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7 Quick Steps To Find Power Bank Real Capacity

Are you confused on how to calculate real output capacity of the power bank? Then you’re in the perfect place!

Many of us calculate that a 10000mAh power bank could charge my 5000mAh mobile phone battery 2 times fully (10000mAh :- 5000mAh = 2 times) and made there purchases. But the calculation is totally wrong!

You need to know that a power bank couldn’t able to provide you their full capacity as actual output no matter what brands and capacity (10000mAh, 20000mAh, 15000mAh, etc.) power bank you have. Every power bank loses their capacity while providing juices to your mobile or devices.

There are several reasons why power bank’s actual output is lower than rated capacity. Those reasons are explained below!

Also read: 15 Important features to consider when buying a power bank

How To Calculate Real Output Capacity Of The Power Bank:

To calculate power bank actual capacity you need to understand and get familiar with some technical things which I have explained below. But you need to read step by step to understand properly.

1. First Let’s Understand Power Bank And Its Components:

A power bank itself is a rechargeable battery that stores and discharges electrical energy.

Power banks has two main components:

  • Rechargeable battery: A power bank has rechargeable batteries or battery (LiPo, li-ion) that stores electrical energy in the power bank. A power bank capacity or rechargeable battery capacity measured in milliamperehour (mAh). For instance 10000mAh,15000mAh, 20000mAh, etc.
  • Circuitry board: A power bank has a circuit board that controls recharging and discharging process of the rechargeable battery.

The quality of these two components (rechargeable battery and circuit board) are important to get high charging conversion from any power bank.

A good power bank must have high quality components for high charging conversion.

2. Understanding Power Bank Battery Capacity and Nominal Voltage:

A power bank usually composed of one or more rechargeable batteries with certain capacity (eg. 10000mAh, 200000mAh) with nominal voltage of 3.7 V.

3. Understanding Power Bank Stored Energy:

As we already know power bank is an electronic device that stores electrical energy in an internal battery. This energy is measured in watt-hours (Wh).

A power bank stored energy is calculated by multiplying the power bank capacity (mAh) by the nominal voltage (V) of its internal battery.

Stored energy (Wh) = [Battery capacity (mAh) x Nominal battery voltage (V)] / 1000

For Example with 20000mAh power bank:

Stored energy (Wh) = [Battery capacity (10000mAh) x Nominal battery voltage (3.7V)] / 1000

  • Stored energy = 74 Wh (watt-hours)

A 20000mAh power bank stores 74Wh energy with 3.7V internal battery.

4. Understanding Power Bank Voltage Conversion:

Now, we already know that a power bank usually stores energy with 3.7V nominal voltage that is 74Wh if 20000mAh power bank.

But during the charging process power bank raises its nominal voltage (3.7V) to the voltage of USB output port (5V standard charging) to charge your mobile or devices.

So for example,

A 20000mAh power bank stores 74Wh with nominal voltage of 3.7V but while charging your mobile or devices it raises their nominal voltage to output voltage that is 5V standard.

Which means when it fills your mobile it fills with 5V, 9V, 12V, etc. So for example,

Power bank stored energy :- output voltage X 1000 = actual capacity with 5V standard charging.

74Wh :- 5V x 1000 = 14800mAh

The actual output capacity of 20000mAh power bank with 5V standard charging is 14800. And this capacity is without losing any capacity. Its just happen due to voltage conversion process.

5. Understanding Power Bank Energy Efficiency:

As we already calculated power bank output energy with 5V, now let’s calculate power bank energy efficiency.

Power Bank Energy Efficiency (%) = Output Energy / Stored Energy

Power bank energy efficiency = 14800mAh :- 20000mAh = 74%

The energy efficiency of 20000nAh power bank is 74%.

6. Understanding Charging Efficiency Of The Power Bank:

Typically quality power banks has 85% – 90% charging efficiency.

Because power bank also lose some energy in charging process in the form of heat.

7. Calculation Of Real Capacity or Usuable Energy Of a Power Bank:

Now after understanding all those things let’s calculate power bank real output capacity.

Output Capacity (mAh) = Efficiency x [Battery capacity (mAh) x Battery voltage (V)] / Charging voltage (V)

Output Capacity (mAh) = 90% x 20000mAh x 3.7V :- 5V

Output Capacity (mAh) = 13320mAh

The actual output capacity of 20000mAh power bank would come 13320mAh if the power is good quality product.

Why Actual Capacity Of The Power Bank Is Lower Than Rated Capacity?

Mainly, these are the reasons for lower real output capacity:

  • Voltage conversion while charging (3.7V nominal voltage to 5V output voltage).
  • Energy efficiency of the power bank with 5V standard output charging.
  • Quality of power bank components that are internal battery and circuitry board.
  • And also power banks lose their some energy while charging process in the form of heat.

Also read: Which power bank is better 10000mah or 20000mah?


The above real output calculation of the power bank may technically incorrect but you can still able to get an idea about power banks capacity calculations. You should know these to correct your misconception that a power supplies its full capacity in your mobile or devices.

It also helps you to properly calculate and find your best capacity need. So always keep in mind that a rated capacity and actual output capacity of any power bank is not same. Real output of any power bank is around 70% or even less if the power bank is made of low quality materials.


What Is Real Output Capacity Of 10000mAh Power Bank?

The real output capacity of 10000mah power banks are around 6660mAh.

What Is Actual Capacity of 20000mAh Power Bank?

The actual output capacity of 20000mAh power bank would come 13320mAh if the power is good quality product.

What Is The Easiest Way To Find Actual Capacity Of Any Power Bank?

Multiply the power bank capacity with ⅔. Example: 10000mAh x ⅔ = 6666mAh (approx. real capacity of 10000mah power banks).

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