What is the real output capacity of a Power bank?

Do you know how many times you can recharge your cell phone with a 10000mAh or a 20000mAh power bank?

A very common mistake among those of us who want to know the number of charges we can get on our devices with a power bank is to divide the capacities of both devices, i.e.:

Number of Charges = Power Bank Capacity (mAh) / Smartphone Capacity (mAh)

For example, if you have a smartphone with a 2500mAh battery capacity and you want to buy a 10000mAh power bank, how many charges could you get?

If we apply the above formula we get 4 full charges:

Number of Charges = 10000 mAh / 2500 mAh = 4 charges

THAT’S A BIG MISTAKE!!

We are sorry to tell you that this calculation would be incorrect because the 10000mAh of the power bank refers to the capacity of its internal battery.

A result closer to reality would be to use the actual capacity available at the output USB port of the power bank:

Number of Charges = Actual Power Bank Capacity / Smartphone Capacity

If we assume that the actual capacity is 6000 mAh we would have enough juice for 2 full charges of our smartphone:

Number of Charges = 6000 mAh / 2400 mAh = 2,5 charges

Although this second formula is still technically incorrect (capacities divide at different voltages) we wanted to make you see in a simple way that the number of charges is much lower than expected despite the fact that the power bank is advertised with a capacity of 10000 mAh.

And this is the main problem that many Amazon users encounter when buying a power bank:

customer reviews amazon power bankIn the following article we are going to explain in depth how to calculate the real capacity and the number of charges of a power bank in any device and for different charging voltages.

Although the content of this article may be a bit technical and boring to read, we have tried to make it as easy to understand as possible for anyone without previous experience.

After reading this article you will become a power bank expert!

Which are the components of a Power Bank?

First of all, you should get familiar with the 2 basic elements that make up a Power bank:

  1. A rechargeable battery of a certain capacity (mAh) and nominal voltage (V).
  2. An electronic circuit that controls the charging and discharging process of the rechargeable battery, as well as performing other important functions such as, for example, protection against over-voltage, over-discharge, temperature control, etc.

As we will see below, the actual output capacity of a power bank will depend on the quality of such components.

Battery capacity and voltage rating

The battery of a power bank consists of lithium-ion (Li-Ion) or lithium polymer (LiPo) cells.

Usually, they use cells with a nominal voltage of 3.7 volts (V) and a capacity ranging from 1500 to 5000 milliampere-hours (mAh). However, cells with other voltages are also available on the market, e.g., 3.6V, 3.8V or 3.85V.

Lithium Ion Battery Cell 2400mAh 3.7V

Additionally, the battery may be made of of a single cell or several cells connected together:

One cell

If the power bank battery is made of a single cell, its capacity and rated voltage will be the same as that of the cell itself.

Multiple cells

If the power bank battery is made up of multiple cells, its capacity and voltage rating will depend on the individual capacity and voltage of each of the cells as well as the configuration used (series or parallel circuits) to connect the cells together.

Generally, a power bank battery uses cells with the same capacity and voltage nominal connected in parallel.

With a parallel connection, the voltage of the power bank battery is the same as the voltage of the cells and its capacity is the sum of the individual capacity of each of the connected cells.

In the previous image we see an example with 4 cells of lithium-ion (Li-Ion) of 3.6V and 3400mAh have been connected in parallel resulting in a battery with a capacity of 13600mAh and a voltage of 3.6V.

Therefore, if we wanted to make a power bank using 3400mAh and 3.6V cells connected in parallel, the battery will have a voltage of 3.6V and its capacity will depend on the number of cells used:

  • 1 cell: 3400 mAh/3.6V
  • 2 cells: 6800 mAh/3,6V (2 x 3400 mAh)
  • 3 cells: 10200 mAh/3.6V (3 x 3400 mAh)
  • 6 cells: 20400 mAh/3.6V (6 x 3400 mAh)
  • 8 cells: 27200 mAh/3.6V (8 x 3400 mAh)

So, from now on, whenever you see the capacity of a power bank advertised, remember that it refers to the capacity of its internal battery!

USB Output port voltage

We have already seen that a power bank is composed of an internal battery of a certain capacity (mAh) and nominal voltage (V) which is usually 3.7 volts (V).

However, when charging a device with a power bank we should know that the USB output port of any power bank works at a standard voltage of 5V.

Even this voltage can be higher (9V, 12V or 20V) if both the connected device and the power bank support fast charging protocols such as Quick Charge (QC) or Power Delivery (PD).

This difference between the battery voltage and the power bank output voltage is the reason why the capacity of a power bank at its USB output port is different from the capacity indicated on its internal battery.

For example, a 10000mAh power bank would have a capacity of 7400mAh at its USB output port at a charging voltage of 5V.

To sum it up

In this section we learned that:

  • A power bank is made up of an internal battery (consisting of one or more Li-Ion or LiPo cells) and an electronic circuit.
  • The capacity advertised on a power bank indicates the capacity of its internal battery and is different from the capacity available at the output port.

How to calculate the actual output capacity of a Power bank?

Once we are familiar with the main components of a power bank, let’s learn a step-by-step method to calculate the actual output capacity of a power bank for any charging voltage (or output port voltage).

Additionally, to better understand the theoretical concepts, each section will have a practical example using the real data of a power bank of 10000 mAh capacity, the Ugreen (model PB178):

The specific data we are interested in knowing about this model are the capacity (mAh) of the device, and the voltage (V) of its battery:

  • Battery Capacity: 10000mAh 3.85V

Later, we will also explain what the Rated Capacity data means, which some manufacturers are starting to include in the specifications of their power banks.

Stored energy

As we already know, a power bank is an electronic device that stores energy in an internal battery to later transfer it to the battery of other devices, or even power some of them.

This energy is measured in watt-hours (Wh) and is calculated by multiplying the capacity (mAh) by the nominal voltage (V) of its internal battery:

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

Example 1: How to calculate the stored energy in a power bank

The power bank in the image had a battery capacity of 10000 mAh and a nominal voltage of 3.85 V. To find out how much energy it stores we apply the above formula:

Power Bank Stored Energy = 10000 mAh x 3,85V = 38500 mWh / 1000 = 38,5 Wh

We see that our power bank stores 38.5 watt-hours (Wh) of energy that we can use to recharge the battery or power other devices.

Voltage conversion

During the process of charging a device, the electronic circuitry of a power bank raises the nominal voltage of the battery (e.g., 3.7V) to the voltage of the USB output port (5V standard voltage).

If we assume that the voltage conversion (from 3.7V to 5V) was an ideal process, i.e., without energy losses, all the energy stored in the power bank battery should be completely transferred to the USB output port.

Therefore, we can define the following equivalence:

Power Bank Stored Energy = Power Bank Output Energy

Let’s deconstruct this equation in terms of capacity and voltage:

Battery Capacity (mAh) x Nominal Battery Voltage (V) = Output Capacity (mAh) x Charging Voltage (V)

From the above equation we know all the variables except the capacity at the USB output port.

If we solve this variable, we get a first approximation to know the real capacity at the output port of a power bank according to the voltage at which the device is charged (5V being the standard voltage):

Output Capacity (mAh) = [ Battery Capacity (mAh) x Nominal Battery Voltage (V) ] / Charging Voltage (V)

Example 2: Calculation of the (ideal) Real Capacity

If we apply this formula with the data of our power bank (10000 mAh and 3.85 V) we will obtain a capacity of 7700 mAh at the output port at a charging voltage of 5V:

Output Capacity = (10000 mAh x 3,85 V) / 5 V = 7700 mAh

Furthermore, we can see that, although the capacities are different (10000mAh and 7700mAh), the energy is conserved at the input (battery) and output (USB port) of the power bank as we have considered that the voltage conversion process during the charging of a device is ideal (without energy loss):

10000 mAh x 3.85 V = 38500 mWh = 7700 mAh x 5 V

Energy efficiency

So far, we have calculated the actual capacity of a power bank at its output port considering that it is able to supply 100% of the energy stored in its battery.

However, we are sorry to tell you that the formula defined in the previous section is never going to be fulfilled in real life:

Power Bank Stored Energy Power Bank Output Energy WRONG!

The energy at the output port of a power bank is ALWAYS going to be less than its stored energy and the amount supplied will depend on the quality of its 2 components:

The Battery

If the battery used is a low quality one, it will loss a fraction of its capacity after a few power bank charge/discharge cycles.

Consequently, the power bank will have less energy available to supply to your devices.

The Electronic Circuit

During the voltage conversion stage performed by the power bank’s electronic circuit, part of the energy stored in its battery will be lost in the form of heat (this is known as The Joule Effect).

For this reason, the less efficient the electronic circuit is, the more energy will be lost and, therefore, the power bank will have less energy available to supply to your devices.

We can define the discharge energy efficiency of a power bank as the ratio between the energy supplied at its USB output port and the energy stored in its battery:

Power Bank Energy Efficiency (%) = 100 x (Output Energy / Stored Energy)

It should be clear that the energy efficiency of a power bank will never be 100%:

Even if we buy a new power bank and its battery is in perfect condition, there will always be an energy loss due to the voltage conversion process performed by the electronic circuit of the power bank in order to charge a device.

This energy loss will be higher or lower depending on the quality of the electronic circuit.

Example 3: Energy efficiency calculation

In Example 2 we calculated the actual output capacity of a power bank assuming that the voltage conversion process (from 3.85 to 5 V) was ideal, i.e., it occurred without energy loss.

Consequently, the power bank supplied 100% of its stored energy:

(Ideal) Energy Efficiency = Output Energy / Stored Energy = 38480 mWh / 38480 mWh = 1 x 100 = 100%

However, we already know that, actually, during the voltage conversion process some of the stored energy is lost, therefore, the efficiency will always be under 100%.

To calculate the actual energy efficiency of our power bank, let’s assume that we have measured the energy obtained at its output port:

(Real) Energy Efficiency = 28875 mWh / 38500 mWh = 0.75 x 100 = 75%

As we can see, the power bank has been able to supply 75% (28875 mWh) of the total energy stored in its battery (38500 mWh) while the remaining 25% (38500 – 28875 = 9625 mWh) has been lost as heat.

Usable Energy and Real Capacity

Once we know the concept of energy efficiency in a power bank, we are ready to calculate the energy and capacity it will have at its USB output port.

With the Energy Efficiency formula as our starting point, we take the Stored Energy variable to the other side of the equation and, in this way, we get the energy available at the output port of the power bank:

Power Bank Energy Efficiency = Output Energy / Stored Energy

(Usable) Output Energy = Energy Efficiency x Stored Energy

The energy calculated with this formula is the usable energy of the power bank, that is, the energy that we will have available to use to recharge our devices.

If we analyze this formula, the only element we don’t know is the energy efficiency of the power bank.

This data must be established by us and, you may wonder, what value should I use?

Based on our experience, after analyzing a large number of models, we recommend using an efficiency of 85% (0.85):

There are power banks that have an efficiency higher than 90% as there are also those that have it below 80%, but if our power bank is of a good enough quality, its discharge efficiency will be around 85% *.

*Note that this efficiency may be slightly reduced when working with fast charging protocols.

We have already seen how to calculate the usable energy of a power bank, if we want to know what is its real capacity at the output port for a given charging voltage, we simply have to express the above equation in terms of capacity and voltage:

Output Energy = Efficiency x Stored Energy

Output Capacity (mAh) x Charge Voltage (V) = Efficiency x Battery Capacity (mAh) x Nominal Battery Voltage (V)

Next, we solve our variable which would be the actual capacity at the output port and finally we get the general formula:

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

This formula is valid for any charging voltage if, for example, we want to calculate the real capacity at the output port of a power bank for a charging voltage of 5V we would apply the following formula:

Actual Output Capacity (5V) = 0.85 x [Battery capacity (mAh) x Battery voltage (V)] / 5V

In case you indicate the power bank capacity in energy terms (watt-hours Wh) you can use the following formula:

Actual Output Capacity (5V) = (0.85 x Stored Energy (Wh) / 5V) x 1000

Example 4: Calculation of Usable/Output Energy and Real Capacity

Finally, we are going to apply these formulas with our power bank:

We have to remember that this model has an internal battery of 10000 mAh capacity and 3.85V voltage and we want to know what is its usable energy and its capacity at the output port for a standard charging voltage of 5V.

Let’s start by calculating the usable/output energy of the power bank assuming it has an energy efficiency of 85%:

Output Energy = Efficiency x Stored Energy = 0.85 x (10000 mAh x 3.85 V) = 32725 mWh / 1000 = 32.725 Wh

This power bank stores an energy of 38.5 watt-hours (Wh) and is capable of supplying 85% of that energy, therefore, its usable energy is 32725 mWh or 32.725 Wh.

If we want to know what is the capacity at the output port for a voltage of 5V:

Actual Output Capacity (5V) = 0.85 x (10000 mAh x 3.85V) / 5V = 32725 mWh / 5V = 6545 mAh

As we can see this 10000 mAh power bank has a capacity of 6545 mAh at its output port for a voltage of 5V.

Furthermore, we check that the energy stored in the power bank (38500 mWh) does not match the energy supplied at the output port (32725 mWh) because part of it is lost as heat:

10000 mAh x 3.85V = 38500 mWh ≠ 32725 mWh = 6545 mAh x 5V

The remaining energy (38500 – 32725 = 5775 mWh) has been lost during the voltage conversion process (from 3.85V to 5V).

To sum it up

In this section we have learned:

  1. That the energy at the output port of a power bank is known as usable energy and is ALWAYS going to be less than the energy stored in its battery.
  2. That the usable energy of a power bank will depend on the quality of its components (battery condition and efficiency of the electronic circuitry).
  3. That the energy efficiency of a power bank is the ratio between the energy supplied at its output port and the energy stored in its battery.
  4. How to calculate the usable energy (theoretically) and the real capacity at the output port of a power bank knowing its specifications and assuming an energy efficiency of 85% when its battery is completely discharged.

Formulas

  • Stored Energy (Wh) = [Battery Capacity (mAh) x Battery Voltage (V)] / 1000
  • Energy Efficiency (%) = [Output Energy (Wh) / Stored Energy (Wh)] x 100
  • Output (or Usable) Energy (Wh) = Stored Energy (Wh) x 0.85
  • Actual Output Capacity (mAh) = [Output Energy (Wh) / Charging Voltage (V)] x 1000

Use Case

We applied the formulas with the data of the power bank used as an example in this section assuming it has an energy efficiency of 85%:

Ugreen Power Bank Data:

  • Internal Battery Capacity of 10000mAh and nominal voltage of 3.85V.
  • Discharge Energy Efficiency of 85%.

Results:

  1. Stored Energy (Wh) = (10000mAh x 3.85V) / 1000 = 38.5 Wh
  2. Usable Energy (Wh) = 38.5Wh x 0.85 = 32.725 Wh
  3. Actual Output Capacity at 5V (mAh) = (32.725 Wh / 5V) x 1000 = 6545 mAh

How to measure the actual output capacity of a Power bank?

So far, we have seen some formulas that allow us to know what would be the usable energy and the real capacity of any power bank before buying it.

These formulas are based on the assumption that a generic power bank has an energy efficiency of at least 85% when fully discharged.

However, as we will see below, energy efficiency may vary from one power bank to another.

Comparison of real vs. theoretical data

If we have already purchased a power bank and we want to know its real capacity, usable energy and energy efficiency, it would be necessary to discharge it completely (from 100% to 0%) by connecting to an output USB port an electronic load at a constant voltage (V) and current (A) and a multimeter to measure the total energy provided.

Let’s discharge our Ugreen power bank with a 10W electronic load (5V/2A) and compare the data obtained by the multimeter with the results of the formulas presented in the previous section:

Discharge Test Stored Energy Output Energy Efficiency Output Capacity
10W (5V-2A) 38.5 Wh 35 Wh 90.78% 6769 mAh
Theoretical (formulas) 38.5 Wh 32.725 Wh 85% 6545 mAh

We see that the real and theoretical data are quite similar and even for this model we obtain better results than predicted by the formulas.

In short:

For the theoretical calculation of the usable energy and real capacity of a power bank we can use another value of energy efficiency, for example 80% or 90%, however, we consider that 85% is a valid average value for pretty much every power bank.

What does Rated Capacity mean?

Although it is not very common to find in the specifications of a power bank information about the actual capacity at the output port, some manufacturers are beginning to include this information.

An example of this is the PB178 model from Ugreen:

  • Rated Capacity: 6000mAh (TYP 5V-3A)

This indicates that the manufacturer guarantees that a power bank of this model, with a battery capacity of 10000 mAh (38.5 Wh), can supply at least 6000 mAh (30 Wh) through one or more USB output ports when a charge of 15W (5V/3A) is demanded.

Moreover, in this case, the 15W matches the total power that this model can supply (Total Output: 5V-3A).

Out of curiosity, we wanted to perform this same test with our power bank and confirmed that the guaranteed power (30Wh) is supplied:

Discharge Test Stored Energy Output Energy Efficiency Output Capacity
Ugreen 38.5 Wh 30 Wh 77.92% 6000 mAh
PowerBank20 38.5 Wh 33.62 Wh 87.32% 6580 mAh

As an additional note you should know that the energy efficiency of a power bank also varies according to the type of discharge performed, i.e., the more current demanded (3A vs. 2A) the more energy will be lost as heat.

More real-world examples: The PowerBank20 project

In order to recommend the best power banks on the market, one of the tests we perform in PowerBank20 is the analysis of the energy efficiency of the power bank when it is completely discharged.

In the following table you can see some of the data we recorded in this test for models analyzed on our website of well-known brands in the market:

BrandModelCell TypeBattery Capacity (mAh)Battery Voltage (V)Input Energy (Wh)USB Output PortOutput Capacity 5V (mAh)Output Energy (Wh)Efficiency (%)
BlitzWolfBW-P6LiPo100003.737.0USB-A647332.387.3
TronsmartPBT10LiPo100003.737.0USB-A628531.384.59
XiaomiPLM02ZMLiPo100003.8538.5USB-A664633.8487.9
XiaomiPLM09ZMLiPo100003.737.0USB-A635831.8286.0
XiaomiPLM01ZMLiPo101803.8539.19USB-A711735.4690.48
AnkerA1261LiPo100003.8538.5USB-A626331.9182.88
RAVPowerRP-PB077LiPo100003.838.0USB-A648033.287.37
ElecjetGen 4Graphene90003.733.3USB-A528626.1978.64
OmarsOMPB10KLiPo100003.737.0USB-A/USB-C688334.6893.73
TqkaKA023LiPo100003.737.0USB-A/USB-C642832.5487.95
KinpsKP-S010LiPo100003.737.0USB-A643932.6888.32
UgreenPB108LiPo100003.838.0USB-A/USB-C686434.8991.82
DuracellPB318650 Li-Ion100503.6336.5USB-A593029.781.37
PoweraddMP-TC018GYLiPo100003.737.0USB-A/USB-C682934.7393.86
AideazID1001LiPo100003.636.0USB-A/USB-C707435.8399.53
TronsmartPBD01LiPo100003.737.0USB-A/USB-C657633.0389.27
AukeyPB-N50LiPo100003.737.0USB-A650732.2387.11
BaseusBS-P10KQ02LiPo100003.737.0USB-A/USB-C667332.6588.24
ZendureZDA3TC18650 Li-Ion100003.737.0USB-A/USB-C550727.4774.24
XnuoyoXP2LiPo100003.737.0USB-A469323.4663.41
CharmastW1040PLiPo104003.738.48USB-A/USB-C660032.985.5
OzkuC1001LiPo100003.737.0USB-A/USB-C653332.9889.14
ZendureZDA3PD18650 Li-Ion100003.737.0USB-A/USB-C594329.9580.95
PoweraddSlim 218650 Li-Ion50003.718.5USB-A292314.6579.19
AukeyPB-Y13LiPo100003.737.0USB-A/USB-C648431.9786.41
AnkerA110918650 Li-Ion50003.718.5USB-A329816.8691.14
AukeyPB-N5418650 Li-Ion50003.718.5USB-A328216.2587.84
AukeyPB-N4118650 Li-Ion50003.718.5USB-A307615.382.7
BonaiBNPBM58-9GN18650 Li-Ion58003.721.46USB-A292814.5767.89
OmarsSlim PackLiPo50003.718.5USB-C328716.8991.3
RomossQS05LiPo50003.718.5USB-A338116.7890.7
RAVPowerRP-PB06018650 Li-Ion67003.724.79USB-A367918.7775.72
XiaomiPLM10ZMLiPo50003.718.5USB-A301915.1782.0
PoweraddEnergyCell21700 Li-Ion100003.737.0USB-A562127.6674.76
PoweraddEnergyCell21700 Li-Ion50003.718.5USB-A272513.8374.76
TeckNetIEP1010LiPo100003.737.0USB-A711035.7496.59
XnuoyoXPB-1W18650 Li-ion100003.737.0USB-A604630.0981.32
OmarsSlim PackLiPo100003.737.0USB-A644932.8888.86
AukeyPB-XN10LiPo100003.737.0USB-A628332.0786.68
XiaomiPLM03ZMLiPo100003.8538.5USB-A719835.4792.13
OmarsOMPB20KPLTLiPo200003.774.0USB-A1415869.1593.45
AideazID1002LiPo200003.672.0USB-A1440570.4497.83
AukeyPB-T10LiPo200003.774.0USB-A1202661.9283.68
ElecjetA5Graphene50003.718.5USB-A314615.9186.0
PoweraddPilot X7LiPo200003.774.0USB-A1353869.193.38
ZMIQB822LiPo200003.774.0USB-A1390270.5895.38
XiaomiPLM07ZMLiPo200003.774.0USB-A1453471.0696.03
AukeyPB-N36LiPo200003.774.0USB-A1199859.8480.86
AukeyPB-Y11LiPo200003.774.0USB-A1357070.0994.72
AukeyPB-Y14LiPo200003.774.0USB-A1380771.3896.46
XiaomiPLM11ZMLiPo100003.737.0USB-A649632.9288.97
RAVPowerRP-PB043LiPo201003.774.37USB-A1114856.475.84
PoweraddEnergyCellLiPo200003.774.0USB-A1403170.7395.58
RAVPowerRP-PB159LiPo201003.672.36USB-A1221060.3283.35
RomossSW20 ProLiPo200003.774.0USB-A1291265.1788.07
UgreenPB132LiPo200003.774.0USB-A1415771.7897.0
TeckNetiEP12000LiPo200003.774.0USB-A1300565.5888.62
LitioniteVulcanLiPo200003.774.0USB-A1279964.7187.45
dodocoolDP13LiPo201003.672.36USB-A1321264.1988.71
BlitzWolfBW-P8LiPo200003.774.0USB-A1367965.788.78
CharmastW2002PLiPo268003.799.16USB-A1384671.4872.09
PoweraddPilot Pro 4LiPo268003.799.16USB-A2126392.2693.04
PoweraddEnergyCell ll18650 Li-ion100003.737.0USB-A551727.5674.49
LitioniteNJF-2LiPo250003.792.5USB-A1151959.2664.06
YaberYR700LiPo220003.781.4USB-A1114556.6669.61
X-DragonXD-PB-021LiPo268003.799.16USB-A1101456.1456.62
OmarsOMPB20PW40GYCJNL18650 Li-ion200003.672.0USB-A1278164.2389.21
RAVPowerRP-PB18621700 Li-ion100003.6336.3USB-A645431.6887.27
EC TechnologyPB0518650 Li-ion268003.799.16USB-A1380467.3867.95
XiaomiPLM06ZMLiPo200003.774.0USB-A1385168.4992.55
BlitzWolfBW-P9LiPo100003.737.0USB-A632932.1486.86
FlyLinkTechJ17BLiPo268003.799.16USB-A1375170.871.4
RedmiPB200LZMLiPo200003.774.0USB-A1393171.9397.2
TackLifeT8LiPo180003.766.6USB-A977249.3774.13
BlavorPN-W12LiPo200003.774.0USB-A1396268.7892.95
PoweraddEnergyCell llLi-ion268003.799.16USB-A1743085.2986.01
UgreenPB178LiPo100003.8538.5USB-A676934.9590.78
CharmastC2023LiPo238003.788.06USB-A1324268.5677.86
BaseusBS-30KP365LiPo300003.8114.0USB-A21284109.4295.98
AnkerA123918650 Li-ion100003.6336.3USB-A608630.8584.99

If you are curious about all the data for each power bank analyzed you can visit this section.

Conclusions

Before buying a power bank:

  1. The usable energy and actual output capacity can be estimated by knowing only their specifications and establishing a theoretical energy efficiency of 85%.
  2. Once we know the usable energy of the power bank we can estimate the number of charges it could perform on a cell phone.

How to calculate the number of charges you can get from a Power Bank

To calculate the number of charges that a power bank can deliver to a device we need to know:

Power Bank Usable Energy

The usable energy of a power bank is the energy available at its USB output port for transferring to a device.

To calculate it, it is necessary to know the energy stored in the power bank’s battery and its energy efficiency when discharged (recharging the device) at a given voltage.

Power Bank Usable Energy = (Stored Energy x Efficiency) / 100

Remember that the energy efficiency of a power bank will depend on the quality of the model and the type of charging performed on the device. However, as a guideline value, we recommend using an efficiency of 85%.

Power for Device Recharging

The power for device recharging is the energy required by its internal battery for a full recharge (from 0% to 100%).

To calculate it, you need to know the energy stored in the device’s battery as well as its recharge energy efficiency (including the charging cable):

Power for Device Recharging = (Stored Energy / Efficiency) x 100

The energy stored in a device can be found by checking the specifications in the user guide or googling ‘mah (or wh) + device name’.

On the other hand, each device will have its own energy efficiency when recharged, with a value between 80 and 90%, logically, it will depend on many factors:

  • Quality and state of conservation of the charging circuit.
  • Charging cable: length, section and state of conservation.
  • Condition of the device’s battery itself.

However, as with a power bank, we recommend using 85% as a guideline value.

Once we know this data, we simply apply the following formula to obtain the number of charges:

Number of Charges = Power Bank Usable Energy / Power for Device Recharging

Remember, this is a general formula, and it works for any device that can be recharged with a power bank (cell phone, tablet, smartwatch…).

On the other hand, its result, logically, is not 100% accurate but it helps us to have an idea of how many approximate charges can the power bank that we are thinking of buying to charge our devices provide.

Example: How many charges are 10000 mAh?

Here, we will estimate the number of charges that we would have on our Bq Aquaris X2 Pro cell phone, which has a 3100mAh battery, assuming that we want to buy the 10000 mAh power bank from Ugreen.

Subsequently, we will compare the result with the actual data obtained from the measurements once we have purchased the power bank.

The formula for estimating the number of charges on the Bq Aquaris X2 Pro cell phone using the Ugreen power bank is as follows:

Number of charges = Power Bank Usable Energy / Power for Device Recharging

The following table shows the specifications of both devices:

Specs Ugreen PB178 Power Bank Bq Aquaris X2 Pro Smartphone
Battery Capacity 10000 mAh 3100 mAh
Battery Voltage 3.85V 3.85V
Energy Stored 38500 mWh = 38.5 Wh 11935 mWh ~ 12 Wh

Let’s calculate the usable energy of the Ugreen power bank assuming it is capable of supplying 85% (0.85) of its stored energy (38.5 Wh):

Power Bank Usable Energy = 10000 mAh x 3.85 V x 0.85 = 32725 mWh ~ 33 Wh

Next, we calculate how much energy our cell phone would need to recharge its battery assuming that the recharging process (cable, voltage conversion…) has an efficiency of 85% (0.85):

Power for Device Recharging = (3100 mAh x 3,85 V) / 0,85 = 14041 mWh ~ 14 Wh

We can see that the battery of the Bq Aquaris X2 Pro needs to receive approximately 2000 mWh extra energy (14000 – 12000 mWh) to fully recharge its capacity of 12 Wh.

Finally, we calculate the estimated number of charges:

Number of charges = 33 Wh / 14 Wh = 2.36 charges

Therefore, we know that if we buy the Ugreen power bank of 10000 mAh capacity we would have for 2 full charges (from 0% to 100%) on our Bq Aquaris X2 Pro mobile and, in addition, we would still have energy for a third partial charge (from 0% to 36%).

For the above calculation we have seen that an efficiency of 85% has been used both for discharging the power bank and for recharging the device.

While the energy efficiency in recharging the device can be checked if you have the necessary measuring equipment, we do not know the efficiency in discharging a power bank before buying it.

For this reason, we say that the calculated number of charges is a guideline but valid enough for anyone who is interested in buying a power bank and wants to have an approximate idea of the number of charges that would have on your device without making the mistake of dividing the capacity of the power bank by the capacity of the device:

Number of charges = 10000 / 3100 = 3.22

However, for our more curious readers, let’s check the actual number of charges we would have on the mobile after purchasing the Ugreen power bank.

Data validation

The following tables show the results obtained from the tests of recharging the mobile and discharging the power bank both for standard charging (the usual charge for any device with USB charging port) and for Quick Charge 3.0 fast charging and Power Delivery (this mobile is compatible with both protocols).

We also verified that the average efficiency obtained in both tests differs from the 85% established in the theoretical formulas.

Bq Aquaris X2 Pro Smartphone

Type of Charge Charging Cable* Stored Energy Power for Device Recharging Efficiency
Standard (5V) USB-A a USB-C 11.94 Wh 14.63 Wh 81.61 %
Quick Charge 3.0 USB-A a USB-C 11.94 Wh 15.20 Wh 78.55%
Power Delivery USB-C a USB-C 11.94 Wh 14.70 Wh 81.22 %
Average 11.94 Wh 14.84 Wh 80.46 %*
  1. We note that this cell phone has an average recharge efficiency of 80% with a required power supply of approximately 15Wh.
  2. The charging cable is taken into consideration for the measurement of the recharging energy of the device.

Ugreen PB178 Power Bank

Discharge Type USB Port Electronic Charge* Energy Stored Output Energy (usable) Efficiency
Standard (5V) USB-A 10W 38.5 Wh 34.95 Wh 90.78 %
Quick Charge 3.0 USB-A 14W 38.5 Wh 34.04 Wh 88.42 %
Power Delivery USB-C 14W 38.5 Wh 34.05 Wh 88.44%
Average 38.5 Wh 34.35 Wh 89.22 %*
  1. We observed that this model is capable of supplying more energy (90%) than what we established for the theoretical calculation (85%).
  2. To measure the energy that a power bank can supply through its output USB port we perform a complete discharge by connecting an electronic load at a constant power. Taking into account the charging power of a cell phone we performed our tests with 10 and 14W for standard and fast charging respectively.

With these data we calculate the actual number of charges by applying the above formula:

Type of Charge Power Bank Usable Energy Power for Device Recharging Number of Charges
Standard (5V) 34.95 Wh 14.63 Wh 2.39
Quick Charge 3.0 34.04 Wh 15.20 Wh 2.24
Power Delivery 34.05 Wh 14.70 Wh 2.32
Average 34.35 Wh 14.84 Wh 2.31
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