Power bank runtime estimate: how to judge it

A power bank listing may promise a certain number of charges or hours, but the time you get depends on what your device is doing. A phone streaming video uses power differently from a laptop with a dim screen and a few tabs open.

Use the Battery Meter to see your device's current battery level, drain rate, and estimated time left. That estimate describes the device's battery, not how long a power bank will last. You can combine it with the bank's watt-hour rating and your usual workload to make a more useful purchase estimate.

Check your device's battery use

1. Open the Battery Meter on the device you plan to charge. It appears only when your device and browser report battery data. Some phones or browsers may not provide it.
2. Unplug the device and note its battery level, drain rate, and estimated time left. The time-left figure is a prediction based on recent use, not a fixed promise.
3. Use the device as you normally would for 30 to 60 minutes. Keep the screen brightness and workload similar to the way you expect to use it with the power bank.
4. Check the readings again. Note how many battery percentage points it used and whether the estimated time left changed. A second test under a different workload can show how much video, gaming, or a brighter screen affects use.
5. Before comparing power banks, check the device battery's capacity in watt-hours (Wh) if the manufacturer lists it, and find the bank's rated capacity or output energy on its label or product information.

For a fair comparison, do not compare a light-use estimate with a bank test or claim based on heavy use. Screen brightness, wireless connections, apps running in the background, and temperature can all change the result.

What the readings mean

The time-left estimate changes when you start using the device

This is normal. The device adjusts its prediction as its recent power use changes. For example, starting a video or turning up screen brightness can make the estimate fall quickly. Treat it as a snapshot of your current workload, not a guarantee of how long a power bank will extend use.

The drain rate stays similar during your test

A steady rate gives you a more useful baseline for that particular workload. If the battery drops 10 percentage points in an hour, that is a rough estimate of the device's battery use under those conditions. It does not tell you the exact power draw in watts unless you also know the battery's capacity.

The drain rate changes a lot

Your workload may be uneven, or the device may be adjusting power use. A short burst of video, an update, or a bright screen can change the reading. Repeat the test for longer, or use an average from a representative period rather than relying on one moment.

No battery information appears

The browser may not expose battery data on that device. Battery Meter is available only when the browser reports a battery, and it cannot read exact battery capacity or health. Use the device maker's battery information and your own observed battery use instead.

Make the estimate match your real use

1. Match your normal workload. Test with the apps and screen brightness you expect to use away from an outlet. A reading from an idle device will overstate runtime during video calls or gaming.
2. Keep the power mode consistent. On Windows 11, check Settings › System › Power & battery. On a Mac, check System Settings › Battery. Power-saving modes and performance settings can change how much energy the device uses.
3. Check the device's battery size. Look for a Wh figure in the manufacturer's specifications. If capacity is shown only in milliamp-hours (mAh), you need the battery's voltage to convert it. Do not assume the browser can provide this exact capacity.
4. Check the power bank's output. Capacity and charging speed are different. Make sure the bank supports the charging standard and wattage your device needs, especially for a laptop. A bank with enough stored energy may still charge slowly or fail to keep up if its output is too low.
5. Allow for conversion losses. Energy is lost as a bank converts its internal battery output to USB power and your device charges. Actual usable energy depends on the bank, cable, output voltage, and charging conditions, so treat a calculated runtime as an estimate.

On a phone, you can also review which apps use the most battery in Settings › Battery. That can help you choose a realistic workload for your test. Phone menus and available details vary by model.

Use watt-hours to estimate added runtime

Watt-hours measure stored energy. They are more useful than mAh for comparing devices that use different voltages. If a power bank lists only its internal capacity, estimate its energy with this formula:

mAh × voltage ÷ 1,000 = Wh

For example, a bank rated at 20,000 mAh using 3.7 volts internally contains about 74 Wh before conversion losses. If, for illustration, about 65 percent reaches the device, that is roughly 48 Wh of usable energy. The actual percentage depends on the model and how you use it. If the maker gives an output-energy figure, use that instead and do not apply a second loss allowance.

To estimate added runtime, divide usable power-bank energy by the device's average power use in watts:

usable power-bank Wh ÷ device watts = approximate hours

Suppose a laptop has a 60 Wh battery and uses about 10 percent of its battery per hour during your test. That is roughly 6 Wh per hour, or about 6 watts. A bank delivering 48 Wh could then provide around 8 hours at that same load. This is a simplified estimate, not a promise: charging and device use can overlap, and the bank may reduce output as it empties.

If you do not know the device's battery capacity in Wh, avoid turning its percentage drain into an exact runtime. Use your test to compare light and heavy use, then compare those patterns with the bank's stated output energy and supported wattage.