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To estimate charge count, convert both batteries to watt-hours (Wh), then account for charging losses: full-charge equivalents = power-bank Wh × overall efficiency ÷ phone-battery Wh. Dividing the printed mAh figures alone can substantially overstate the result because the two ratings may use different voltages.
What the number means
A charge-count estimate is the number of full-charge equivalents a power bank contains. For example, an estimate of 3.3 means the bank has enough usable energy for about three complete zero-to-100% battery fills plus roughly 30% of another, assuming the stated conditions. It does not promise three uninterrupted charging sessions or a fixed number of charges for every phone.
The result is an energy estimate, not a laboratory measurement. Conversion losses, the phone’s activity, battery age and the bank’s operating conditions can all reduce the energy that reaches the phone battery.
Why dividing mAh by mAh gives the wrong answer
Milliamp-hours (mAh) measure electrical charge, not energy by themselves. Energy depends on voltage as well as charge. A power bank’s internal lithium-ion cells are commonly rated at a nominal 3.7 volts; its USB output is normally converted to a different voltage before it reaches the cable. The phone then converts that incoming power again for its battery.
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TP-Link describes the problem directly: One common misconception is that we can simply divide the capacity of a battery by the capacity of a smartphone/tablet to figure out how many cycles we can use this power bank to charge the smartphone/tablet. However, that is not the correct algorithm.
The statement appears in its support FAQ, updated October 15, 2014: Understanding the capacity (mAh) and the charge efficiency of a power bank.
For instance, dividing a 20,000 mAh bank by a 5,000 mAh phone battery suggests four charges before allowing for voltage conversion or any losses. The energy-based method may produce a materially smaller figure.
The two energy values you need
Power-bank energy
If the label gives watt-hours, use that number. If it gives only mAh, convert it with:
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchPower-bank Wh = (rated capacity in mAh ÷ 1,000) × nominal cell voltage in volts
Anker’s support documentation uses 3.7 V as the nominal cell voltage for most of the lithium-ion power banks it discusses. Its worked example converts 20,000 mAh at 3.7 V to 74 Wh: (20,000 ÷ 1,000) × 3.7 = 74 Wh. Treat 3.7 V as the voltage stated or supported for the particular bank, not as a universal value for every battery chemistry or product. See Anker’s explanation of rated power-bank capacity.
Phone-battery energy
Use the phone manufacturer’s Wh rating when it is available. Otherwise convert the phone’s mAh and nominal battery voltage in the same way:
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Phone Wh = (phone capacity in mAh ÷ 1,000) × nominal battery voltage in volts
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Do not substitute the USB output voltage for the phone’s internal battery voltage. If the phone label gives mAh but no nominal voltage, the calculation is incomplete; look for a Wh specification or a manufacturer-stated nominal voltage instead of silently assuming one.
Step-by-step calculation
- Record the bank’s rated capacity. Write down its Wh rating, or its mAh rating and nominal cell voltage. Capacity printed on a bank is a rated value under the maker’s specified conditions.
- Convert the bank to Wh. Divide mAh by 1,000 and multiply by nominal volts. Keep the voltage attached to the number in your notes.
- Record the phone’s battery energy. Use Wh directly, or convert its mAh and nominal voltage with the same equation.
- Choose an overall efficiency assumption. This must represent the complete path from the bank’s cells to energy stored in the phone, not merely one circuit. If the maker publishes a product-specific figure, use it with its conditions. Otherwise calculate a range rather than presenting one percentage as certain.
- Apply the full-charge-equivalent formula. Multiply bank Wh by the chosen overall efficiency, then divide by phone Wh.
- Label the output as an estimate. Round sensibly; a decimal such as 3.3 is useful for planning, but it is not a guarantee that the phone will complete exactly three charges.
Worked example: a 20,000 mAh bank
1. Convert the bank capacity
Using the 3.7 V nominal cell voltage in Anker’s example:
(20,000 ÷ 1,000) × 3.7 = 74 Wh
That 74 Wh is stored energy in the cells, before the bank’s output conversion and the phone’s charging circuitry consume energy.
2. Compare several efficiency assumptions
Anker gives illustrative figures of about 82% bank-side conversion efficiency and about 82% device-side conversion efficiency in one explanation. Multiplying those illustrative values gives roughly 67% overall (0.82 × 0.82 = 0.6724). The same documentation also describes bank-circuit efficiency of about 80–85%. These are explanatory examples, not a universal specification. The table shows how much the assumption changes the estimate for a phone with a 15 Wh battery.
| Overall efficiency assumption | Estimated energy reaching phone | Full-charge equivalents for a 15 Wh phone |
|---|---|---|
| 60% | 44.4 Wh | 2.96 |
| 67% (Anker’s illustrative combined example, rounded) | 49.6 Wh | 3.31 |
| 80% | 59.2 Wh | 3.95 |
At the 67% illustrative assumption, the calculation is 74 × 0.67 ÷ 15 = 3.3 full-charge equivalents. The 15 Wh phone value in this example is an explicit hypothetical input; replace it with your phone’s stated battery energy.
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3. Understand the range
The table is not saying that every bank performs at exactly 60%, 67% or 80%. It demonstrates why a single answer based only on the 20,000 mAh label is misleading. A bank’s efficiency can vary with output current, temperature, cable and port, and the phone’s charging behavior. Use the maker’s measured or stated conditions when they exist; otherwise present a range and identify it as an estimate.
Manufacturer example: why output ratings differ from cell ratings
TP-Link’s TL-PB10400 explanation provides a concrete, product-specific example. Starting with a 10,400 mAh, 3.7 V cell rating, it describes about 90% discharge efficiency at a 1 A load and estimates 6,926 mAh available at 5 V. TP-Link notes that efficiency is lower at 2 A. It then explains that phone-side losses mean fewer than 2.4 cycles for a 2,600 mAh phone under the idealized setup described in that FAQ.
Those figures should not be transplanted unchanged to another bank. The 90% value is bank-side performance for the stated current and product example; it is not the total energy-storage efficiency of every power bank. The phone-cycle statement includes additional losses and the conditions TP-Link specifies. Read the complete explanation at TP-Link’s capacity and charge-efficiency FAQ.
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When the manufacturer supplies a complete figure
Use a documented overall-efficiency or delivered-capacity figure only when the manufacturer identifies what it includes and the test conditions. Record the output current, voltage, port and any stated temperature or device assumptions. A bank-side number alone cannot answer how much energy the phone battery will receive.
When only partial figures are available
Keep the stages separate. For example, an 82% bank-conversion figure and an 82% device-conversion figure can be multiplied as an illustrative estimate, but neither number guarantees the result for a different model. Do not combine a bank-only figure from one product with a phone-side figure from another source and describe the product as tested.
When no figure is published
Calculate a planning range using clearly labeled assumptions, such as 60%, 67% and 80%, and show how the result changes. A range is more honest than a precise decimal based on an unverified percentage. The output should remain a planning estimate, not a promise.
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Factors that reduce the number you actually see
- Voltage conversion: The bank converts its cell voltage to USB output, and the phone converts incoming power for its battery. Each conversion has losses.
- Phone use while charging: Screen-on time, radios, navigation, gaming and background work consume some incoming energy before it can increase the battery’s stored energy. Anker and TP-Link both explain why device-side consumption lowers the apparent charge count.
- Charging current: Efficiency can change with load. TP-Link specifically says its example is less efficient at 2 A than at 1 A.
- Battery age and use: A bank’s available capacity changes as it ages and as it is used. The printed rating describes the product under its rating conditions, not an unchanged capacity forever.
- Partial charges: Two sessions that each add 50% to the phone represent approximately one full-charge equivalent, but losses and phone use still apply to each session.
- Multiple devices: If the bank charges a phone and another device at the same time, the available energy is divided between them; the phone’s count cannot be calculated as if it received the whole bank.
Reading power-bank labels correctly
| Label item | What it tells you | How it affects the estimate |
|---|---|---|
| Rated capacity (mAh) | Charge capacity of the cells under the maker’s rating conditions | Must be paired with nominal cell voltage to derive Wh |
| Energy (Wh) | Direct energy rating, when provided | Use it instead of converting mAh |
| Output voltage and current | Electrical conditions available at a port | Helps assess compatibility and load-dependent efficiency; it is not the phone’s internal battery voltage |
| Output power (W) | Maximum or supported charging power | Affects charging speed and potentially efficiency, not the stored Wh by itself |
| Ports and connector type | Which cables and charging modes are supported | Determines whether the bank can deliver power to your phone as intended |
When comparing products, prioritize a stated Wh value or mAh value paired with nominal voltage, then check output power, ports, cable compatibility, portability and whether you will charge one device or several. Lenovo’s buyer guidance discusses these comparison dimensions at How to evaluate a battery power bank for different tasks. Specifications and labels can change, so use the current label for the product you are considering.
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A Huawei support result states a wired-mode formula of 1.3 × rated power-bank capacity ÷ phone battery capacity for applicable Huawei products. That is a product-specific method, not a general law of power banks. The factor incorporates assumptions chosen by that manufacturer and should not be applied to another bank unless its documentation says to do so. See the stated context at Huawei support.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Quick worksheet
Copy these lines into a note or spreadsheet:
- Bank capacity: ____ mAh
- Bank nominal cell voltage: ____ V
- Bank energy: (____ ÷ 1,000) × ____ = ____ Wh
- Phone capacity: ____ mAh or ____ Wh
- Phone nominal voltage (if converting): ____ V
- Phone energy: (____ ÷ 1,000) × ____ = ____ Wh
- Overall efficiency assumption: ____%
- Estimated full-charge equivalents: bank Wh × (efficiency ÷ 100) ÷ phone Wh = ____
If either battery’s voltage or Wh value is missing, mark the result as incomplete rather than filling the gap with the USB voltage or an arbitrary efficiency.
Troubleshooting a lower-than-expected charge count
The bank’s mAh ratio looked much higher
Recalculate in Wh and include an efficiency assumption. The raw ratio ignored voltage differences and conversion losses, so it was an upper-bound-style comparison rather than an energy estimate.
The bank’s own documentation quotes a different number
Check whether its number refers to cell capacity, output capacity at a particular voltage, or complete phone charging. Also check current, port and device conditions. Bank-side output efficiency and energy stored in the phone are different measurements.
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Results vary between phones
Use each phone’s own Wh value. Different battery energies, charging circuits and patterns of use produce different full-charge equivalents from the same bank.
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The estimate was accurate when the bank was new but is lower now
Capacity changes with age and use. Recalculate only as an estimate; a label rating does not guarantee the bank still stores its original energy.
The phone was being used heavily
Repeat the comparison with the phone idle or lightly used if you want a cleaner planning figure. During active use, part of the bank’s output powers the phone directly instead of being stored in the battery.
How to report your answer responsibly
A useful answer includes the inputs and assumptions: “This bank stores approximately 74 Wh from its 20,000 mAh, 3.7 V rating. At an assumed 67% overall delivery efficiency and a 15 Wh phone battery, that is about 3.3 full-charge equivalents.” It should also identify whether the efficiency is a manufacturer illustration, a product-specific measurement or a planning assumption.
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Bottom line
Use watt-hours, not a simple mAh division. Convert the bank and phone to Wh, apply an efficiency assumption that matches the documented conditions, and label the result as an estimate. Check the bank’s current output, ports and cable compatibility, and expect fewer equivalents when the phone is in use or the bank has aged.
Quick Recap
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