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

The mAh rating is measured at the cell voltage of 3.7V, not at the 5V USB output. Converting from 3.7V to 5V wastes 10-20% of the energy as heat. A 20,000mAh bank at 3.7V holds 74Wh but delivers roughly 60Wh at the USB port. A 4,000mAh phone battery (about 15.4Wh) needs roughly 15Wh per charge, giving you about 3.9 full charges — not 5.

Can I use a power bank to charge a laptop?

Yes, if the power bank supports USB-C Power Delivery at 45W or higher. Most laptops need at least 30W to charge while in use, and 45-65W for full-speed charging. A 20,000mAh (74Wh) power bank gives a 50Wh laptop about one partial charge at best. For serious laptop charging, look for power banks in the 25,000-30,000mAh range with 65W+ PD output.

Does fast charging drain a power bank faster?

Fast charging delivers the same total energy to your device — it just does it quicker. The trade-off: higher charging speeds generate more heat in both the power bank and your device, and heat reduces conversion efficiency by a few percent. You might get 3-5% fewer total charge cycles from a fast-charging power bank compared to slow-charging at 5V/2A, but the time savings usually outweigh this small loss.

Power Bank Runtime Calculator — Free

4 min read

Power bank labels show capacity in mAh, but that number is misleading. A 20,000mAh power bank does not charge a 4,000mAh phone five times. Voltage conversion and heat losses eat 15-25% of the rated capacity. This calculator accounts for real-world efficiency to give you an honest runtime estimate.

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

Typical Device Power Draw

Device	Typical Draw	Full Charges from 20,000mAh
iPhone 15 (3,349mAh)	5-10W	~4 charges
Samsung Galaxy S24 (4,000mAh)	5-15W	~3.5 charges
iPad Air (28.6Wh)	10-20W	~2 charges
Nintendo Switch (16Wh)	8-18W	~3.5 charges
AirPods Pro case (0.52Wh)	1-2W	~100+ charges
GoPro Hero 12 (6.4Wh)	3-5W	~9 charges
Laptop (USB-C, 50Wh)	30-65W	~1 partial charge

"Full charges" assumes 85% conversion efficiency from the power bank's 3.7V internal cells to the 5V USB output. Actual results vary by cable quality and ambient temperature.

Power bank capacity comparison chart showing runtime for phones, tablets, laptops, and cameras. — A 20,000mAh power bank lasts 31 hours for a Bluetooth speaker but barely over an hour for a laptop.

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. When the internal 3.7V is boosted to 5V USB, you lose 10-20% to heat. A 74Wh bank delivers roughly 59-67Wh at the USB port. For USB-C PD at 20V, losses are similar because the boost ratio is higher.
Divide by your device battery. Your phone battery in Wh = mAh x voltage / 1000. Most phones run at 3.85-3.87V internally. A 4,000mAh phone battery is about 15.4Wh.

Worked Examples

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

Context

Your power bank is rated 20,000 mAh at 3.7V. Your phone battery is 4,000 mAh at 3.7V (about 15 Wh). You want to know the real number of full charges.

Calculation

Power bank energy: 20,000 x 3.7 / 1,000 = 74 Wh

After 85% conversion loss: 74 x 0.85 = 62.9 Wh usable

Charges: 62.9 / 15 = 4.2 full charges

Interpretation

You get about 4 full phone charges, not the 5 you might expect from dividing 20,000 by 4,000. The 15% conversion loss at the voltage boost step eats roughly one full charge worth of energy.

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 via USB-C. You have a 20,000 mAh / 74 Wh power bank rated for 65W PD output. How long will it keep the laptop running?

Calculation

Usable energy: 74 x 0.80 = 59.2 Wh (laptop charging has ~80% efficiency)

Runtime: 59.2 / 45 = 1.3 hours

Interpretation

About 1 hour and 18 minutes of active laptop use. That might bridge a flight layover, but it will not replace your charger for a full workday.

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

Power banks store energy at 3.7V but output at 5V (USB-A) or up to 20V (USB-C PD). The voltage conversion wastes 10-20% of the stored energy as heat, which is why actual runtime is always less than the raw mAh suggests.

Voltage Boost

The process of converting the power bank's internal 3.7V cell voltage up to the output voltage (5V, 9V, or 20V). Higher output voltages require more boost, which increases conversion losses.

Curious how long a solar panel takes to refill your power bank? Check the <a href="/solar/solar-battery-charge-time-calculator">solar battery charge time calculator</a>.

Power bank marketing overstates real capacity by 20-40% because they quote cell-level mAh without accounting for conversion losses. Always convert to watt-hours for honest comparisons, and expect about 60-70% of the headline mAh to reach your device in practice. 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.

Last updated: March 20, 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.