Why does my 20,000mAh power bank only charge my phone 3-4 times instead of 5?

The mAh on the label is measured inside the cells, and the energy is converted twice before it reaches your phone's battery. Anker puts each conversion at about 82%, which multiplies out to roughly 67% end to end. A 20,000mAh bank against a 4,000mAh Galaxy S24 therefore returns about 3.4 charges, not the five the raw division suggests.

Can I use a power bank to charge a laptop?

Yes, if the bank has a USB-C Power Delivery output your laptop accepts. Apple ships a 70W adapter with the 14-inch MacBook Pro and states that fast charging needs a 96W or higher USB PD power source, so a small bank charges it slowly or only while it sleeps. On energy alone a 20,000mAh (74Wh) bank returns about 0.7 of a charge to that laptop's 72.4Wh battery, which makes it a top-up rather than a spare charger. Look for 25,000mAh or larger with a high-wattage PD output if you want more than that.

Does fast charging drain a power bank faster?

Fast charging moves the same energy, it just moves it in less time. The trade-off is heat: both the bank and the device run warmer, and a warmer converter returns less of the stored energy, so the charge count slips a little. No manufacturer publishes a figure for how much, so treat it as a small loss rather than a number you can plan around.

Power Bank Runtime Calculator — mAh, Wh & USB-C PD

5 min read

Power bank labels quote capacity in mAh, and that number is measured inside the cells rather than at the USB port. Anker prints about 82% efficiency for the conversion out of a power bank, and about 67% once your phone converts that power again into its own battery (0.82 × 0.82). That is why a 20,000mAh bank returns roughly three and a half charges to a 4,000mAh phone instead of five. This power bank runtime calculator applies those printed figures to your own numbers. The arithmetic underneath, usable Wh divided by device watts, is the same general battery runtime formula used for any battery; what is specific to power banks is the boost-conversion loss this calculator factors in.

Portable power bank capacity stats showing 20000mAh, 74Wh energy, and phone charge count.

Charges by Device Battery Size

Anker publishes the method for this and works an example with it: multiply the bank's capacity by the combined efficiency, then divide by the device battery. Both capacities have to be in the same unit.

Device battery	Capacity its maker prints	Charges from a 20,000mAh bank
Anker's own worked-example phone	3,200mAh	4.2 charges
Samsung Galaxy S24	4,000mAh	3.4 charges
Samsung Galaxy S24 Ultra	5,000mAh	2.7 charges
MacBook Pro 14-inch	72.4Wh	0.7 charges

Every capacity above is printed by the company named in its row. Anker's example runs smaller: a 10,000mAh bank against that 3,200mAh phone gives (10,000 × 0.67) / 3,200 = 2.1 charges, and the same arithmetic against the Galaxy S24's larger battery gives 1.7. Anker's own product pages round that combined figure to 0.65 in places, which takes about 3% off every count above. Where a battery is quoted in mAh the comparison assumes both packs sit at a similar cell voltage, which is how Anker works its own example. Your own count lands lower on a cold day or through a thin cable.

Power bank runtime chart showing hours delivered from a 20,000 mAh bank across loads from 2 W to 45 W. — A 20,000mAh bank holds 74Wh; at the 82% conversion efficiency Anker prints, about 61Wh reaches the USB port.

How to Find Your Power Bank's Real Capacity

Find the mAh rating on the label. This is the cell-level capacity at the internal voltage (usually 3.7V). A "20,000mAh" bank stores 74Wh (20,000 x 3.7 / 1000).
Check for a Wh rating. Some manufacturers list the real energy in watt-hours. This is a more honest spec. Airlines require Wh for lithium battery limits (100Wh carry-on, 160Wh with airline approval).
Apply the conversion loss. Anker prints about 82% efficiency for the step out of the bank, so a 74Wh pack delivers about 61Wh at the USB port. The rest leaves as heat.
Divide by your device battery, in the same unit. Anker's published example divides mAh by mAh: (10,000mAh × 0.67) / 3,200mAh = 2.1 charges. The 0.67 is the two conversions multiplied together, one in the bank and one in the phone.

Worked Examples

How Many Phone Charges from a 20,000mAh Power Bank?

Context

Your power bank is rated 20,000 mAh at 3.7V. The phone is a Galaxy S24, which Samsung rates at 4,000 mAh. You want the real number of full charges.

Calculation

Combined efficiency: 0.82 x 0.82 = 0.67, the two conversion steps Anker prints

Charges: (20,000 x 0.67) / 4,000 = 3.4

Energy side: 20,000 x 3.7 / 1,000 = 74 Wh; 74 x 0.82 = 60.7 Wh usable, which the tile rounds to 61

Interpretation

You get about three and a half charges, not the five you would get by dividing 20,000 by 4,000. Two conversion steps take the difference, one inside the bank and one inside the phone.

Takeaway

For longer trips where you need to track total energy across power banks and batteries, convert everything to watt-hours using our Wh to Ah calculator.

Running a USB-C Laptop from a Power Bank

Context

Your laptop draws 45W over USB-C. You have a 20,000 mAh / 74 Wh power bank. How long will it hold the laptop up?

Calculation

Usable energy: 74 x 0.82 = 60.7 Wh

Runtime: 60.7 / 45 = 1.3 hours

Interpretation

About 1 hour and 21 minutes of active laptop use. The charge-count tile hides itself at this load: 45W is past the 25W Samsung prints for the Galaxy S24, so counting phone charges would say nothing useful about a laptop.

Takeaway

For all-day laptop power off-grid, you need a proper battery bank, not a USB power bank. Check our laptop battery runtime calculator to find the right capacity for your usage pattern.

Frequently Asked Questions

Glossary

Milliampere-Hours

A unit of charge capacity equal to 1/1000 of an amp-hour. Power banks use mAh ratings because the numbers sound larger — 20,000 mAh is more marketable than 20 Ah, even though they are the same capacity.

Conversion Efficiency

The share of stored energy that survives the step-up to the USB output. Anker prints about 82% for that step and the same again inside the device, so roughly 67% of the energy in the cells reaches a phone battery. That is the gap between the mAh on the label and the charges you actually get.

Voltage Boost

The converter that lifts the pack's cell voltage up to the voltage the USB output has to supply. Every boost costs energy as heat, which is where the 82% figure comes from. Higher output voltages need more boost, so a laptop-class port is not free either.

Curious how long a solar panel takes to refill your power bank? Check the solar battery charge time calculator.

Power bank marketing quotes cell-level mAh, which is why the headline number never survives the trip to your device. Anker's own product page puts the end-to-end loss at 30% to 45% of rated capacity, so expect 55% to 70% of the headline mAh to reach whatever you are charging. Convert to watt-hours for any honest comparison between packs. For larger portable batteries used on e-bikes, the same Wh-based math applies — see our e-bike battery range calculator for range estimates that factor in terrain and assist level.

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Last updated: September 22, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. He works professionally as Commercial Director at Ezoic Inc., leading revenue strategy across digital publishing.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.

Methodology reviewed by Doc. dr. sc. Damir Topić, Assistant Professor, FERIT Osijek.