You have an original real capacity 10,000 mAh power bank, and that should charge your 1500mah phone about 6 times, right? Well, here is where the confusion begins.

What the advertised capacity actually tells you.

The rated capacity is what is actually inside the powerbank, the physical battery.


Inside a 10,000 mAh power bank.
Inside a 10,000 mAh power bank.

This powerbank has 4 3.7V 2,500mAh batteries, together that equals 10,000mAh and this is correct.

But USB is 5V!

Inside powerbanks are 3.7V batteries, but the USB standard is 5V.  Between the battery and the USB socket is a conversion circuit and this changes the 3.7 V into USB friendly 5V. When converting into a higher voltage, you must also convert the mAh into the new voltage.

How to calculate theoretical USB output capacity

A simple equation can be used to convert the 3.7V into 5V.

ACTUAL 5V mAh = 3.7 X Advertised Capacity / 5

For a 10,000mAh powerbank – 3.7 X 10,000 / 5 = 7,400 mAh

So a 3.7V 10,000 mAh powerbank really only supplies 7,400 mAh at the 5V USB connection. So straight out of the box is a 23% reduction in the stated mAh.  This is not the actual level as there is also conversion loss.

What is conversion loss?

As you use your powerbank the circuit inside that converts 3.7V to 5V USB uses some energy and also creates heat. During this conversion, you lose some extra mAh.  There is a wide range in conversion efficiency and most brands don’t state the losses, Xiaomi has prized themselves on their conversion efficiency chips which are up to 98% efficient, meaning you only lose 2% off your battery power in the conversion. Some others can consume as much as 10% during conversion.

Why don’t they just state the actual output?

They don’t need to as technically that is what’s inside the box and most people have no idea. By not giving the actual 5V output brands can reap these benefits;

  • Manufacturers can have a higher number of mAh for their powerbank and sound more powerful.
  • It avoids the talk or testing of conversion loss and brands with low efficiency conversion chips can still market them in the same league as other efficient brands.

Some manufacturers will state in the manual or in small print on the device but most will not state anything other than the 3.7V mAh. As a general rule I would recommend taking 25% – 30% off the advertised capacity straight away and then you have a more realistic indication of performance.

This power bank actually states both capacity’s, you wouldn’t notice it tho unless you are one of the people who actually read product labels. This one has 16,000 mAh but only has 10,200mAh at 5.1V.

It is a very gray area and not common knowledge but I hope this has shed a bit of light on how powerbanks are advertised and busted the classic assumption that dividing the rated capacity by your phone battery capacity is an accurate measurement of the actual number of recharges, in reality, it is far from it.

You are now a powerbank expert and next time you are on the prowl for a new one have a look through our range.

Leave a Reply

Notify me of

I’m glad someone finally wrote about this. Thank you.


Oh I didnt know that,thats a really good information to share!


Gee, now that’s a taste of honesty. Few other retailers would dare make that info public. A very good explanation. Well done. Very similar to the wattage output of amplifiers. TMO Vs RMS etc. Only fostered by manufacturers trying to outdo their rivals and still used today.

The USB socket needs 5V. If the voltage of the cell is 3.7V we need a circuit to change 3.7V to 5V: a step-up circuit So, there’s a law in physics that I know like “law of conservation of energy”. Nothing is destroyed but all changes. Let’s see another physics law: Electric Power We can say that P = V * I, where P is power, V is voltage and I is Current. Example: if we have some cells and the total capacity is 10 Ah we can say that they can generate 10A in about 1 hour. The voltage… Read more »

Yeah that’s a great way to make the power universal, and should be something that responsible brands use. Some AC chargers use the W as the power rating but I have not seen many power banks using this term.
One reason I see is phone battery’s are rated at mAh so its easier for people to understand as the average consumer without the knowledge we have would be confused with a W power rating but everyone knows mAh.


Yes, you’re right.
But like said in the article, the capacity on the cover of the power bank isn’t its real capacity. There’s always a loss due to the conversion from 3.7V to 5V. The usage of Wattage can resolve this misunderstanding.

While it’s great what you’re doing with this article, I saw that there’s one very important bit of information that you’ve missed out, and one that I was hoping you’d cover concerning the use of “mAh” to label a battery’s capacity. While the conversion equation and efficiency consideration you’ve pointed out definitely gives a much clearer picture of a power bank’s capacity, the labelled mAh capacity of a power bank is technically more “correct” before the conversion process you’ve provided. The reason for this is because a smartphone’s battery’s mAh rating is also done at 3.7V, even though it charges… Read more »
Nitin Chauhan

Does it mean that the power bank of 10000 mah capacity at 3.7 V will actually charge a 2000 mah 3.7 V battery in phone, 5 times. Without any loss?

Yeah you are right in you analysis, the reason I didn’t include it is that it involves introducing a third unit variable, Wh. It is a much more simpler way of comparing power however it adds to the complexity of an average Joe as now they need to also understand another type of measurement. mAh is widely used in devices and a familiar term, Wh is rarer and would require deeper explaining (like you have done for us above). I will plan to create another article that explains why we should be using Watt hours as it is truly the… Read more »