Engadget
If you need to charge your phone repeatedly while away from an outlet, a big power bank could be a good investment.
20,000 mAh power banks tend to be chunky and are considered unwieldy for most pockets — but despite their size, there are plenty of situations in which you might rely on one. Large power banks are perfect for camping trips or site visits where your phone may not have access to an outlet for an entire day or multiple days at a time. They're also great for intensive use, like when your phone acts as a wireless hotspot or navigational tool. Bringing one along can help give you peace of mind from constantly checking your devices' battery percentages.
If you don't need the ability to store multiple charges, companies like Anker have smaller 5,000 mAh ones that are lighter and occupy less space. Some even attach to your phone with magnets. Here's a breakdown of how much energy your devices consume and when a high-capacity power bank is the right one for you.
Battery capacity is often measured in milliamp-hours (mAh). In theory, a 20,000 mAh power bank delivers 20,000 milliamps of current for an hour. However, factors like internal resistance, heat and energy loss while charging/discharging mean its usable capacity hovers around 60 to 70 percent.
A better way to measure a power bank's capacity is energy, which is measured in watt-hours (Wh). This number is derived from the internal battery voltage, which is usually 3.7V for lithium-ion batteries. Naturally, 10,000 mAh and 5,000 mAh power banks store less energy in Wh than a 20,000 mAh one.
A 20,000 mAh power bank operating at 3.7V stores about 74 Wh of energy. You'll get this number when you multiply the milliamp-hours by the voltage and divide the result by 1,000. This makes for an apples-to-apples comparison while charging tablets or laptops that operate at higher voltages. The jump in voltage also comes with some heat loss, further impairing efficiency.
No, a power bank of this capacity is not overkill. Its effectiveness depends on how much energy your devices can store. Here are some examples of devices with different capacities and how many charges they'll get out of a 20,000 mAh power bank:
An Apple iPhone 17 (3,692 mAh at 3.88V) stores 14.3 Wh, meaning it could get between three and four full charges.
A larger phone like the Apple iPhone 17 Pro Max (4,823 mAh at 3.88V) stores 18.7 Wh, meaning it can be sustained for two to three charges.
The Steam Deck (5,313 mAh at 7.7V) stores 40.9 Wh, and it can get one to one and a half charges out of the power bank.
Laptops like the M5 MacBook Air 13-inch usually have their batteries measured in Wh. Its 53.8 Wh capacity will drain the power bank with one charge.
If you're only charging your smartphone, a 20,000 mAh battery is a great investment as it can keep it powered up for multiple days. It also comes under the 100 Wh airline limit, meaning you can take it along on flights in your carry-on luggage. For lighter use on daily commutes, consider a smaller power bank. While buying one, also keep an eye on charging speeds, as the best power banks will support fast charging.