourable scenario the count moves from 1.7 to 1.1, a difference of about 0.6 of a charge. For a trip plan, plan on the lower figure and treat anything above it as a margin.

80%: 74.0 × 0.80 = 59.2 Wh   ÷ 17.3 = 3.42 charges, about 3.4
60%: 74.0 × 0.60 = 44.4 Wh   ÷ 17.3 = 2.57 charges, about 2.6

Note that the count is not the bank’s capacity divided by the phone’s capacity. The phone’s 4,500 mAh battery is smaller than the bank’s 10,000 mAh, but the count is still under two because the bank’s cell voltage and the efficiency both reduce the energy that arrives. Comparing counts across banks is safe when the phone and efficiency stay the same.

What a partial top-up gives

Most trips involve top-ups rather than full charges. A top-up from 20% to 80% draws 60% of the phone battery’s energy, because the state of charge rises by 60 percentage points:

top-up 20% → 80% = 0.60 × 17.3 Wh = 10.38 Wh, about 10.4 Wh

At 80% efficiency the 10,000 mAh bank delivers 29.6 Wh, so it covers 29.6 ÷ 10.38 = 2.85 of those top-ups, which is about 2.9. The count is an average, because a phone does not take a top-up at the same rate at every level of charge, and the screen and radios draw different amounts. Treat the count as a planning figure rather than a schedule.

The same arithmetic shows why small top-ups are easy to underestimate. Each top-up looks small on the phone’s own display, but the sum of a day’s top-ups, multiplied by the number of days, is what the bank has to supply.

Measuring your own numbers

A USB power meter placed between the bank and the phone shows the energy that leaves the bank and reaches the phone. It is the most direct way to replace the efficiency assumption. Record the watt-hours at the start and end of a full charge, and record the phone’s percentage change over the same period. The ratio of the two gives the phone battery energy as the meter sees it, which includes charging losses.

Use the measurement with care. A meter reading from one charge, on one cable, from one bank, applies to that set-up only. Change any of the three and the efficiency may change. Run the measurement once with the cable and bank you intend to carry.

The power budget tool also takes a daily device load in watts, which is a different question from charge counting. A 2 W average for 3 hours a day over 3 days gives a demand of 2 × 3 × 3 = 18 Wh. That number is an example input, not a claim about any phone. The tool then compares it with the bank’s usable energy, which is 29.6 Wh at 80%, so the balance without solar is 29.6 − 18 = 11.6 Wh. The 18 Wh demand is about 1.04 times the phone’s 17.3 Wh battery (18 ÷ 17.3), so the trip needs roughly one full phone charge of energy.

Planning a multi-day trip

For a trip, decide three things first: the phone’s daily use in watts and hours, the number of days, and the efficiency you will assume. Then run the power budget with those values. If the balance is negative, the bank alone does not cover the trip, and you need either more stored energy or a reduced load.

Keep the margin visible. The tool’s balance has no reserve built in, and it does not account for a cold morning, a bank that has been left in a hot car, or a phone that is used more than planned. The batteries in cold weather guide covers the cold case in detail. Solar recharging is covered separately in the solar panel guide, and the offline trip planning checklist places batteries alongside the other planning items.

Water treatment can share the same battery budget. A UV device draws from a cell as well, and it competes with the phone for the same energy. The UV water treatment guide covers when that choice makes sense.

Using the estimate in the field

Check the bank’s charge level and the phone’s before you leave for each day, and note the difference. If the phone is drawing faster than planned, recalculate the day’s demand using the measured watts rather than the planned figure. A simple log of the bank’s and phone’s percentages each morning gives you the real rate faster than any calculation.

Carry a way to reach help that does not depend on the bank. A charged phone is a convenience on a trip, and the model above is for planning, not for deciding when the phone is essential. Use the official guidance for emergency communication in the area you visit, and treat the battery as a budget that can run out.