Power bank purchase: compare your device's drain rate

A power bank that looks large on the box may still fall short during a long day out. The amount you need depends on how quickly your phone, tablet, or laptop uses its own battery, and how fast the bank can recharge it.

Compare battery drain during the kind of use you expect, then check how well a power bank keeps up. The Battery Meter can show your device's charge level, charging state, and drain rate when your browser provides that information.

Measure battery drain before you shop

1. Open the Battery Meter on the device you plan to charge. If your browser asks for battery access, allow it. Some browsers and devices do not expose battery information, so the tool may not show readings.
2. Start with a comfortable charge level and unplug the device. Note the percentage and the drain-rate reading. Keep the device in the same place and use it as you normally would, such as browsing, streaming, or using navigation.
3. After 30 to 60 minutes, note the percentage and drain rate again. Repeat the check during a second activity if your use varies a lot. Keep screen brightness, network connection, and apps as consistent as you can.
4. If you already own a power bank, connect it and repeat the same activity. Check whether the device reports that it is charging and whether its percentage rises, holds steady, or continues to fall. For a direct charging-speed comparison, use the Charger & Cable Test with each bank and the same cable.

A short test can be misleading. Battery percentages update in steps, and a brief burst of video, navigation, or a weak cellular connection can change the drain rate. Compare longer periods under similar conditions rather than treating one reading as a promise about every day.

What the results mean

Your device loses charge at a steady rate

Use the change as a rough estimate of your daily need. For example, if your phone drops 12 percentage points in two hours, that is about 6 percentage points per hour in that situation. Four similar hours would use roughly 24 percentage points. Add a margin for heavier use, poor signal, cold weather, or a day that runs longer than planned.

This tells you how much of your device's battery you may need to replace. It does not tell you the power bank's usable capacity. To estimate energy more closely, you also need your device battery's capacity in watt-hours, usually listed in its specifications.

The drain rate changes between activities

That is normal. A bright screen, video, games, navigation, hotspot use, or a weak mobile signal can increase power use. Base your purchase on the demanding activity you actually expect, not only on the lowest drain reading you see while the device is idle.

The device still loses charge while connected to the bank

The bank may be supplying less power than the device uses, or the device may be using more power than usual. Repeat the test with the screen dimmed and demanding apps closed. If it still loses charge during ordinary use, compare the bank's output rating and charging-speed result with another bank using the same cable.

The Battery Meter has no reading

Your browser may not provide battery data. This is a browser limit, not proof that the device battery or power bank is faulty. You can still note the charge percentage, wait a fixed period, and check the change manually. If you check battery usage in system settings, Windows 11 has Settings › System › Power & battery, and macOS has System Settings › Battery. On Android, look under Settings › Battery; on iPhone, open Settings › Battery.

Choose a power bank for your routine

1. Estimate your top-up need. Multiply the percentage points you expect to use by the number of hours you will be away from an outlet. Use a demanding but realistic activity as your reference. Treat the result as a planning guide, not a capacity conversion.
2. Check the rated capacity and energy. Compare the power bank's watt-hour rating when available, and do not assume that its advertised milliamp-hour figure will all reach your device. Conversion losses mean usable energy is lower than the battery-cell rating.
3. Match the output to your device. Check that the bank supports a charging standard and output power your device can accept. A high-capacity bank can still charge slowly if its port, cable, or output mode is unsuitable.
4. Compare charge rate under the same conditions. Use the Charger & Cable Test to compare power banks with the same device, cable, and approximate starting charge. Run each test when the device is cool, since heat and a high charge level can slow charging.
5. Keep a reserve. Choose more capacity than your minimum estimate if you need a full extra charge, share the bank, or cannot recharge it during the day. A reserve also helps account for conversion losses and changes in your routine.

Why the percentage is not the power bank's capacity

A battery percentage is a fraction of your device's battery, not a universal unit of energy. Losing 20 percent on a small phone battery does not represent the same energy as losing 20 percent on a large tablet battery. The Battery Meter can help you compare your own usage patterns, but it cannot identify a power bank's actual capacity or certify its advertised rating.

For a useful buying comparison, combine your measured drain with the device's published battery capacity, the power bank's watt-hour rating, and a charging-speed test. Leave room for real-world losses instead of expecting a precise number of full recharges from the label.