A power bank’s advertised battery capacity is not its effective battery capacity
When we buy power banks or portable batteries, we all have that misconception that the advertised capacity will be the same as what our phone — or whatever devices we plan to charge with it — will eventually receive. If you’re looking at a 10,000mAh power bank and you have a phone with a 5,000mAh battery, then that’s two times a full charge. Right?
Well, no. I used to make that mistake, too, until multiple bad experiences made me look into it and realize that there’s an efficiency metric or a real capacity that’s much lower than the advertised number. This is due to two reasons.
One is the voltage conversion. Most internal battery cells inside a power bank operate at a nominal voltage of around 3.7V, but to charge your phone, they have to boost that up to 5V at least. This drastically lowers the effective mAh output. A better measure would be Wh (Watt-hours), but no brand uses that. In short, a 10,000mAh battery at 3.7V gives off 37,000mWh or 37Wh, which in turn would only be 7,400mAh at 5V. That’s already a 25% drop. At 7.2V, which is what both UGREEN and Baseus show off on their packaging, this drops to around 5,000mAh — half the capacity! I just had to look for that number in the tiny specs tables on the side of the box while ignoring the big bold 10,000mAh on the front of the box and product listing.
The other reason is inefficiency and energy loss due to heat when the battery converts its 3.7V to 5V or more. Even over USB-C, the best power banks will have an efficiency of 85%-90%, thus losing an extra 10-15% of their capacity. This explains why there’s only 10% or so remaining battery at the end of my USB-C charging tests.
Be sure to check the full specs or packaging of a power bank; it'll tell you the real capacity. You can expect that to be half of the advertised one.
The problem with wireless charging, however, be it the first-gen Qi, Qi2, or Qi2 25W (aka Qi2.2), is that this same energy loss is a lot more pronounced over the electromagnetic induction of wireless charging. The battery will generate more heat when pushing its charge across plastic and metal in order to reach the coil inside your phone.
Even with the advancement of Qi2, which uses magnets for perfect alignment and reduced energy loss, you still have a gap with no direct contact between emitting and receiving coils, as well as a double conversion process. This results in a lower efficiency score, usually around 25%. And this explains why there’s even less remaining battery percentage when I charged my phones over Qi2 and Qi2.2 than over USB-C.
So, which power bank should you buy for your Pixel 10?
Honestly, the answer to this question really depends on you. What you need to do is go in with wide-open eyes, knowing that the advertised capacity you’re paying for will translate to about half of that number when charging your Pixel 10, 10 Pro, or 10 Pro XL, regardless of whether you’re charging wirelessly or over a USB-C cable.
A 5,000mAh power bank will top up your Pixel 10 to about 70% in my experience, in the best-case scenario. A 10,000mAh power bank will fill it once, with a few percentages left to spare. And a 20,000mAh unit will charge it twice with some juice left for somewhere between a quarter and a half-charge. These are the general numbers. Wireless charging, even over Qi2.2 25W, will be slower, cause more heat, and use up your portable battery more. But the difference isn’t too drastic to avoid using it altogether, unless you’re in a rush and need your phone topped up as fast as possible.
When buying a power bank, you just need to keep the basic math in mind. Divide what you see on the box by two and that's approximately what you'll get.
The more important choice to make, in my opinion, is whether you actually
need wireless charging or not. Sure, it’s more convenient and doesn’t require you to think about the USB-C cable, whether integrated or as an extra item to carry, but it also makes any power bank thicker, heavier, and much more expensive. If you’re happy with USB-C, skip Qi2 or Qi2.2 25W altogether, and get a good USB-C-only portable battery. You’ll save some money, shave off a few grams, and get a superior charging speed and efficiency.
Personally, I’ve gone with this teeny tiny Baseus Picogo 5,000mAh Qi2 15W power bank (
$35.99 at Amazon) as my daily carry. It’s not as fast and not as capable as the two larger models I tested here, but it’s so incredibly convenient with its integrated kickstand and very compact design. Plus, going in with my eyes open, knowing what to expect from the more barebones Qi2 15W and the limitations of the smaller 5,000mAh battery, means that I’m not disappointed by its performance. The larger 10,000mAh UGREEN and Baseus power banks are still there for me on extremely busy days.