Choosing a Portable Solar Power Bank

Choosing a Portable Solar Power Bank


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If you are thinking of choosing a portable power bank for your mobile devices, but just aren’t sure what to buy, or if you have one and it isn't performing as you thought it might, you are in the right place.  So many spec. So many details.  It can be confusing.  Let’s break it all down. This conversation is exclusive to lithium ion batteries, primarily used in portable power banks and portable electronic devices.

Let’s start with mAh:  It stands for milliampere-hour, or in other words, it is the amount of energy available that could possibly be stored in a given battery.  When selecting a portable power bank, start by comparing the mAh rating of your mobile device to that of your portable power bank. 

Now, it is tempting to do some “easy” math.  If you have a 5,000mAh power bank, and your cell phone internal battery rating is 3,200mAh, you might think that 5,000 / 3,200 would give you about 1.5 charges.  But you would be wrong.  Transferring stored energy from one battery to another is not 100% efficient. 

There are two things to consider when determining how much energy can be transferred from your power bank to your battery.

#1 – Power Bank Efficiency

Performance power banks are only about 85-90% efficient.  When a power bank is charging a phone, typically 10-15% of the charge is lost in the transfer of power in the form of heat due to the resistance of the battery and USB connection.  If you drain the power bank at low power for a long time (e.g. 1 Amp), you get better efficiency and less heat than if you drain it quickly at high power (e.g., 2-3 Amps).

#2 – Difference Between Storage/Output Voltage

More importantly, the voltage of a power bank’s stored energy is different from its output. Take, for example, a 5,000mAh capacity power bank. Its stored energy is 5,000mAh x 3.7V = 18.5 watt-hours (Wh).  Yes, although the Li-ion battery outputs 5V, the storage voltage is 3.7V.

Since the output capacity of your phone's battery is listed in mAh at 5V, you need to apply a conversion to be able to figure out how many full charges you can get. Using the Conservation of Energy principle (remember that from high school?), we can calculate the capacity of this power bank at 5V given that the transferred energy remains constant as 18.5 Wh /5V = 3.7 Wh/V = 3,700mAh.

In our example, a power bank listed with 5,000mAh at 3.7V corresponds to 3,700mAh at 5V. The former voltage corresponds to your power bank capacity and the later voltage to your phone charging voltage. This leads us to a conversion factor of 74% (74% of 5,000mAh = 3,700mAh).

Real-World Application

If you multiple the efficiency of the power bank (90%) times the 74% conversion factor based on the difference between storage/output voltage, you obtain the actual charger capacity:

              90% x 74% = 66.7%

In the real world, a 5,000mAh power bank would provide approximately 3,335mAH (5,000mAh x 66.7%), which would barely be enough to charge your 3,200mAh cell phone battery once. To determine which portable solar bank best suits your needs, use this formula:

              (mAh capacity of your cell phone / 66.7%) x desired number of charges

For example, the iPhone6 Plus has a 2,915mAh capacity battery.  If you wish to charge it twice, you would need a power bank with a capacity of at least 8,744mAh.

              (2,916mAh/66.7%) x 2 = 8,744mAh

A Nexus 10 (tablet) has a 9,000mAh capacity battery.  If you charge it twice, you would need a power bank with a capacity of at least 28,616mAH.  (I am using 62.9% efficiency in the formula below because the high Amp charging is less efficient.) 

              (9,000mAH/62.9%) x 2 = 28,616mAH

Keep in mind, now, that this math represents best case scenarios for charging your devices.  Other factors, such as age and degradation of lithium ion battery cells over time, and what you are doing with your device during a charge, will further reduce the number of times a portable power bank can charge your device. 

Alas! Don’t feel defeated!  Shop our most popular portable solar power sources to keep your mobile devices as charged and on the go as possible.

Shop Most Popular Portable Solar Power 

Meantime, stay tuned for our future blogs, where we will discuss solar panels in relation to charging your portable devices, power banks, and batteries, both small and large.  

Here is a table of common portable devices and their associated battery capacity expressed in mAh and watt hours.


Device Brand Model Capacity (mAh) Capacity (Wh)
Phone Apple iPhone 6 Plus 2915 14.6
Phone Apple iPhone 6 1800 9
Phone Apple iPhone 5S 1570 7.9
Phone Apple iPhone 5C 1507 7.5
Phone Apple iPhone 5 1440 7.2
Phone Apple iPhone 4S 1432 7.2
Phone Apple iPhone 4 1420 7.1
Phone Apple iPhone 3GS 1219 6.1
Phone Apple iPhone 3G 1150 5.8
Tablet Apple iPad 1 6613 33.1
Tablet Apple iPad 2 6944 34.7
Tablet Apple iPad 3 11560 57.8
Tablet Apple iPad 4 11560 57.8
Tablet Apple iPad Air 8820 44.1
Tablet Apple iPad Air 2 7340 36.7
Tablet Apple iPad Mini 1st 4440 22.2
Tablet Apple iPad Mini 2nd 6471 32.4
Tablet Apple iPad Mini 3rd 6471 32.4
Phone Samsung Galaxy S2 1650 8.3
Phone Samsung Galaxy S3 2100 10.5
Phone Samsung Galaxy S4 2600 13
Phone Samsung Galaxy S5 2800 14
Phone Google Nexus 6 3220 16.1
Phone Google Nexus 5 2300 11.5
Phone Google Nexus One 1400 7
Phone Google Nexus S 1500 7.5
Phone Google Nexus 4 2100 10.5
Phone Google Galaxy Nexus 1750 8.8
Phone Google Galaxy Nexus LTE 1850 9.3
Tablet Google Nexus 7 (tablet, 2012) 4325 21.6
Tablet Google Nexus 7 (tablet, 2013) 3950 19.8
Tablet Google Nexus 10 (tablet) 9000 45
Tablet Google Nexus 9 (tablet) 6700 33.5
Phone HTC HTC One 2300 11.5
Phone HTC HTC One (M8) 2600 13
Phone HTC HTC One (M7) 2300 11.5
Phone HTC HTC One mini 1800 9
Phone HTC HTC Desire 320 2100 10.5
Phone HTC HTC Desire 510 2100 10.5
Phone HTC HTC Desire 2100 10.5
Phone HTC HTC Desire C 1230 6.2
Phone HTC HTC One X 2100 10.5
Phone HTC HTC One V 1500 7.5
Phone HTC HTC One S 1650 8.3
Phone HTC HTC One X+ 2100 10.5
Phone HTC Windows Phone 8X by HTC 1800 9
Phone HTC Windows Phone 8S by HTC 1700 8.5
Phone Sony Xperia Z3 3100 15.5
Phone Sony Xperia Z3 Compact 2600 13
Phone Sony Xperia Z3 Dual 3100 15.5
Phone Sony Xperia Z3v 3200 16
Phone Sony Xperia Z 2330 11.7
Phone Sony Xperia Z Ultra 3050 15.3
Phone Sony Xperia Z2 3200 16
Phone Sony Xperia Z2a 3000 15
Phone Sony Xperia T3 2500 12.5
Phone Sony Xperia T3 LTE 2500 12.5
Phone Sony Xperia T2 Ultra 3000 15
Phone Sony Xperia M Dual 1750 8.8
Phone Sony Xperia M2 LTE 2300 11.5
Phone Sony Xperia M2 Dual 2300 11.5
Phone Sony Xperia Z1 3000 15
Phone Sony Xperia Z1 Compact 2300 11.5
Phone Sony Xperia Z1s 3000 15
Phone Sony Xperia C 2390 12
Phone Sony Xperia C3 2500 12.5
Phone Sony Xperia SP 2370 11.9
Tablet Sony Xperia Z2 Tablet 6000 30
Tablet Sony Xperia Z3 Tablet Compact 4500 22.5
Tablet Sony Xperia Z Tablet LTE 6000 30


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