How to Check Capacity of Power Bank: Rated vs Real Output

Learning how to check the capacity of a power bank reveals a simple truth: a charger almost always delivers less than the number printed on the case. The good news is that you can measure the real figure yourself. This guide walks through the tools, the formulas, and what your results actually mean, so you can judge any power bank with confidence.

Quick Summary

Short on time? Here is the essence before we get into the methods.

• Rated capacity is usually based on the cell’s nominal voltage, commonly around 3.7 V or 3.85 V. Power banks then convert this energy to 5 V or higher for charging devices, which reduces the usable mAh.
• Expect roughly 60% to 75% of the rated mAh as usable output, depending on voltage conversion efficiency and load conditions.
• A USB power meter is the easiest accurate tool; a multimeter works with more effort.
• Compare watt-hours (Wh), not just mAh, for a fair measurement.
• No tools to hand? Multiply the rated capacity by about 0.65 for a quick estimate.

Why Do Power Banks Deliver Less Than Advertised?

Before measuring, it helps to understand where the missing capacity goes.

Voltage Conversion and Energy Loss

The biggest reason is a change in voltage.

A power bank stores energy at around 3.7 V, then converts it to 5 V or higher for device charging. This conversion process causes some energy loss as heat. Because the rated mAh is quoted at 3.7 V, the mAh delivered at 5 V is naturally lower, even before other losses.

Conversion Efficiency Rates

The electronics inside add a second, smaller loss.

Good power banks convert energy at roughly 80% to 90% efficiency, with some premium models achieving higher efficiency. Voltage conversion changes the way capacity is measured, while conversion losses reduce the actual energy delivered. Together, these factors mean the usable output is typically lower than the rated capacity.

A 10,000 mAh power bank stores around 37 Wh of energy. After conversion losses, it typically provides around 30 to 35 Wh of usable energy, which is roughly equivalent to 6,000 to 7,000 mAh at a 5 V output. Knowing how to check the capacity of a power bank lets you see this gap for yourself rather than guessing.

Rated capacity	Rated Wh (3.7 V)	Typical usable output (at 5 V)
5,000 mAh	~18.5 Wh	~3,000 to 3,500 mAh
10,000 mAh	~37 Wh	~6,000 to 7,000 mAh
20,000 mAh	~74 Wh	~11,000 to 13,000 mAh
25,000 mAh	~92.5 Wh	~15,000 to 16,000 mAh

How to Check Battery Capacity of a Power Bank Using a USB Power Meter

This is the most reliable home method, and the meter costs very little.

A USB power meter sits between the power bank and the device being charged, then records the total mAh and Wh delivered. Here is how to check battery capacity of power bank units step by step:

1. Charge the power bank fully to 100%.
2. Plug the USB power meter into the power bank's output port.
3. Connect a load, such as a phone, tablet or a USB resistive load, to the meter.
4. Allow the power bank to discharge fully while the meter records the cumulative output.
5. Read the total mAh and Wh recorded when the power bank switches off.

Because the meter measures output at around 5 V, compare the measured Wh value with the rated Wh value for the most accurate result. Watt-hours account for voltage differences, making them a better way to compare the real energy delivered by a power bank.

Alternative Method: Using a Multimeter

If you already own a multimeter, you can approximate capacity with a little more effort.

A multimeter only shows voltage and current at a specific moment. It cannot directly measure accumulated capacity, so it needs to be combined with a controlled load. Measure the average discharge current and the total discharge time. Then estimate the output capacity using average current × discharge time × 1,000. It works, but a USB power meter is quicker and more accurate for most people.

How to Estimate Capacity Without Specialist Tools

No meter? A quick calculation gets you close enough for everyday decisions.

• For a quick estimate with typical 3.7 V power banks, multiply the rated capacity by around 0.6 to 0.7. A 20,000 mAh power bank, for example, gives roughly 13,000 mAh of usable output.
• Alternatively, estimate usable capacity by multiplying your device’s battery size by the number of full charges, while allowing for charging losses.
• Check the printed Wh rating, then divide by 5 and multiply by 1,000 to estimate usable mAh at 5 V.

Factors That Affect Measurement Accuracy

Several variables can nudge your reading up or down, so keep them in mind.

• Cable quality and length: a thin or long cable adds resistance and loses energy.
• Load level: very high or very low draw changes efficiency.
• Temperature: extreme heat or cold reduces measured output.
• Meter accuracy: budget meters carry a small margin of error.
• Battery age: older cells hold less than when new.

How to Interpret Your Results

Once you have a figure, here is how to judge whether it is healthy.

At the pack level, a measured value of roughly 75% to 95% of the rated Wh may indicate normal performance, depending on test conditions. When converted to a 5 V output, usable capacity is often around 60% to 70% of the rated mAh due to voltage conversion and energy losses.

If your power bank falls well short of that, the cells may be ageing, or a very low-cost, unbranded unit may simply be mislabelled. A quality power bank should maintain performance reasonably well over its expected service life.

Tips to Maintain and Prolong Power Bank Battery Life

A few habits keep your measured capacity high for longer.

• Keep the charge between about 20% and 80% for daily use, and store it near 50% if unused for a long spell.
• Avoid full, repeated deep discharges, which age lithium cells faster.
• Use quality cables and chargers to limit heat.
• UK tip: Avoid leaving a power bank in an unheated car or garage during winter. Also keep it away from direct heat, such as a sunny windowsill. Moderate temperatures help lithium batteries last longer. Recycle an old power bank at a UK battery collection point rather than the household bin.

Recommended Anker Power Banks for Different Capacity

Whatever capacity you need, accurate ratings and efficient conversion matter most. Below are three Anker high capacity power banks designed for different needs, from everyday phone charging to high-power laptop use.

Model	Rated capacity	Approx. Wh	Usable output*	Max output
Anker MagGo Power Bank (10K, 35W)	10,000 mAh	~37 Wh	~6,000 to 7,000 mAh	35W
Anker Prime Power Bank (20K, 220W)	20,100 mAh	72.36Wh	~11,250 mAh	220W
Anker Prime Power Bank (26K, 300W)	26,250 mAh	99.75Wh	~15,200 mAh	300W

Usable output after voltage conversion and energy loss, based on Anker testing.

Best for Everyday Carry: Anker MagGo Power Bank (10K, 35W)

The Anker MagGo Power Bank (10K, 35W) is designed for users who prioritise portability and convenience. It is a practical option for commuters, travellers and Apple users. Its compact design reduces the number of accessories they need to carry.

• 35W maximum output supports fast charging for compatible phones and tablets.
• Built-in USB-C cable reduces the need to carry an extra charging cable.
• An integrated 5W Apple Watch wireless charger supports compatible Apple Watch models.
• The USB-C port and built-in cable support up to 30W output for devices such as iPhone, iPad and compatible Android phones.

Best for Laptop Users and Work Travel: Anker Prime Power Bank (20K, 220W)

The Anker Prime Power Bank (20K, 220W) is designed for professionals and frequent travellers. It provides enough power for laptops, tablets and phones while remaining portable.

• 220W maximum total output supports charging multiple devices simultaneously.
• One USB-C port delivers up to 140W output for compatible laptops.
• USB-C input supports up to 100W recharging, reducing downtime between uses.
• Includes USB-C and USB-A ports for charging laptops, tablets, phones and accessories together.
• Smart display and app support show charging status and power distribution information.

Best for Maximum Power and Multi-Device Charging: Anker Prime Power Bank (26K, 300W)

The Anker Prime Power Bank (26K, 300W) is built for users who need maximum capacity and higher charging performance. It is suited to creators, business users and long-distance travellers who carry high-power devices and need longer battery support.

• 300W maximum total output supports simultaneous charging of multiple high-power devices.
• Single USB-C port delivers up to 140W output for compatible laptops.
• Two USB-C ports and one USB-A port allow charging up to three devices directly.
• Supports up to 250W combined USB-C input for faster recharging when using compatible chargers.
• 99.75Wh battery capacity is designed to stay within the common 100Wh airline carry-on battery limit.

Conclusion

With the methods and the maths in hand, the label no longer has any secrets.

In short, how to check the capacity of a power bank output comes down to measuring real delivery, not trusting the printed number alone. A USB power meter gives the most accurate reading. A multimeter or a simple 0.65 estimate can help when tools are unavailable. Comparing watt-hours also provides a fairer comparison. Every Anker power bank here is rated according to its stated specifications and designed for efficient power conversion, so what you measure closely matches what you paid for.

FAQ

What is the actual capacity of a 20000mAh power bank?

After voltage conversion and energy losses, a 20,000 mAh power bank typically provides around 40 to 50 Wh of usable energy, depending on efficiency. The Anker Prime Power Bank (20K, 220W), rated at 20,100 mAh, provides about 11,250 mAh in practice. The rest is lost as heat during the step-up from 3.7 V to 5 V.

Is 100Wh the same as 10000mAh?

No. Watt-hours and milliamp-hours measure different things. To convert, use Wh = mAh x voltage / 1,000. At 3.7 V, a 10,000 mAh power bank equals about 37 Wh, not 100 Wh. A 100 Wh power bank would be roughly 27,000 mAh at the same voltage, so always note the voltage.

Do I need 10000mAh or 20,000mAh power bank?

It depends on your day. A 10,000 mAh power bank gives about one to two phone charges, which suits daily top-ups. A 20,000 mAh power bank offers three to four charges and can help with tablets or a light laptop boost, making it the better pick for travel and longer days away from the mains.