A power bank’s headline mAh rating usually describes charge stored in its internal cells at about 3.7 volts—not the energy that reaches your phone’s battery. The bank must convert that energy to USB voltage, loses some as heat and electronics overhead, and your phone consumes some while charging. Compare watt-hours (Wh), rated output, voltage, current and test conditions to estimate what you can actually use.
What the mAh number actually measures
Milliampere-hours (mAh) measure electric charge. They do not, by themselves, state how much energy a battery stores or delivers. Energy depends on both charge and voltage, so the useful unit for comparing power banks is watt-hours (Wh).
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Most portable banks contain lithium-ion cells with a nominal voltage around 3.7 V. The large number printed on the case—10,000 mAh, 20,000 mAh or another figure—normally refers to those cells at that internal voltage. USB output is typically 5 V or a higher negotiated voltage. Because the same energy produces fewer ampere-hours at a higher voltage, cell-side mAh cannot be compared directly with a phone’s battery mAh.
Anker Support summarizes the distinction this way: “The mAh (milliampere-hour) value printed on a power bank represents charge capacity, not actual usable energy.” That is why a 20,000 mAh bank should not be expected to provide 20,000 mAh to a 5 V phone.
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Convert the rating to watt-hours first
Use this calculation:
Wh = (mAh ÷ 1,000) × voltage
For a 20,000 mAh bank rated at a nominal 3.7 V:
(20,000 ÷ 1,000) × 3.7 = 74 Wh
The 74 Wh figure describes theoretical energy in the cells. If that energy could be converted perfectly to a 5 V USB output, the equivalent output charge would be:
74 Wh ÷ 5 V × 1,000 = 14,800 mAh
That 14,800 mAh is already lower than 20,000 mAh because it is expressed at a higher voltage. It is still an ideal value: it assumes no conversion loss, no heat loss and no energy used by the bank’s control electronics.
TP-Link gives a model-specific example for its TL-PB10400_V1. The stated 10,400 mAh at 3.7 V equals 38,480 mWh, or approximately 38 Wh. At 5 V, the ideal equivalent is about 7,696 mAh. TP-Link then applies approximately 90% discharge efficiency at a 1 A load and calculates roughly 6,926 mAh of output. The company says efficiency is lower at 2 A. Those figures describe that model and those test conditions, not every power bank.
The three stages between the label and your phone
1. Energy stored in the cells
The internal cells hold chemical energy. Their nominal voltage is used for the advertised mAh calculation, but actual voltage changes during charging and discharging. The printed capacity is therefore a standardized rating, not a promise that every milliamp-hour can be extracted under every load.
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2. Energy delivered at the USB port
A boost or regulation circuit changes the cell voltage to the port’s output voltage. That circuit has resistance, switching losses and control electronics that consume energy. Some energy becomes heat. Output efficiency also changes with current, cable resistance, temperature, battery state and the selected charging profile.
3. Energy stored by the device
Your phone, tablet or other device has another charging stage. Its power-management circuitry converts USB input for the battery, while the device may be running its display, processor, radios, camera or background services. The phone can therefore draw power from the bank without storing all of it in the battery.
These stages must be kept separate. A manufacturer’s output-rated capacity normally describes energy available at the bank’s port. It does not necessarily describe energy that ends up stored in a phone.
Why usable output is lower than the headline rating
- Voltage reference: Cell-side mAh and USB-side mAh refer to different voltages. Wh provides the common basis for comparison.
- Conversion losses: Voltage-conversion electronics consume energy and dissipate heat.
- Load dependence: A bank can be less efficient at a higher current. TP-Link’s example reports about 90% efficiency at 1 A and less than 90% at 2 A.
- Device consumption: A phone that is active while charging may use part of the incoming power immediately.
- Cable and connector losses: Resistance in the cable and contacts reduces the energy arriving at the device.
- Operating conditions: Temperature, battery state, output mode and automatic shutoff thresholds affect how much energy is available.
The result is that “how many phone charges?” cannot be answered from the bank’s mAh number alone. A phone’s own battery capacity is also a cell-side figure, and its voltage, charging efficiency and usage during the session matter.
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Worked estimates from published manufacturer examples
| Example | Cell rating and energy | Output assumption | Estimated result |
|---|---|---|---|
| Anker Support example (page accessed 2026) | 20,000 mAh at 3.7 V = 74 Wh | Ideal 5 V conversion, before losses | 14,800 mAh equivalent at 5 V |
| TP-Link TL-PB10400_V1 (2014 support example) | 10,400 mAh at 3.7 V ≈ 38 Wh | 5 V output; approximately 90% efficiency at 1 A | About 6,926 mAh output; lower efficiency at 2 A |
| Energizer calculation example (page accessed 2026) | 10,000 mAh at 3.7 V = 37 Wh | 5 V output and 85% efficiency | About 6,290 mAh estimated output |
The Energizer formula is equivalent to:
rated output mAh = battery Wh ÷ output voltage × efficiency
Its 37 Wh example divided by 5 V and multiplied by 85% produces approximately 6,290 mAh. This is a calculation example, not a universal efficiency promise.
What independent evidence says about the gap
A 2020 CALCE study summary from the University of Maryland examined two power banks. For those case studies, it reported theoretical available output capacity of 74% to 76% of the advertised capacity and experimental results as much as 40% below the advertised figure. Those percentages apply to the studied units and conditions; they are not a market-wide performance range or a prediction for every current model.
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- 45W Pro Speed Portable Phone Charger: This 45W fast charger boosts an iPhone 17 Pro Max to 76%, a Galaxy S25 Ultra to 84%, or an iPad Pro to 60% in just 30 minutes (charging from 20%). Whether you are a driver needing a quick portable charger for car, a commuter relying on a desk phone charger, or a gamer wanting a seamless Switch battery pack and Steam deck charger portable, you will spend less time tethered to a wall and more time enjoying your day.
- Detachable Cable for Multiple Devices: Unlike a standard portable charger with built in cable that becomes useless if the cord breaks, INIU's 0.4ft detachable braided USB-C cable can be swapped anytime to save your investment. It effortlessly transforms into a versatile travel charger for multiple devices, ensuring your family or group travelers always have a reliable portable charger for all cell phones to keep everyone connected without carrying messy cords.
- Airline-Safe Travel Essentials: This TSA-compliant, flight safe power bank provides nearly 2 full charges for your smartphone, making it the perfect battery pack flight safe for business trips, cruise vacations, and airplane travel essentials. Beyond flights, it empowers campers, hikers, and outdoor users with ultimate camping essentials and road trip essentials—so you can explore freely without battery anxiety, making it ideal gifts for travelers women and men.
- Trusted Emergency Backup: As the SAFE Fast Charge Pro trusted by over 38 million users, we ensure the safest charging with the highest-grade materials. When unexpected storms or hurricanes strike, it acts as a lifeline—providing a dependable power outage battery backup and emergency phone charger for emergency users.
How to read a power-bank specification
Look for Wh or output-rated capacity
A label that states both cell energy and output-rated capacity is easier to interpret. If Wh is printed, use it as the starting point. If only mAh appears, check the voltage used for that number and whether a separate “rated output” figure is provided.
Check each port’s voltage and current
Read the output table for the exact port you will use. A port may support 5 V at one current and a different voltage or power profile when a compatible device negotiates fast charging. The available energy and efficiency can differ between profiles.
Find the test conditions
Look for rated current, cutoff voltage, temperature, cable assumptions and whether the figure is measured at the port or after charging a device. A number without these conditions is difficult to compare fairly with another product.
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Capacity (Wh or mAh) indicates how much energy can be delivered over time. Output power (watts) indicates how quickly it can be delivered. A bank can have substantial capacity but insufficient power for a laptop, or high power with a smaller reserve.
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A practical estimate for your own device
- Record the bank’s energy: Convert its cell rating to Wh, or use the stated Wh value.
- Apply a realistic bank efficiency: Use the manufacturer’s rated output figure or stated efficiency at your intended current. Do not substitute a laboratory or ideal value for a different load.
- Account for device-side use: Expect less battery gain when the screen is on, radios are active or the device is performing intensive work.
- Compare with the device battery: Convert the device’s battery mAh to Wh if its voltage is known. Comparing Wh to Wh avoids mixing voltage references.
- Leave a margin: Automatic shutoff, cable losses and changing load mean a calculation is an estimate, not a guaranteed charge count.
For example, if a bank provides 74 Wh in its cells and the output path is 85% efficient, the energy available at the port is about 62.9 Wh before the phone’s own losses. The device will store less than that, especially if it is being used during charging.
Common misunderstandings and how to avoid them
“The bank is defective because 20,000 mAh did not charge my 4,000 mAh phone five times.”
That comparison mixes cell-side mAh on the bank with the phone’s battery rating and ignores voltage conversion. Convert both to Wh, then account for both devices’ losses and active use.
“A higher mAh number always means more usable energy.”
Only when the voltage reference and test basis are comparable. A lower-mAh product at a higher voltage can contain similar energy, while an inflated cell-side figure can have a modest output-rated capacity.
“Every efficiency percentage is a universal benchmark.”
No. TP-Link’s approximately 90% example is tied to one model and a 1 A load; Energizer’s 85% figure is a calculation assumption. Efficiency varies with conditions.
“Any difference proves deceptive labeling.”
Some of the difference is an expected consequence of voltage conversion and circuit loss. Ambiguous or inaccurate claims can also exist, so compare output-rated specifications and test conditions rather than judging from the headline mAh alone.
Checklist before buying or comparing a bank
- Convert the advertised cell mAh and voltage to Wh.
- Find rated output capacity or output Wh for the port you will use.
- Confirm the output voltage, current and power profiles your device supports.
- Check whether the rating is measured at the port or represents energy stored by a device.
- Read the stated load, temperature and cutoff conditions.
- Expect less stored charge if the device is operating during the session.
- Treat manufacturer examples and published case studies as condition-specific, not guarantees.
The bottom line on advertised capacity
A power bank’s prominent mAh figure is a cell charge rating, usually at about 3.7 V. The bank must convert that energy to USB voltage, and both the bank and the connected device consume part of it. Wh, rated output and test conditions provide a more honest basis for estimating usable energy than mAh alone. Use the label as the first input to a calculation—not as a promise of the number of full phone charges.
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