Critical Points

Ignore mAh

mAh is what power banks are usually advertised under, and may give you a rough idea of capacity, but it doesn’t tell the whole story. The specification is incomplete as it does not include Voltage.

And mAh listed can be used to hide the true capacity of a battery. Ignore mAh!

Look at Wh (Watt hours)

Instead, look at the Wh (Watt Hour) spec., which will be a better indicator of total power capacity, making it far easier to compare different power banks to each other, then estimate how much you can charge your other electronic devices.

Capacity Loss Affects All Batteries: My 80% Rule of Thumb

Take into consideration that no battery delivers the amount of power spec’d because of inefficiencies throughout your electronics. As a rough way to estimate this, I take 80% of the Wh number from my battery and then compare the Wh capacity of my electronics. 

And that’s all you need to know! Dive deep down below for the delicious details:

Introduction

For long distance, self-powered trips where I’m trying to optimize everything, the electronics I bring are often in my critical sights. As small as they are, they’re heavy for their volume – especially when compared to other items like clothing. 

But electronics can also be extremely useful. My satellite communicator lets loved ones know where I am and allows me to call for help in an emergency. My phone is also my map and navigation system, my music player, my camera, and many other utilities. My head lamp allows me to cover some miles at night. My action video cameras allow me to share my mis-adventures upon an unsuspecting audience. 

To keep all these powered, I also bring a small USB power bank. Going overboard on how much power capacity to bring is easy to do (packing your fears!), but bringing too little capacity may mean you won’t have enough power to recharge all of these conveniences we just mentioned.

And believe me: I have been on the side of the road, sun threatening to set, trying to contact someone (anyone!) for a ride from the trailhead to town after many days in the Wilderness, racing the “1% battery” warning on my phone, wondering if my imminent future has me sprawling on a motel bed, devouring a large pizza or if I’m making due with the bitter dregs collected from my leftover food bags, huddled in a fetal position to stave off the cold desert night air relegated to attempting a hitch in the morning.

So, how do you pick out the right power bank size – or at least keep your expectations realistic?  

USB Power Banks Are Often Mislabeled

The naive approach – the approach I’ve used myself – is to purchase a lightweight power bank based on the very optically obvious mAh label. mAh is often used as the de-facto specification to market the capacity of a power bank.

For example, a 10,000-mAh sized power bank seems about right for a few days rambling given the rest of what I’m bringing. For example, my phone is spec’d at 3,000mAh, so I can estimate that I should get three full charges out of a 10,000mAh power bank. Easy!

With that out of the way, I can just compare weight of the power bank itself and make my purchase choice on how much each option weighs.

And I would be very wrong. 

Why is mAh the Wrong Specification to Look At

Several problems can be pointed out in this approach. One is that mAh is just the wrong specification to look at when comparing two components, since mAh doesn’t tell us the whole story of how much power capacity is in a battery. But it is often used as the most important spec. of a power bank — sometimes with mAh being integrated with the name of the product itself. Observe all these different power banks and their use of mAh in their marketing copy:

Why is it used? Probably because marketing using a big number is effective. 

Use Wh Instead

To be more accurate, we also have to know the battery pack’s native Nominal Voltage. Batteries are chemistry and the voltage they produce varies depending on many different factors, so the average voltage of the battery could also be different between different batteries.

With both mAh and Nominal Voltage, we can get a better idea of the true capacity in Watt Hour (henceforth: Wh) of a power bank. Wh can be found just by multiplying mAh and Voltage. If you’re lucky, Wh should also be listed in the spec. sheet of the power bank you’re wishing to purchase – check to see if it’s listed on the power bank itself as well. And if you’ve won the Power Bank Truth Lottery, all three of these specifications will line up.

mAh x Voltage = Wh

Voltage is often similar on power banks advertised with similar mAh ratings, but small discrepancies add up to big difference in actual, total capacity – and you: the consumer have to be careful. Let’s explore how these small differences impact actual power bank capacity, and how it matters in the long run:

I have two battery packs that I’ll be using to illustrate these concepts. I’ve picked these two because they’re what I have on hand (and full disclosure: that I’ve bought myself at retail), They’re reasonably similar in capacity if we look at mAh and I can test them directly. I originally bought these because they were small, high capacity, and pretty lightweight. Weight – or really: power capacity for the amount of weight I am to carry – is what’s most important to me for light and fast self-powered trips.

The two batteries are the following: 

Power Bank	Listed mAh	Listed Nominal Voltage	Listed Wh	Weight (grams)	Listed Capacity/grams
GoalZero Flip 36 Gen 5	9600	3.6	35.52	194	.183 Wh/gram
Nitecore NB10000 Gen 3	10000	3.85	38.5	151	.254 Wh/gram

The GoalZero Flip 36 is listed as a slightly smaller capacity battery both in mAh and WH, with a lower nominal voltage compared to the Nitecore NB10000 Gen 3. If we look at just mAh (9600 v 10000) it should have 96% of the capacity.

If we look at Wh, it should have 92.26% the capacity of the NB 10000 Gen3. But strangely, Goal Zero’s own spec sheet doesn’t line up, as 9600mAh x 3.6V does not come to 35.52Wh, but 34.56Wh or 89.8% the capacity of the NB10K Gen3. We’ll keep that in mind, but specifications not lining up could be a red flag. Confusing, but I’ll use 35.52Wh for our discussion.

Now that we have the Wh specifications for our two power banks, how can we approximate how useful they are? For example, how many times will these two charge my phone?

Goal Zero’s own spec. sheet for the Flip 36 does state that it will charge a phone approx. 3x. But phones come in all shapes and sizes, and this is no more than a rough measure. It does coarsely track: my phone’s own battery has a 11.97 Wh capacity, and 35.53 Wh is roughly 3x larger. I could change thrice, and have charge left over!

But,

Take Into Account Capacity Deprecation From The Loss of Efficiency

As I’ve mentioned, this will be incorrect, because we haven’t taken into consideration the loss of power efficiency between the internal battery in the power bank and the phone. This comes in many forms: electrical resistance in the charging cables, loss from heat, as well as loss of efficiency from changing voltage. 

That last one needs more explanation: Many electronics – like your phone – charge at 5V, but the nominal voltage of both of these power banks is lower — again: batteries are chemistry, not really a always-available source of electrons. A chemical reaction has to happen to turn the energy into electricity. So, the voltage needs to be normalized and converted to 5V to charge your phone from the power bank. 

As a rule of thumb, you can expect a loss of efficiency of your battery pack upwards of 20%. That means our 35.52 Wh battery pack is probably closer to being a 28.4 Wh battery pack, while our 38.5 Wh battery pack is actually a 30.8 battery pack. You may start to see how all these advertised specs. aren’t helping with getting straight answers!

With these new figures, we can see that neither a 28.4 Wh or 30.8Wh power bank will recharge a 11.97 Wh capacity phone battery three times. Instead, we need to temper our expectations. If we really need to charge our phone completely from 0% to 100% three times, we’ll need a larger power bank than either of the power banks I have on hand.

Abide by the Three Rules When Choosing a Power Bank

So, using three rules: ignore mAh, Use Wh, and only trust 80% of capacity, we can make far more informed decisions when buying a power bank, whittling down our choices for what we want, then being able to compare other features (like weight, price). 

Testing Capacity in Power Banks

Can we validate the above information? Well, we can substantiate much of this by performing a discharge test on our power banks using an inline USB power meter that shows how many total Wh are being taken out of the power bank, until the power bank stops pushing power out. I’ve done this to our two power banks: 

Here are the results: 

Battery Pack	Listed Nominal Voltage	Listed mAh	Measured mAh @ 5V	Listed Wh	Measured Wh	Wh Diff
GoalZero Flip 36	3.6	9600	5710	35.52	28.16	79.28%
Nitecore NB10000 Gen 3	3.85	10000	6761	38.5	33.92	88.10%

In our two examples, you’ll note the difference in Actual Wh I recorded and the Listed Wh being advertised. As I stated, we’ll chalk this up to inefficiencies, rather than thinking either manufacturer is misleading us with false advertising and selling us a smaller capacity battery illegitimately (this does happens, but it’s usually pretty easy to spot a counterfeit). And to be clear again: this inefficiency happens to all power banks. 

But our test results are suggesting that the inefficiency is different between the two power banks with the GoalZero reporting 79.28% the Wh than it advertises compared to the 88.10% compared to the NiteCore. 

This could be explained in a few ways. To be fair (and to be clear, I didn’t start this article to disparage Goal Zero), my Goal Zero is a year older than the NiteCore, and batteries lose capacity over time. Another explanation is that the Goal Zero design is really just more inefficient. Internally, it does use 3, 18650 batteries wired up in parallel, whereas the Nite Core uses one battery. I would love to re-do my test with a fresh Goal Zero battery.  Another theory is that the NiteCore is actually selling a slightly larger batter than the spec.

Since weight – and weight to power capacity is important to me when choosing a power bank, the NiteCore would be the better pick. With the actual capacity in Wh put into play, the NiteCore now gives me .224 Wh/gram compared to .145 Wh/gram of the Goal Zero. As we’re starting to more clearly see who small differences in advertised specifications become larger differences once we start using the power banks in the real world.

What if the Wh on my Power Bank is Not Advertised?

Most electronics used for backpacking charge at 5V¹  and hopefully list at least its mAh, which we can use if all else fails. 

Nominal voltage is the general voltage a power bank pushes out before the voltage is stepped up to 5V, so we need to convert our power bank specifications to 5V. This again is why mAh is so tricky to use: since nominal voltage is perhaps different between power banks, actual Wh capacity will also be different even if the mAh spec. is the same – and as we proved above, small differences lead to big differences in actual, measurable capacity.  

Here are some formulas to use: 

Conversion Formulas

Calculating “Actual” mAh from Advertised mAh

To get the actual usable mAh from the advertised mAh use this: 

(Advertised mAh * (Nominal Voltage  / 5V )) * Efficiency = Actual mAh. 

Then we can compare different electronics using mAh, so long as they are charged at 5V. 

For example, we measured the efficiency of our NiteCore at 88.10%, (I use 80% for “Efficiency” when I’m not certain). I know that the nominal voltage of the NiteCore NB10000 Gen3 battery is 3.85V according to its spec sheet, we can fill in everything for mAh: 

(10000mAh * (3.85V / 5V)) * .88 = 6776 mAh 

This closely matches our test results of 6761 mAh. Only do this if all else fails — stick with Wh to get a better estimate.

Calculating Wh from mAh and Nominal Voltage

As we mentioned, Wh is just mAh multiplied by Nominal Voltage. Then, just multiply this with the efficiency:

((Advertised mAh *  Nominal Voltage) / 1000) * Efficiency = Actual Wh

So,

((10000mAh * 3.85) / 1000) * .88  = 33.88 Wh

Which closely matches our measured Wh of 33.92 Wh.

Comparing Power Banks with Electronics

Armed with this information, we can then finally compare electronics against each other. The trick is to compare Watt hours (Wh) only!

For example, the NiteCore Nu20 Classic head lamp has an internal battery specified as at 500 mAh, 3.7V, so we can figure out the Wh as so:

((500 * 3.7) / 1000) = 1.85 Wh

We can take 33.92 Wh and divide it by 1.85 Wh, giving us 18.3x we could recharge this headlamp.

If you don’t know the nominal voltage, use 3.6V for your estimates for lithium ion batteries.

Power Bank Charging Estimates of Example Electronics

Given my own two power banks, here are reasonable estimates on how many times I can charge some of the electronics I use, when using their spec’d Wh or by estimating their Wh given their stated mAh. I added another column to show how this compares to using the naive approach of using mAh, without taking into consideration nominal voltage or inefficiencies:

Product	mAh	Nominal Voltage	Wh	Naive 1000mAh	With Goal Zero Flip 36 (28.16 Wh)	With NiteCore NB10k Gen3 (33.92 Wh)
iPhone 13 Pro	3095	3.83	11.97	3.2x	2.4x	2.8x
Garmin inReach Mini 2	1200	3.6	6	8.3x	4.7x	5.7x
Fenix HM62-T Headlamp	3400	3.6	12.24	2.9x	2.8x	2.3x
Black Diamond  Distance LT 1100	2200	3.7	8.14	4.5x	3.46x	4.17x
NiteCore NU20 Classic	500		1.85	20x	15.6x	18.3x
GoPro Max Enduro Battery	1600	3.85	6.16	6.25x	3.6x	4.2x
Insta360 x4 Battery	2290	3.85	8.82	4.4x	2.5x	3x
GoLab Air Pop Earbuds	350	3.6 (estimate)	1.26 (estimate)	28.6x	22.3x	26.9x

Here’s a Google Sheet of the above and you can play with the numbers as well.

And that’s how I can more accurately estimate how much power capacity to bring and what to really expect from that power capacity. 

Glossary of Terms

I am not an Electrical Engineer, and one of the reasons for me to write about things like power banks is to understand the gear I use a little more closely. Thankfully, the Water Analogy of Electricity has been useful for me as a visual learner who holds an Art Degree in trying to understand concepts that I wasn’t taught in school. Here is how I interpret the concepts: 

• Volts: Electrical Potential. The pressure built up of electrons. I imagine this like the water pressure created by a body of water that’s held back by a dam. And with a dam, we can control the amount of water that flows out. If we had a small pipe as the only outlet for water, it would take a long time for all the water to drain out at a controlled rate. But if the dam failed, it would drain out in a stupendous rush, only taking a few minutes. The capacity of that dam is the same (more on that). 

• Amps: Unit of measure for electrical current (flow rate). I imagine this similar to the amount of water that can come out of the dam, which is dependent of the size of the pipe coming out. A larger diameter of pipe would allow more flow, but this doesn’t quite tell me how much water has come out, because we don’t know how big my dam is.  

• Watts:  The rate at which electrical energy is transferred. Watts = Volts x Amps. If my dam was full to the brim (a lot of pressure!), even with a small diameter pipe at the bottom (low amps), water would still come out quite quickly, delivering to me a lot of water. If that pipe had a larger diameter, I could get more water still. But, if the dam’s water pressure was halved, the amount of water coming out of either pipe would be halved too.

• Watt Hours: power in (Watts) over time (Hours). Similar to the total volume of water that we delivered through our pipe. You can think of this as an actual capacity (volume) we measured and then can easily compare between two different containers. If we were able to fill an entire town’s swimming pools with what water was in our dam (leaving it bone dry), we know that the capacity of all the swimming pools and our one dam are equivalent. 

• mAh (milliampere hour): power measurement (amp) over time ( hour). This is a problematic measurement. If you look back on my water dam analogy, amps are the size of the pipe, but we have no idea how large the dam (potentiality/pressure) is! Once Volts (potential energy) are known, we then have a good idea of actual capacity, which is what the Watt Hour is. 

If you would like to have someone smarter than I, who didn’t fail high school Physics explain this to you, this video was what unlocked the concepts for me the best,

Footnotes

1. It’s true, that not ALL accessories charge at 5V. Some electronics are intelligent enough to negotiate a different voltage power delivery (PD). Just another reason to use Wh, rather than mAh! ↩︎