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To estimate how many full charges a power bank can provide, compare energy in watt-hours (Wh), not the two printed mAh numbers. Convert the power bank and phone batteries to Wh, then account for charging losses:
Estimated full-charge equivalents = (power-bank Wh × overall delivery efficiency) ÷ phone-battery Wh.
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The result is a planning estimate, not a guarantee of that many zero-to-100% charges. Conversion losses, phone use while charging, cable and port limits, temperature, and battery age can all reduce the practical count.
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The quick calculation
If the power bank label already states watt-hours, use that figure. Otherwise calculate its stored energy:
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Power-bank Wh = (rated capacity in mAh ÷ 1,000) × nominal cell voltage.
Calculate the phone’s battery energy in the same way when its Wh rating is unavailable:
Phone-battery Wh = (battery capacity in mAh ÷ 1,000) × nominal battery voltage.
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Full-charge equivalents = (power-bank Wh × overall delivery efficiency) ÷ phone-battery Wh.
For example, a 20,000 mAh bank using a 3.7 V nominal lithium-ion cell stores about 74 Wh. If you use a clearly labelled, illustrative 67% overall delivery assumption, about 49.6 Wh reaches the phone. A phone with a 4,000 mAh battery rated at 3.85 V stores 15.4 Wh, producing an estimate of about 3.2 full-charge equivalents:
(20,000 ÷ 1,000 × 3.7 × 0.67) ÷ (4,000 ÷ 1,000 × 3.85) = 3.22
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That does not mean the phone will necessarily go from empty to full 3.22 times. It means the bank’s estimated usable energy is roughly 3.22 times the phone battery’s stored energy under the assumptions shown.
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Why dividing mAh by mAh gives an inflated answer
Milliamp-hours measure electric charge, not energy. Energy depends on both charge and voltage. A power bank’s advertised capacity normally describes its internal cells at a nominal voltage around 3.7 V, while USB output is commonly 5 V or a negotiated higher voltage. The phone battery has its own nominal voltage. Dividing the printed capacities treats those different voltage bases as if they were identical.
TP-Link describes this as a common misconception: “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 TP-Link Australia’s FAQ, updated October 15, 2014: Understanding the capacity (mAh) and the charge efficiency of a power bank.
Energy conversion creates another difference. The bank must convert the cell voltage into USB output, and the phone must convert incoming power back into the voltage its battery uses. Electronics, cable resistance, heat, and charging overhead consume some energy. A phone that is running a display, mobile radio, navigation, gaming or a hotspot while connected also uses part of the incoming power before it can become stored battery energy.
Find the numbers on both devices
Power-bank information
Look on the bank, its packaging or its technical specification for:
- Rated capacity in mAh.
- Nominal cell voltage, if supplied.
- Stored energy in Wh, which is preferable to an mAh-only label.
- USB output voltage and maximum current or wattage for each port.
- Whether the capacity is a cell rating or a specified output rating.
Most lithium-ion power banks discussed by Anker use a 3.7 V nominal cell voltage. Its support explanation gives the worked conversion of 20,000 mAh at 3.7 V to 74 Wh: Why is the Rated Capacity of Power Banks Lower Than Expected? Treat 3.7 V as the value for a bank whose documentation identifies that nominal cell voltage, not as a universal number for every chemistry or product.
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Phone-battery information
Check the phone maker’s specifications, regulatory label or service documentation for battery capacity and nominal voltage. Some specifications provide Wh directly. If the phone lists only mAh, multiply by its nominal voltage and divide by 1,000. Do not use a charger’s output voltage in place of the battery’s nominal voltage.
If you cannot find the voltage, any resulting figure is less certain. A capacity in mAh by itself cannot establish an exact energy comparison, so label the result as a rough estimate rather than a measured charge count.
Worked calculations
Example 1: Both devices provide Wh
Suppose a bank is labelled 74 Wh and the phone battery is labelled 15.4 Wh. If you choose an illustrative 70% overall delivery efficiency:
(74 Wh × 0.70) ÷ 15.4 Wh = 3.36
The estimate is about 3.4 full-charge equivalents. You may obtain several complete charges if you top up from 20% to 80%, because a “full-charge equivalent” counts the energy of one complete battery capacity, not the number of times a cable is plugged in.
Example 2: Convert mAh ratings first
Assume the power bank is 10,000 mAh at 3.7 V and the phone is 4,500 mAh at 3.85 V:
- Power-bank energy:
(10,000 ÷ 1,000) × 3.7 = 37 Wh. - Phone-battery energy:
(4,500 ÷ 1,000) × 3.85 = 17.325 Wh. - With a 70% overall efficiency assumption:
(37 × 0.70) ÷ 17.325 = 1.50.
The calculation predicts roughly one and a half full-charge equivalents. Dividing 10,000 by 4,500 would suggest 2.22 and therefore overstate the likely result because it ignores the different voltage bases and conversion losses.
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Example 3: The TP-Link illustration
TP-Link’s FAQ uses a 10,400 mAh bank whose cells are rated at 3.7 V. It estimates about 6,926 mAh at 5 V after approximately 90% discharge efficiency at 1 A, and notes that efficiency is lower at 2 A. For a 2,600 mAh phone, TP-Link says phone-side losses mean fewer than 2.4 cycles in the idealized setup it describes. This is an example tied to that product and test conditions, not a promise for every 10,400 mAh bank: TP-Link’s explanation.
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Anker gives illustrative figures of about 82% bank-side conversion efficiency and about 82% device-side conversion efficiency, producing a combined estimate of about 67%. It also describes bank-circuit efficiency of about 80–85%. Those percentages explain why a 74 Wh bank does not normally deliver 74 Wh into a phone battery, but they are not universal guarantees for every port, cable, load or temperature. See Anker’s explanation of unexpected charge counts and its rated-capacity article.
Choosing an efficiency assumption
There is no single percentage that applies to every power bank and phone combination. Your estimate should state which efficiency figure you used and where it came from.
| Efficiency figure | What it describes | How to use it |
|---|---|---|
| About 80–85% | Anker’s description of bank-circuit efficiency. | Useful for understanding losses inside the bank, but it does not represent all energy reaching the phone battery. |
| About 82% bank-side and about 82% device-side; about 67% combined | Anker’s illustrative two-stage example. | Can serve as a clearly labelled planning assumption; it is not a universal test result. |
| About 90% at 1 A | TP-Link’s bank-side discharge example for the TL-PB10400; efficiency is lower at 2 A. | Apply only when explaining that example’s conditions, not to another model automatically. |
| Product-specific formula: 1.3 × rated bank capacity ÷ phone battery capacity | A formula shown by Huawei Support for wired mode on applicable products. | Do not generalize it to every power bank. The support page is How many times can a HUAWEI power bank charge a phone? |
If a manufacturer publishes a measured output energy or a tested charge-count figure for your exact bank and conditions, prefer that information over a generic percentage. Otherwise, calculate a range rather than one falsely precise number. For instance, calculating the same bank at 60%, 67% and 75% efficiency shows how much the assumption controls the answer.
Full-charge equivalents are not the same as charging sessions
A full-charge equivalent represents energy equal to one complete phone-battery capacity. Five sessions that each add 20% consume about one equivalent, although the bank may report five separate charging events. Conversely, charging from 10% to 90% uses roughly 80% of one equivalent, subject to the phone’s charging behavior.
Phones also taper charging near the top of the battery range. The phone may draw less power after reaching a high state of charge, while the bank’s electronics remain active. If you need a practical travel plan, leave margin instead of assuming every estimated equivalent becomes a perfect zero-to-100% cycle.
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Why your actual result may be lower
The phone is being used
Screen brightness, processor load, cellular reception, GPS, video calls and hotspot operation consume power concurrently with charging. The bank can be delivering energy correctly while the phone’s percentage rises slowly because some energy is running the phone rather than filling the battery.
Output power and cable limitations
A bank may have enough stored energy but still charge slowly if its port, cable or phone negotiates a lower power level. Check the output wattage, supported charging standards, connector type and cable rating. A USB-C port does not by itself guarantee a particular charging speed or power-delivery profile.
Battery age and storage conditions
Rechargeable cells lose usable capacity with age and cycling. Heat, cold, prolonged storage at an extreme state of charge and heavy loads can also affect performance. A calculation based on the new label rating therefore tends to be optimistic for an older bank or phone.
Multiple devices connected
When two or more devices share one bank, the same stored energy is divided among them. Add the Wh requirements of the devices you intend to charge and compare that total with the bank’s estimated usable Wh; do not apply the one-phone result to each device independently.
A step-by-step method you can reuse
- Record the bank’s rating. Write down its Wh value, or its mAh value and nominal cell voltage.
- Convert the bank to Wh. Multiply mAh by voltage and divide by 1,000 when Wh is not printed.
- Record the phone’s battery rating. Use Wh if available; otherwise obtain both mAh and nominal voltage.
- Convert the phone to Wh. Use the same mAh-to-Wh calculation.
- Select an efficiency assumption. Use a manufacturer figure only within its stated scope, or show a range such as 60%–75% for planning and label it as an assumption.
- Calculate. Multiply bank Wh by the selected efficiency, then divide by phone Wh.
- Round conservatively. Report a practical whole-number expectation below the mathematical result when travel reliability matters.
- Record the conditions. Note output port, cable, charging current, phone use, temperature and whether another device was connected.
How to compare power banks before buying
Charge-count arithmetic is only one part of choosing a bank. Lenovo’s buying guidance recommends evaluating the specifications against the task: How to Evaluate the Best Battery Power Bank for Different Tasks. Use these comparison points:
| Specification | What to check | Decision it informs |
|---|---|---|
| Energy rating | Wh, or mAh together with nominal cell voltage. | Provides the fairest comparison of stored energy. |
| Output power | Maximum wattage and per-port limits. | Determines whether the bank can charge your phone at the desired speed. |
| Ports and standards | USB-C or USB-A connectors, power-delivery support and simultaneous-output rules. | Shows whether your existing cables and devices are compatible. |
| Portability | Physical size and weight relative to capacity. | Balances extra charge count against what you are willing to carry. |
| Multi-device behavior | How output is divided when several ports are active. | Prevents assuming the single-device output applies while sharing. |
| Documentation | Published output energy, efficiency conditions and safety information. | Indicates how much confidence to place in the label and any stated charge-count claim. |
A larger mAh number is not automatically better if the bank is too heavy for your use or cannot supply the required output power. Conversely, a compact bank may be adequate when you need only one partial top-up rather than several full equivalents.
Troubleshooting an unexpectedly low charge count
Check the calculation inputs
- Confirm that you did not divide bank mAh directly by phone mAh.
- Verify that the voltage belongs to the battery cell, not the USB output.
- Check whether the label’s Wh value is stored energy or an output specification.
- Make sure the phone’s capacity refers to the battery, not a software estimate or a charger rating.
Check the charging setup
- Try a known-good cable rated for the required current and charging standard.
- Use the bank port that supports the phone’s expected input power.
- Test with the screen off and radios or hotspot features minimized to separate phone use from conversion loss.
- Avoid judging capacity from a single short session; note the phone’s starting and ending percentages and whether the bank stopped early.
Check environmental and age factors
- Allow the bank and phone to operate within their specified temperature range.
- Compare an older bank with its original behavior; capacity can decline as cells age.
- If the bank becomes unusually hot, swells or behaves erratically, stop using it and follow the manufacturer’s safety instructions rather than continuing a capacity test.
What the number can and cannot tell you
The estimate is useful for deciding whether a bank is likely to cover a commute, flight, outage or day away from a wall outlet. It lets you compare products on a common energy basis and exposes why two banks with similar mAh labels may deliver different practical results.
It cannot certify battery health, predict an exact percentage on a particular phone, or replace a manufacturer’s test under defined current and temperature conditions. Treat a stated efficiency as an assumption with a scope, carry a margin for real use, and update the estimate when the bank, cable, phone or charging pattern changes.
Quick Recap
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