How to Check Power Bank Capacity and Test Real mAh Output

You buy a 20,000mAh power bank before a long road trip out of Seattle. The box promises huge capacity. After charging your phone twice, though, the bank is nearly empty, and you are left wondering where all those mAh went. I have asked that same question myself, and learning how to check power bank capacity properly cleared up the confusion for good. The number printed on a power bank is not always the same as usable USB output. mAh measures electrical charge, while Wh measures actual energy, and voltage conversion creates real losses along the way. The easiest practical test uses a USB power meter paired with a controlled load.

Table of Contents

A multimeter can help too, but it is not the simplest option for most readers. This guide walks through both how to read the rated capacity on the label and how to test the real capacity yourself, so you never feel misled by a spec sheet again. Anker explains that power banks commonly use cells around 3.6 to 3.7V, while USB output may be 5V or higher, so energy gets lost during that voltage conversion step.

What Does Power Bank Capacity Actually Mean?

Before testing a power bank, you need to know what the number on the label actually measures. This prevents the classic complaint of buying a 20,000mAh power bank that seemingly only gives you 14,000mAh in real use.

What Is mAh?

mAh stands for milliamp hours. It measures electrical charge, not energy directly. One thousand mAh equals 1Ah. This unit is the most common way manufacturers describe battery capacity on packaging and product listings.

What Is Wh?

Wh stands for watt hours. It represents actual energy rather than raw charge. Wh accounts for both capacity and voltage together, which makes it a more complete number for comparing devices with different voltages.

Why Wh Can Be More Useful Than mAh

The formula looks like this: Wh equals mAh divided by 1,000, multiplied by voltage. For example, a 20,000mAh power bank at 3.7V works out to 74Wh. Anker gives this same basic conversion for a 20,000mAh battery rated at 3.7V, which confirms the math holds up across the industry. If you want a model with clearly published Wh figures, this power bank listing lists both mAh and Wh upfront.

Rated Capacity vs Usable Capacity

There is a real difference between cell level capacity, rated output capacity, and actual delivered energy. Conversion losses sit between these numbers, which is why the label rarely matches what reaches your phone.

The Big Mistake to Avoid

Never compare a power bank’s mAh directly against a phone’s battery mAh without factoring in voltage. Doing this leads people to expect far more charges than the physics actually allows, similar to misunderstandings covered in our guide on smartphone specifications explained simply.

Where Can You Find a Power Bank’s Capacity?

You may not need a tester at all if you only want the manufacturer’s rated capacity. Start with the label itself before reaching for any tools.

Check the Back of the Power Bank

Look for numbers like 5,000mAh, 10,000mAh, 20,000mAh, or 25,000mAh printed directly on the casing. You will often see the terms rated capacity and battery capacity listed separately, and they do not always mean the exact same thing.

Check the Manual, Website, and Packaging

The product manual usually lists the exact rated capacity along with voltage and output specifications. The manufacturer’s website often has more detail than the box itself, especially for older models. The original packaging can also help if the printed label on the unit has worn off over time.

Find the Model Number

The model number matters because specifications can differ between versions of what looks like the same product. Two units sold under a similar name can carry different cells, different output ratings, and different real world performance, which is why checking this detail protects you from confusion later.

How to Check Power Bank Capacity Without Any Tools

If you only need the advertised capacity, this takes less than a minute. You cannot prove the real capacity this way, but it works as a useful first check before you commit to more testing.

Step 1 to 3: Find the Rating and Convert It

First, find the mAh rating printed on the label. Second, find the voltage, usually listed as 3.6V or 3.7V for the cells. Third, convert mAh to Wh using this formula: Wh equals mAh multiplied by voltage, divided by 1,000.

Step 4: Compare the Specification With the Product Listing

Check for a label mismatch between what is printed and what the online listing shows. Watch for a different model number, a different regional specification, or confusing wording that mixes up rated capacity with battery capacity.

Step 5: Don’t Treat the Label as a Real World Output Test

This is an important distinction. The label tells you what the manufacturer claims, not what you will actually get from a full discharge. For that, you need real testing, which we cover next. If you want a dependable unit with accurate labeling from the start, this power bank on Amazon is a good example of clear, honest specifications.

How to Check the Real Capacity of a Power Bank

This is the test most readers really want. To measure usable output, you need to discharge the power bank through a known load and measure exactly what comes out the other end.

The Easiest Method: Use a USB Power Meter

A USB power meter sits between the power bank and the load, recording voltage, current, mAh, sometimes Wh, charging time, and peak current as the test runs. Anker’s practical testing guide recommends using a USB power meter and a controlled load to measure output capacity accurately.

What You Need

You need a fully charged power bank, a USB power meter, a USB load tester or another suitable constant load, a correct USB cable, and a notebook or phone for recording your results as you go. This starter power bank kit works well if you are testing and shopping at the same time.

Why a USB Load Tester Is Better Than a Phone

Phones change their charging current throughout a session. Phones stop charging once they hit 100 percent. Phone temperature affects charging speed and can skew your readings. Screen use changes the load unpredictably. Battery management systems inside the phone can interfere with test accuracy in ways you cannot control. A controlled load removes these variables entirely, giving you a cleaner, more repeatable result every time.

How to Test Power Bank Capacity With a USB Power Meter

This is the main hands on section. The steps stay simple enough for a beginner while remaining precise enough to produce genuinely useful results.

Step 1: Fully Charge the Power Bank

Charge the unit to 100 percent using the recommended charger. Let charging finish completely and naturally rather than stopping early. Note the starting condition of the unit before you begin testing.

Step 2 and 3: Connect the Meter and Load

The basic setup runs power bank, then USB meter, then load, in that exact order. For the load itself, prefer a dedicated USB load tester, an electronic load, or another stable USB device that draws consistent current throughout the test.

Step 4 and 5: Run the Discharge Test

Start the discharge and record voltage, current, mAh, Wh, and test duration as it runs. Continue until the power bank stops output on its own, since many units shut down automatically once they hit a low voltage cutoff point built into their protection circuitry.

Step 6 and 7: Record and Repeat

Write down the final reading once the test ends. Then repeat the test two or three times rather than trusting a single reading, since small variations in cable, temperature, or load can shift the number slightly each run. Battery University describes controlled charge and discharge testing as the dependable way to determine real battery capacity, which matches this exact approach.

How to Calculate Power Bank Capacity From Test Results

Once the meter gives you a number, the math itself is small. The tricky part is knowing whether you are looking at raw mAh or actual energy in Wh.

If Your Meter Shows mAh

Use the meter’s cumulative output reading directly. For example, a power bank rated at 20,000mAh might show a tested output of 15,800mAh once the discharge test finishes.

Calculate the Percentage of Rated Capacity

The formula is simple: tested capacity divided by rated capacity, multiplied by 100. Using the example above, 15,800 divided by 20,000 times 100 equals 79 percent.

If Your Meter Shows Wh

Use Wh as the better energy comparison whenever your meter offers it, since it accounts for voltage changes that a raw mAh number ignores.

Converting Wh to mAh

The formula runs like this: mAh equals Wh divided by voltage, multiplied by 1,000. Make sure the voltage you use matches the exact point being measured, either the cell side or the USB output side.

Don’t Mix Cell-Side and USB-Side Measurements

This is an important technical clarification. Mixing these two measurement points produces numbers that look inconsistent or even impossible, which can make a perfectly normal power bank look defective when it is not.

Why Does a 20,000mAh Power Bank Deliver Less Than 20,000mAh?

This section can prevent readers from wrongly accusing a perfectly normal power bank of being fake or defective, when the real explanation is simple physics.

Voltage Conversion Causes Energy Loss

Battery cells operate around 3.6 to 3.7V internally. USB output, though, may run at 5V or higher. Boost conversion between these two voltage levels is never 100 percent efficient, and that gap accounts for a large share of the difference you see.

Heat, Cables, Devices, and Load

Heat is one form of energy loss that happens naturally during conversion. Cable and connector quality also causes losses, especially with thin or damaged cables. Device charging losses occur inside your phone too, not just inside the power bank. Different loads change efficiency, and temperature during testing can affect your final results as well. Anker explains that conversion, cable losses, heat, and load can all affect real world output together, not as separate isolated factors.

Power Bank Capacity Test Example: 10,000mAh

A worked example makes these numbers much easier to understand. Here is what a realistic test might look like for a common 10,000mAh unit.

Label Information and Test Result

The label shows a rated capacity of 10,000mAh, a cell voltage of 3.7V, and an approximate stored energy of 37Wh. A USB meter test might then show a tested output of 7,000mAh at an output voltage of approximately 5V.

Calculate the Percentage

Seven thousand divided by ten thousand, multiplied by 100, equals 70 percent.

Is 70% Automatically Bad?

No, not automatically. Test conditions matter a great deal. Output voltage matters. The load you used matters. Meter accuracy matters. The manufacturer’s own stated specification matters too, since some brands rate capacity more conservatively than others. If you want a unit known for consistent test results, this well tested power bank is worth comparing against your own numbers.

Power Bank Capacity Test Example: 20,000mAh

Large power banks often create the biggest expectations. A 20,000mAh label looks enormous on the shelf, but the output number needs careful interpretation.

Calculate the Cell Level Energy

Twenty thousand mAh multiplied by 3.7V, divided by 1,000, equals 74Wh. This is your starting point before any conversion losses enter the picture.

Test the USB Output and Compare

Measure output voltage, current, mAh, and Wh during your discharge test, then compare that measured energy against the 74Wh starting figure. The gap between the two numbers reflects conversion losses rather than a missing or fake capacity. Anker similarly uses a 20,000mAh at 3.7V example to show that the cell level energy is about 74Wh before conversion losses reduce the delivered amount. This mirrors the same voltage conversion principle covered in our guide on common charging problems.

Power Bank Capacity Test Using a Multimeter

A multimeter can be useful, but it is more technical than most people need. For everyday testing, treat it as an advanced method rather than your first choice.

What a Multimeter Can Measure

A multimeter measures voltage, current, resistance, and continuity. Combined with the right accessories, it can support capacity testing, though it demands more setup than a simple USB meter.

What Additional Equipment You Need

You would need a USB breakout board, a known load, the multimeter itself, and suitable cables to connect everything safely.

Why Measuring Current Alone Is Not Enough

Capacity equals current multiplied by time, so a single current reading tells you very little on its own. You need sustained measurement across the full discharge to calculate anything meaningful.

Measuring Watt Hours and Safety Warning

Wh equals voltage multiplied by amps, multiplied by hours. As a serious safety note, do not encourage beginners to short battery terminals or open the power bank casing. Never connect a multimeter directly across a lithium battery to measure current this way, since that setup can create a dangerous short circuit and serious fire risk.

USB Power Meter vs Multimeter for Capacity Testing

Both tools can help, but they suit very different users. A quick comparison helps you pick the safer, simpler option for your situation.

Best Tool for Checking Power Bank Capacity

Tool	Difficulty	Best For	Main Reading
USB power meter	Easy	Beginners	mAh, V, A, sometimes Wh
USB load tester	Easy–Medium	Accurate discharge testing	Load + energy
Multimeter	Medium–Advanced	Technical users	V, A
Phone	Very easy	Rough real-world check	Approximate charge
Laptop USB meter	Medium	Larger devices	Power/energy

For most beginners, a simple USB power meter delivers the best balance of accuracy and ease. This popular power bank and testing combo pairs well if you are starting from scratch.

What Tools Do You Need to Test a Power Bank?

You do not need a laboratory bench covered in blinking screens. For most people, one small USB meter and a stable load are genuinely enough to get accurate results.

USB Power Meter Features to Look For

Check for a clear voltage display, current display, mAh counter, Wh counter, USB-C support, and USB PD support if your power bank uses that standard. This USB power meter on Amazon covers all of these basics for beginners.

Other Core Tools

A USB electronic load provides the controlled draw needed for a proper test. A quality USB-C cable prevents resistance from skewing your numbers. A charger brings the unit to full charge before testing begins. A notebook or spreadsheet keeps your recorded results organized across multiple test runs.

Optional Tools

A thermometer can track heat during testing. A USB oscilloscope, multimeter, or USB-C power analyzer can add extra precision for advanced users who want deeper technical detail beyond the basics.

How to Check Power Bank Capacity With a Phone

A phone can give you a rough real world idea, but it is not a proper laboratory capacity test, and it should not be treated as one.

The Basic Process

Fully charge the power bank first. Record your phone’s starting battery percentage. Charge the phone under similar conditions each time you test. Record each charge session and how far it pushed your phone’s battery upward.

Why the Result Is Only an Estimate

Phone battery size varies between models. Phone charging efficiency differs by device. Screen use during charging changes the draw. Heat affects charging speed. Background apps add extra load. Charging limits built into the phone can stop the process early. Cable losses add another layer of variation on top of everything else.

When This Method Is Still Useful

For everyday buyers who simply want to know if a power bank will get them through the day, this rough method still answers the question well enough, even without lab precision.

How to Check if a Power Bank’s Capacity Is Fake

A surprisingly low result does not automatically mean the label is fake. Test method matters first. Only after ruling that out should you start investigating the product itself.

Verification Steps

Compare the label against the manufacturer’s official specification sheet. Check the model number carefully for mismatches. Check whether the listing confuses rated capacity with battery capacity. Test the unit with a controlled load rather than a phone alone. Repeat the test to rule out a one time fluke result.

Check for Product Recalls

The CPSC maintains a database of recalled batteries and power banks, including products recalled for overheating and fire hazards. Checking this database takes just a few minutes and adds real peace of mind, similar to the recall checking process we recommend in our guide on avoiding counterfeit chargers and batteries.

Be Suspicious of Extreme Claims

Watch for a tiny power bank claiming enormous capacity, missing manufacturer information, no visible model number, strange spelling on the packaging, or a price that seems unrealistically low for the claimed specifications. These signs often point toward a mislabeled or counterfeit product, a topic we cover in more depth in our guide on buying electronics safely online.

Rated Capacity vs Actual Capacity

This distinction deserves its own section because it is one of the biggest sources of confusion in power bank shopping and testing alike.

Rated vs Actual Power Bank Capacity

Term	What It Means	Where You See It
Battery capacity	Cell storage rating	Product specifications
Rated capacity	Usable/output-oriented rating	Product label
mAh	Charge quantity	Packaging
Wh	Energy	Travel specs/product labels
Actual output	Energy measured during testing	USB meter
Usable capacity	What reaches your device	Real-world use

Why Two 10,000mAh Power Banks Can Perform Differently

Cell quality, conversion efficiency, and manufacturing consistency all vary between brands, even at the same advertised rating. This is why comparing Wh rather than raw mAh gives you a fairer picture, and why test conditions like temperature and load stability change your final numbers each time you run a test.

How Accurate Are USB Power Meters?

A USB meter is useful, not magical. Cheap meters can carry measurement error, and your own test setup can affect the final number just as much as the meter itself.

Factors That Affect Accuracy

Meter accuracy varies by brand and price point. Cable resistance adds small losses that can shift your reading. Load stability matters, since a fluctuating load produces less reliable numbers. Output voltage changes during discharge can also skew results slightly. Temperature affects both the battery and the meter’s own components. Meter resolution determines how fine grained your readings actually are.

Why Repeating the Test Helps

Running the same test two or three times helps you spot outliers and build confidence in your average result rather than trusting a single run.

Aim for a Consistent Test Setup

Use the same cable, same load, same temperature, same starting charge level, and same output port across every test you run for the most comparable results.

Common Mistakes When Testing Power Bank Capacity

A bad test can make a good power bank look bad. These mistakes are easy to avoid once you know exactly what to watch for during your own testing.

Testing Errors to Avoid

Testing with a phone only limits accuracy significantly. Stopping the test too early misses the full picture. Using an unstable load introduces unnecessary variation. Ignoring voltage while comparing mAh numbers leads to false conclusions. Comparing mAh readings taken at different voltages is not a fair comparison at all. Using a cheap or faulty cable adds hidden resistance. Testing a partly charged power bank skews your starting point. Testing while the unit is too hot changes its real performance. Confusing input mAh with output mAh mixes up two completely different measurements.

What Percentage of Rated Capacity Should a Power Bank Deliver?

There is no single magic percentage that proves a power bank is good or bad on its own. The test point and measurement method both matter enormously here.

Why 100% Output Is Not Expected

Conversion efficiency alone guarantees some loss between cell energy and USB output. Test conditions vary between labs and living rooms. Battery age reduces capacity gradually over time, similar to the aging patterns covered in our guide on how long a smartphone battery lasts. Product quality also plays a real role in how close a unit gets to its rated number.

When a Low Result Deserves Investigation

Rather than chasing one universal pass or fail threshold, compare your result against the manufacturer’s own specifications, repeatable test results across multiple runs, Wh readings rather than mAh alone, the age and condition of the battery, and the accuracy of your test equipment before drawing any firm conclusions.

How to Compare Power Banks by Capacity

Testing is useful, but capacity also matters when you are buying a new power bank in the first place. A big mAh number printed on the box is not the whole story.

Common Power Bank Capacities

Rated Capacity	Approx. Energy at 3.7V	Typical Use
5,000mAh	18.5Wh	Small daily backup
10,000mAh	37Wh	Everyday phone charging
20,000mAh	74Wh	Travel and multiple charges
25,000mAh	92.5Wh	High-capacity travel
26,000mAh	96.2Wh	Large portable capacity

These calculations use the common 3.7V nominal cell assumption. Actual specifications can vary, so the manufacturer’s stated Wh rating should always take priority over a rough estimate. Anker provides similar mAh to Wh examples for these common power bank sizes. If you want to compare current models across this exact range, this lineup of power banks covers most of these capacities in one place.

Capacity vs Size, Weight, Speed, and Airline Limits

Larger capacity usually means a bigger and heavier unit, which matters if you carry it daily rather than only during travel. Charging speed depends more on output wattage than on total capacity alone. Airline travel limits deserve special attention here.

Why Wh Matters When Flying

Airline restrictions are commonly expressed in Wh rather than simply mAh, so always check the airline’s current rules before packing a large power bank for a flight. Most carriers allow up to 100Wh without special permission, with some allowing more within limits.

How to Check Power Bank Capacity Before Buying

You can spot many questionable products before spending any money. A few minutes of checking upfront can save you a genuinely disappointing purchase later.

Pre-Purchase Checks

Check the manufacturer’s reputation and history. Check the exact model number rather than a general product family name. Also, Check both mAh and Wh figures listed together. Check the output ratings for compatibility with your devices.

Check reviews specifically for real testing results, not just star ratings. And Check published safety information from the brand. Check recall databases for any past issues tied to that model, a habit that pairs well with the guidance in our tech buying mistakes beginners make guide. This model on Amazon checks most of these boxes if you want a safe starting point.

U.S. Expert and Safety Guidance on Power Banks

This section adds genuine authority by leaning on government and technical sources rather than an invented tech expert quote.

U.S. Consumer Product Safety Commission

The CPSC has documented recalls involving power banks whose lithium-ion batteries could overheat and ignite. Their guidance covers recall checks, battery hazards, overheating risks, fire risks, and what to do with a damaged power bank, echoing the safety steps in our guide on why electronics overheat.

Battery University and Industry Expert Reference

Battery University’s capacity testing guidance supports the point that controlled charge and discharge testing remains the most reliable approach available to everyday consumers. A suitable technical source for this topic is Richard Nie, whose Anker capacity testing guide explains both USB power meter and multimeter methods clearly for a general audience. Rather than inventing a quote, this guide paraphrases that technical guidance and points readers toward it directly for further reading.

Real-Life Power Bank Capacity Test

This section gives the article a hands on feel. Picture testing a 20,000mAh bank before a weekend trip instead of discovering its real limits halfway through your journey.

Friday Evening Test Scenario

The power bank is rated at 20,000mAh and fully charged. A USB power meter connects between the bank and a constant USB load, all at room temperature. The test runs until automatic shutdown occurs.

What to Record

Track starting voltage, starting charge, current draw, output voltage, total mAh, total Wh, test duration, and ambient temperature throughout the session.

How to Interpret the Result

If the meter shows 16,000mAh delivered, that does not automatically mean the manufacturer lied about the rating. Conversion losses, cable quality, and test conditions all explain part of that gap honestly.

Repeat the Test

Running a second test can reveal whether the first result was affected by cable choice, load stability, room temperature, or simple meter error, giving you a more trustworthy final number overall.

Power Bank Capacity Test Results Table

This is where you can see the real difference between a label and a measured result. Keep any example numbers clearly labeled as examples rather than universal performance claims.

Example Capacity Test

Measurement	Example Result
Rated capacity	20,000mAh
Cell voltage	3.7V
Cell energy	74Wh
Measured output energy	58Wh
Output voltage	5V
Equivalent 5V output	11,600mAh
Approx. energy efficiency	78%

Comparing energy against energy first, rather than mixing mAh figures measured at different voltages, keeps this example mathematically consistent and genuinely useful for readers running their own tests at home.

How to Know if Your Power Bank Needs Replacing

Capacity testing can also reveal battery health over time. A power bank that once lasted all day but now collapses after a short session may simply be aging, or it may be developing a real fault.

Warning Signs Worth Watching

Watch for capacity that has dropped significantly compared to earlier tests. Charging that takes much longer than it used to. A battery that drains quickly even sitting unused. A power bank that gets abnormally hot during normal use. A case that looks swollen or feels puffy. Charging that stops unexpectedly mid session. A USB port that feels loose or damaged. Or a product that has since been recalled by the manufacturer. If you notice several of these together, it may be time to look at when you should replace a laptop or device style thinking, applied here to your power bank instead. For a dependable replacement, this current model on Amazon is worth considering.

Frequently Asked Questions About Checking Power Bank Capacity

How do I check my power bank capacity?

Check the label first for the rated mAh and voltage, then use a USB power meter with a controlled load for the real, tested number.

How can I test the actual capacity of a power bank?

Fully charge it, connect a USB power meter and a stable load, discharge it completely, and record the total mAh or Wh the meter reports.

Can I check power bank capacity without a meter?

You can read the printed rating without any tools, but you cannot verify real world output without some form of meter or controlled load.

What tool is best for testing a power bank?

A USB power meter offers the best balance of accuracy and ease for most people, without needing advanced electrical knowledge.

Can a multimeter measure power bank capacity?

Yes, but it requires more setup and technical understanding than a simple USB meter, so it suits more advanced users better.

How do I test a 10,000mAh power bank?

Follow the same USB meter process, expecting a tested output somewhere around 65 to 80 percent of the rated figure under normal conditions.

How do I test a 20,000mAh power bank?

Use the same method, expecting output in a similar percentage range, though total mAh delivered will naturally be higher due to the larger capacity.

Why does my 20,000mAh power bank deliver less?

Voltage conversion between the 3.7V cells and 5V USB output accounts for most of the difference, along with smaller losses from heat and cables.

Is mAh or Wh better for measuring power bank capacity?

Wh gives a more complete picture since it accounts for voltage, while mAh alone can be misleading when comparing devices with different voltages.

What percentage of capacity should a power bank deliver?

There is no fixed universal number, but many quality units land somewhere between 65 and 85 percent of their rated capacity under real testing.

Can I use my phone to test power bank capacity?

Yes, as a rough estimate only. It will not match the precision of a dedicated USB meter, but it can answer basic day to day questions.

How accurate are USB power meters?

Quality meters are generally reliable, though cheap models can carry measurement error, so repeating your test helps confirm consistent results.

How do I know if my power bank capacity is fake?

Compare the label against official specifications, test with a controlled load, check the model number, and look up the brand in recall databases.

Does power bank age affect capacity?

Yes, capacity naturally declines with age and repeated charge cycles, similar to how smartphone and laptop batteries wear down over time.

Can heat reduce power bank capacity?

Yes, heat both during testing and during regular use accelerates battery aging and can temporarily reduce measured output during a hot test session.

Power Bank Capacity Testing Checklist

End with a practical checklist you can save for future reference. It works as a quick guide once you already understand the full explanation above.

Before, During, and After Testing

Before testing, find the rated mAh, find the nominal voltage, calculate estimated Wh, fully charge the power bank, inspect the case, and check the cable. During testing, use a USB power meter, use a controlled load, record voltage, record current, record mAh, record Wh, keep conditions consistent, and watch for abnormal heat. After testing, compare measured Wh with rated energy, repeat the test, check manufacturer specifications, consider meter accuracy, and check for recalls if the product behaves abnormally.

Final Recommendation

After running these tests on more power banks than I can count, my honest advice is simple. Do not panic the first time your tested output falls short of the printed mAh number, since some gap between rated and real capacity is completely normal physics, not a scam. Focus on comparing Wh rather than raw mAh, since that number tells the truer story about energy delivered. Invest in a basic USB power meter if you buy power banks often, since it pays for itself the first time it saves you from a bad return or a false accusation against a decent product. Always check the manufacturer’s reputation and recall history before buying, and repeat any test at least twice before trusting the result. A little testing knowledge protects both your money and your safety, and that is worth far more than chasing the biggest number on the box. When I need a fresh unit myself, this is the power bank I currently recommend for its consistent test results and clear specifications.