Revealed: the power banks that deliver the power they promise
Lab testing shows that there's a big difference in how much energy portable power banks put into your devices and how much gets wasted.
Energy conversion efficiency varies among models. In other words, this is the percentage of the power you put into the power bank that actually finds its way into your phone or tablet.
Higher efficiency power banks mean more charge for your devices and better returns on the electricity you pay for.
If you're looking for a great deal on a power bank, read our guide to the best cheap power banks which features product selections updated every day.
What is power bank efficiency?
When you use a power bank, energy is expended in ways that don't charge your device. For example, when your power bank gets warm to the touch, this is internal resistance causing energy to dissipate from the power bank.
When we test a power bank, we look at the usable energy you get from it and divide it by its capacity. This is what we call energy conversion efficiency.
We don't take a manufacturer's word for it and we don't take the nominal figures they supply at face value. We observe the power banks charging our devices and measure what happens. This is why we also check what the real capacity of a power bank is versus the stated capacity. Sometimes, there's a big difference.
Which power bank brands are best and worst for efficiency?
We've looked at some of the most popular power bank brands with multiple products currently for sale.
When we average all 60 power banks we've reviewed, the average efficiency is 75%. Some brands are averaging as much as 77% while other big-name brands fall below this mark.
| Brand | Average power bank efficiency | Best power bank efficiency | Worst power bank efficiency |
|---|---|---|---|
| INIU | 77% | 83% | 73% |
| Xiaomi | 77% | 82% | 72% |
| Hama | 77% | 80% | 73% |
| Belkin | 75% | 79% | 71% |
| Anker | 74% | 81% | 64% |
| Juice | 74% | 79% | 72% |
| Griffin | 74% | 79% | 65% |
There's a lot of variance on a product-by-product basis. While Anker has a power bank that's 81% efficient, it also has one that's a terrible 64% efficient. To put this in perspective, if you take a 20,000mAh power bank, at a poor 64% efficiency, you could fully charge a tablet once with the energy you lose through inefficiency.
What does this mean? While you're more likely to get a better deal with some brands, brand quality doesn't guarantee product quality.
Discover 8 easy ways to improve battery life on iPhone and Android so your phone lasts for longer.
Are bigger or smaller power banks more efficient?
We've run the same calculations as before, but instead of sorting by brand, we've sorted by the capacity of the power bank. We define the different sizes like this:
- Small power banks have less than 7,500mAh
- Medium-sized power banks have between 7,501mAh and 14,999mAh
- Large power banks provide 15,000mAh or more.
| Power bank capacity | Average efficiency | Best efficiency | Worst efficiency |
|---|---|---|---|
| Small | 72% | 79% | 65% |
| Medium | 74% | 82% | 60% |
| Large | 77% | 83% | 64% |
Our analysis shows that there's a correlation between power bank size and efficiency.
Large power banks are likely to be more efficient than average and small power banks to be less efficient than average. Since large power banks also cost more money, this is reassuring as it means a high-quality product is giving you more juice for the energy you put into it.
A small power bank that's inefficient is a problem to be avoided. They already lack capacity, so if you buy a bad power bank this size, losing even more energy could mean you can't even fully charge a modern smartphone once.
Three power banks to consider
We've picked three power banks from the three top-ranking brands for efficiency. But to get a full overview of your shopping options which includes charging time, features and ergonomics tests, take a look at our power bank reviews.
INIU Power Bank 22.5W Fast Charging 20,000mAh
We tested this new INIU power bank this summer. It's a big-sized battery for long trips without access to any mains and it's designed to charge your devices as quickly as possible.
20,000mAh is as big a capacity as we review across power banks. You'll find that power banks this big are quite hefty and not easily pocketed, but the good ones should be able to charge a phone at least six times.
- Read the full review of the INIU Power Bank 22.5W Fast Charging 20,000mAh to find out more.
Xiaomi 10,000mAh Mi Power Bank 3 Ultra Compact
10,000mAh is a medium-sized power bank and in lots of circumstances, it's the Goldilocks option. It should be pocketable and not too heavy, but it should have enough capacity to fully charge a big device like a tablet once, or a small device like a smartphone two to three times.
When we reviewed it, we remarked that it was quite pricey, but it's often on sale for less now that it's over a year old.
- Read the full review of the Xiaomi 10,000mAh Mi Power Bank 3 Ultra Compact for a complete verdict.
Hama Slim 5HD Power Pack 5,000mAh
While our research has shown that small power banks trend less efficient than bigger ones, Hama is a brand with a good record of creating efficient chargers.
The joy of a small power bank is how easy it is to carry it around and how light it is. This one's only 109 grams, about as heavy as a bar of soap. Small power banks are useful if you take day trips or if you need a small boost for a flight or a long car journey. If you can get back to the mains before long, the small capacity is fine.
- Read the full review of the Hama Slim 5HD Power Pack 5,000mAh to see what we found out about it on test.
The best power banks with five stars for efficiency
Which? members can see some of our top-scoring power banks that scored five stars in our independent efficiency tests. If you're not yet a member, get instant access to our expert reviews by joining Which? today.
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