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A power bank rarely delivers its printed mAh figure to a phone because that number usually describes charge stored in the bank’s internal cells at their nominal voltage. USB output uses a different voltage, and the bank’s conversion electronics consume energy and produce heat. The phone then loses additional energy in its own charging circuitry and may use power while charging.
For a meaningful estimate, convert the bank’s cell rating to watt-hours (Wh), apply the stated output efficiency under its test load, and treat the result as energy available at the USB port—not necessarily energy stored in the phone battery.
What the advertised mAh number actually measures
Milliampere-hours (mAh) measure electric charge. They do not, by themselves, say how much energy a battery contains, because energy also depends on voltage. Watt-hours (Wh) measure energy and include both quantities:
Wh = Ah × V
For mAh ratings, use:
Wh = (mAh ÷ 1,000) × voltage
The large capacity number printed on a power bank normally refers to its internal lithium-ion cells at a nominal voltage, commonly about 3.7 V. A phone does not receive that cell-side charge directly. The bank must convert it to the voltage used by its USB port, so the same amount of energy appears as fewer mAh at the higher output voltage.
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Anker Support summarizes the distinction this way: the mAh value on a power bank represents charge capacity, not actual usable energy. That is why comparing a bank’s 20,000mAh label directly with a phone’s 5,000mAh battery can produce an unrealistic charge-count estimate.
The three stages between the label and your phone
1. Energy stored in the internal cells
The first figure is the chemical energy held by the cells. A 20,000mAh bank rated at 3.7 V contains approximately:
(20,000 ÷ 1,000) × 3.7 = 74 Wh
That 74 Wh is a cell-level calculation. It is not a measurement of energy that can be delivered from a USB socket, and it does not include the bank’s electronics.
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2. Energy delivered at the USB port
The bank’s converter raises or regulates the cell voltage for USB output. The converter itself uses some energy, and resistive losses release some as heat. The output therefore contains less energy than the cells started with. Efficiency also changes with operating conditions such as output current.
At an ideal, lossless 5 V output, the 74 Wh example would correspond to:
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74 Wh ÷ 5 V = 14.8 Ah = 14,800 mAh
That is already 26% lower than the printed 20,000mAh, before real conversion losses are included. The difference is a voltage conversion issue, not automatically evidence that the label is false.
3. Energy ultimately stored by the device
The phone has its own power-management and charging circuitry. Those circuits consume part of the incoming energy. The phone can also use energy for its screen, processor, radios and background activity while it is connected. Consequently, energy measured at the bank’s USB port will exceed the energy that ends up stored in the phone’s battery.
A charge estimate that does not distinguish these stages cannot promise a fixed number of full phone charges.
Worked calculations from published examples
The following examples show how the same capacity label changes when voltage and efficiency are made explicit. The figures are manufacturer calculations for the stated models or scenarios, not universal performance guarantees.
| Example | Cell rating and energy | Ideal 5 V output charge | Stated efficiency and estimated output |
|---|---|---|---|
| 20,000mAh example | 20,000mAh at 3.7 V = 74 Wh | 74 Wh ÷ 5 V = 14,800mAh | No measured efficiency supplied in the cited Anker example; 14,800mAh is an ideal pre-loss figure. |
| TP-Link TL-PB10400_V1 | 10,400mAh at 3.7 V = 38,480mWh, approximately 38 Wh | 38.48 Wh ÷ 5 V ≈ 7,696mAh | TP-Link gives about 90% discharge efficiency at 1A, producing approximately 6,926mAh; it says efficiency is lower at 2A. |
| Energizer calculation example | 10,000mAh at 3.7 V = 37 Wh | 37 Wh ÷ 5 V = 7,400mAh | Using 85% efficiency gives about 6,290mAh at 5 V. |
The TP-Link and Energizer figures illustrate why two banks with similar printed capacities can have different usable output. Their estimates use different efficiencies and loads. The 6,926mAh and 6,290mAh figures describe charge available at a 5 V output under the assumptions in each manufacturer’s example; they do not describe charge stored in a particular phone.
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Why usable output is lower than the label
The voltage reference changes
Cell-side mAh and USB-side mAh refer to different voltages. Increasing voltage while preserving the same energy necessarily reduces the corresponding ampere-hour figure. A bank can therefore show 20,000mAh at about 3.7 V and legitimately provide a smaller mAh number at 5 V even before losses.
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Boost and regulation circuits are not ideal. Their switching components and other circuitry draw power, while electrical resistance produces heat. The percentage that reaches the USB port depends on the design and operating point.
Output current affects efficiency
Efficiency is not one fixed property of every bank. TP-Link’s TL-PB10400_V1 example specifies approximately 90% discharge efficiency at a 1A load and says the efficiency is below 90% at 2A. A higher-current device or a different output profile can therefore produce a different result from a low-current test.
The phone consumes part of the incoming energy
Even if a meter reports a particular amount at the USB port, the phone’s charging circuitry takes a share. A phone that is being used while charging also spends energy at the same time. Screen-on use, wireless connections, processor activity and background tasks can reduce the net increase in battery percentage.
Published capacity can differ from measured capacity
A 2020 summary from the University of Maryland’s CALCE describes two studied power banks whose theoretical available output capacity was 74–76% of their advertised capacity. In those examples, experimental measurements were as much as 40% below the advertised figure. Those percentages apply to the study’s two case-study products and conditions; they are not a universal failure rate for every current power bank.
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How to estimate what your bank can really provide
- Find the cell-side rating and voltage. Use the printed mAh and the nominal cell voltage if the label provides it. If the label gives Wh, use that directly.
- Convert to cell energy. Calculate
(mAh ÷ 1,000) × V. For example, 10,000mAh at 3.7 V equals 37 Wh. - Identify the output you will use. Note the port’s voltage and current profile. A 5 V, 1A estimate is not interchangeable with a 5 V, 2A estimate or another fast-charge profile.
- Convert energy to output charge. Divide Wh by the output voltage. The 37 Wh example becomes 7,400mAh at an ideal 5 V output.
- Apply a stated efficiency figure. If the manufacturer supplies an efficiency for your load, multiply by it. At 85%, 7,400mAh becomes approximately 6,290mAh.
- Allow for the phone’s own losses and use. The result is still energy at the bank’s port. The phone will store less, especially if it is active during charging.
- Compare energy, not just charge counts. To estimate a phone charge, compare the bank’s output energy with the phone battery’s watt-hours, while recognizing that the phone-side conversion and usage losses are not represented by the bank’s printed mAh.
If no efficiency or output-rated capacity is published, you can calculate an ideal upper bound but not a reliable delivered figure. Present the result as a range or qualification rather than promising two, three or four full charges.
How to read a power-bank label or product page
Use the following checklist before comparing models. The most useful specification is the one that states energy and output conditions together.
| Specification to check | What it tells you | Why it matters |
|---|---|---|
| Cell mAh and nominal voltage | Charge stored by the internal cells and the voltage used for that rating | Lets you calculate cell-level Wh and avoid comparing unlike mAh figures. |
| Cell energy in Wh | Energy represented by the internal battery | Wh is the clearest starting point for cross-voltage comparisons. |
| Rated output capacity or output energy | What the manufacturer expects at a specified port | It is more relevant to a connected device than the headline cell mAh. |
| Output voltage, current and power profiles | The electrical conditions under which output is available | Efficiency and usable charge can change with current and charging mode. |
| Load and cutoff conditions | The current, endpoint voltage and other limits used for the rating | Without these conditions, two capacity figures may not be comparable. |
| Whether the figure is bank output or device-stored energy | The stage represented by the claim | A USB-output estimate will be higher than the amount ultimately added to a phone battery. |
A label that states both cell energy and output-rated capacity is easier to interpret than one that presents only a large mAh number. When comparing products, inspect Wh first, then the rated output at the port you intend to use, supported voltage and current profiles, and the manufacturer’s test conditions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What capacity standards and studies can—and cannot—tell you
The CALCE summary says UL Standard 2056 instructs measurement at output ports at rated current, 20°C and specified cutoff conditions. The current text of that standard was not independently verified here, so treat this as the way the 2020 research summary describes the standard rather than as a fresh certification claim about every product.
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Manufacturer examples are similarly bounded. The TP-Link efficiency figure is for the TL-PB10400_V1 and named loads. The Energizer 85% calculation is an example for a 10,000mAh, 3.7 V battery at 5 V output. Anker’s 74 Wh calculation demonstrates the mAh-to-Wh conversion, not a laboratory measurement of USB delivery. Use each figure for the conditions stated, not as a category-wide benchmark.
When a result looks unexpectedly low
The apparent shortfall is close to the voltage-and-efficiency estimate
If a 10,000mAh, 3.7 V bank is expected to provide roughly 6,290mAh at 5 V using the stated 85% efficiency, a much smaller phone-side increase can be normal once the phone’s charging losses and active use are included. Compare your expectation with output-rated energy rather than the headline cell mAh.
The bank is being used at a different load
Check whether the device is drawing more current than the published example. TP-Link specifically reports lower than 90% efficiency at 2A compared with approximately 90% at 1A. A different current or charging profile invalidates a simple one-number comparison.
The comparison uses two mAh figures at different voltages
Recalculate both ratings in Wh. A phone’s battery mAh and the bank’s cell mAh may each be accurate while representing different voltage references. Wh puts them on a common energy basis.
The claim does not identify its measurement stage
Look for wording such as cell capacity, rated output capacity, output energy, load current and cutoff. If a listing supplies only a nominal mAh number and no output conditions, the available evidence cannot establish how much energy will reach your particular device.
The practical rule for choosing and comparing capacity
Use the printed mAh number as a description of the internal cells, not as a promise of phone battery charge. Start with Wh, verify the output-rated figure and supported output profile, and read the load and cutoff conditions. Then account for the phone’s own conversion losses and whether it will be running while charging.
This approach explains ordinary differences without assuming deception: voltage references and conversion losses are real. At the same time, ambiguous or inaccurate output claims can make a product appear to deliver more than it does. The most transparent specification is an output-based capacity or energy rating with its voltage, current and test conditions stated.
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