Which is better for hiking the JMT - a solar panel, or a USB battery? Or both?

Let's look at one of the better solar panels that you might want to bring on the JMT - the Suntactics 5. 


SunTactics sCharger-5

Why is it called the "5" ? Because it can make 5 Watts of power. That's usually abbreviated to 5W. Is that good? Yes, as it turns out, that is very good for a hiker solar panel. It is the same amount of power that you get from your iPhone wall charger.

So lets plug the Suntactics 5 into an iPhone;

 The iPhone went "bloop" and the little lighting bolt symbol came up, so we are good, right? It's putting 5 Watts into the iPhone, so in a couple of hours my iPhone will be charged? 

Well, maybe. The trouble is, you don't really know what's going on. Maybe the phone is charging fast, maybe it's charging slow, maybe it's not doing much of anything. The only way to know is to measure how much juice is flowing. We need a tester:

USB tester


This little box measures the voltage and current being delivered to the iPhone. You just connect it in between the solar panel and the iPhone:

So we can see that in this particular case, with the sun at a certain angle in the sky, and the iPhone at a certain level of discharge, we happen to be seeing 4.7 Volts and 654mA of current. This panel is actually capable of delivering 1,000mA if it is facing the sun..


So is that the end of the story then?

Suntactics 5 = iPhone wall wart? 

Unfortunately there are a few things that are going to complicate matters.  

First of all, you are only going to get 5W of power out of the panel when the sun is shining. On the JMT that is probably all day, every day. But "probably" is not the same as "definitely". It is common to have overcast days, perhaps several in a row. Occasionally a whole week. On an overcast day you get basically nothing out of these solar panels, so the only guaranteed solution to this is to bring a USB battery as backup. More on this later.

The second problem is that the solar panel has to be pointing at the sun to get the full 5 Watts. So if you have it on your backpack you are going to be getting less. If you are southbound on the JMT, your best bet is to put the panel on top of the pack, but even then you are going to be getting much less power, especially in the early morning and late evening, when you will be getting essentially nothing. You can do a lot of complicated math to do with the cosine of the elevation and the azimuth, but the long and the short of it is you only get decent power when the sun is "up" ie late morning / early afternoon. Outside of that window, you will get very little power. 


The solution to this problem is to stop for lunch and set up the panel pointing at the sun, and to let the phone charge properly for an hour or so. An hour every day will be more than enough to keep the phone charged no matter how long you are out on the trail. More on that later.


Solar panel angled toward sun for increased power

The third problem is that iPhones do not like solar panels. iPhones expect to be charged by wall chargers. They like the steady voltage. They do not like it when a cloud passes over and interrupts the current. It sends them into a state of confusion, and they stop charging and go into sulk mode. 

 iPhone in "sulk" mode drawing only 89mA

In sulk mode they basically stop charging, and refuse to start charging until they feel like it, which might be in a few hours. They don't tell you they are sulking. Without a tester you would have no idea. You might notice your phone going bloop every time a cloud passed by, or every time you went under a tree, but apart from that you would be none the wiser. The Suntactics tries to get around this problem by resetting the power every few minutes, but I found that this did not really fix the issue. After a lot of experimentation, my recommendation is: 

Don't bother trying to solar charge your iPhone while you are hiking. It doesn't work.

The best solution to this is the same as before - just wait until lunchtime and set it all up properly and let it work uninterrupted. In fairness to Suntactics, that's exactly what they tell you to do in the instructions.

How much juice do you need to charge up an iPhone? I have an iPhone 6 Plus. The battery inside that is specced at 2910mAh. That means that if I were to charge that battery at 1 Amp (from a wall charger or solar panel) then it would take 2.91 hours to charge it up from 0% charge to 100% charge. So if you charged it for one hour every day from the solar panel you would get about 33% of a full charge each day. That is enough to keep an iPhone 6 charged indefinitely, since it only uses about 30% per day (in Airplane mode). 

If you are slow hiker, you could probably just stop right here. My recommendation to you is to bring a Solartactics 5, a USB tester to see if it's charging, and that's it. No battery. No wall-wart charger. Just set it up every lunchtime and let it do it's thing for an hour, and you can hike the whole JMT taking as long as you like. One hour per day is all you need. The Suntactics 5 weighs 8.75 ounces and the USB tester weighs 0.5 ounces. You could leave the tester behind, but then you wouldn't really know if your phone is charging, so for me it's worth bringing. 

 Would it be better to bring a battery instead? 

You might be looking at that solar panel and thinking 9oz sounds quite heavy, and maybe you could find a USB battery that weighs less than that. So then your first question is going to be "How big a battery?".

This is where most people start to get confused by all the technical jargon used in the USB battery ads. The reason it's confusing is that the manufacturers are deliberately trying to confuse you. They use a unit called mAh, short for milliampere-hours. I'm not going to get into too much on what that is (if you want to know, look here). The important thing to know is, it's not a measure of energy. It's definitely not a measure of what you can get out of the battery. But if you are savvy you can correct the mAh number on the battery into something real that you can actually use to figure out how many charges you can get for your iPhone.  So let's do that first.

 Here is a typical USB battery that I happened to have lying around:

It is the IntoCircuit 15,000mAh, and it was one of the first really good USB batteries to come on the market. By "good" I mean it has a 2 Amp recharge input, a digital readout of power remaining, and it uses high quality name-brand batteries inside. I like it a lot, and it has been a good choice for the JMT, keeping my iPhone going for 6 days between MTR and Whitney Portal. You can still buy it on Amazon, link here, and it is quite a good choice today, although there are better options. Here is what it looks like inside:


Those green things are Samsung 18650 batteries. That is a common type of battery, probably the most common (it powers the Tesla for example). The Samsung version is pretty good. Someone did a detailed test on it here, and the results are quite acceptable. Each of those 5 batteries holds a charge of 3,000mAh. The spec on the side of the battery says 15,000mAh. That comes from multiplying 3,000mAh by 5 batteries. This is the first gotcha. That figure of 3,000mAh applies to each battery, but those batteries are 3.7 Volts. The voltage coming out of the USB connector is 5V, because that's the USB standard and that's what your phone needs to charge. If you convert all the energy of the 3.7V battery to 5 volts, the capacity becomes 3,000mAh x 3.7 / 5 = 2,220mAh. For all five batteries the total is 11,100mAh.  There is a 26% reduction of the mAh due to the difference between 3.7V and 5V. Yes, the manufacturer is pulling your leg. A 15,000mAh battery is really an 11,100mAh battery.

That assumes the conversion from 3.7V up to 5V is done with perfect efficiency. But nothing is perfect. If you hold the USB battery while it is charging a phone, you will notice it gets warm. That heat has to come from somewhere, and it's coming from the batteries, which means it's energy not going into your phone. That inefficiency is unavoidable, and it typically runs anywhere from 80% to 98% depending on how much current you are taking out, and how good the circuit design is. A typical figure is probably somewhere around 90% at a current of 1 Amp. 

So the 11,100mAh battery is really only capable of delivering 11,100mAh x 0.9 = 9,990mAh. Quite a difference from the 15,000mAh printed on the side of the battery.

To summarize that, the mAh number for any USB battery is just the capacity of the internal batteries at 3.7V. To convert that to 5V equivalent, you must reduce that number by 26%. You then take off an additional 10% to account for the ineffiency of the voltage convertor, leaving you with about 64% of the original number.

We can confirm this by measurements. I charged up the "15,000mAh" battery and discharged it into a resistor and measured what I got out:


Charge out: 8,189mAh

So the 15,000mAh battery is really a 8,189mAh battery. I was expecting it to be a bit higher than that, based on the numbers above. 64% of 15,000mAh  is 9,600mAh. But this battery has been used a fair bit, so maybe the capacity is not what it was originally. Let's just allow that maybe it starts out at 9,600mAh and goes down from there. 

Anyway, the lesson is - don't believe the mAh number on the battery. The real figure is at least 36% below what it says it is.

Now that we know the real capacity of our USB battery (ie its 64% of whatever it says on the label) it becomes pretty straightforward to figure out how many times we can charge our devices. For example, if I have the battery above and an iPhone 6 Plus I do the following calculation:

Battery capacity (claimed): 15,000mAh

Battery capacity (actual): 15,000mAh x 0.64

= 9,600mAh

Capacity of iPhone 6 Plus battery: 2,915mAh (googled it)

Number of recharges = 9,600 / 2915 = 3.3

How many days hiking is that good for? Well, I get 2 days per charge, so if I start with a full battery and get 3.3 additional recharges I am good for 4.3 charges x 2 days = 8.6 days. That assumes the battery will be flat at the end of the hike, and that is when you most need some juice to arrange shuttle rides, hotels, etc, so factor in some margin for that. Let's say its good for 7 days.

So which is better - that USB battery or the solar panel? They are both similar weight (battery is 11oz, solar panel is 9oz). The battery will run out of juice on day 7 or 8, but the solar panel is good for an entire hike. On the other hand, the solar panel needs to be strapped to your pack, set up at lunchtime, and generally fussed with, whereas the battery just sits in your pack and you can use it whenever is convenient, like at night. And more importantly, the solar panel needs sun, and its not impossible on the JMT to run into several cloudy days in a row, which could leave you flat. These are factors you will have to weigh up yourself, trading off weight, convenience, and duration of hike. For me, the answer is to take a battery for hikes where the longest section is 7 days or less, and to go solar for longer hikes.

For hiking the JMT, my strategy is usually:

Happy Isles: Charge phone at store. Depart with no solar or battery.

Tuolumne Meadows:  Recharge phone at the store or permit office.

Red's Meadow: Collect small USB battery in resupply eg 3300mAh. Recharge phone at restaurant.

VVR/MTR: Collect larger USB battery in resupply eg 10,000mAh. Recharge phone at power strip. Leave discharged small battery in hiker barrel with the charger (with a note explaining it is good, but I didn't have time to wait for it to recharge).

That might seem a bit elaborate to some, but I am usually hiking with a base weight of maybe 12lbs and trying to do the whole hike in 10 days, so trimming ounces in the early part of the hike is important to me. If you are going heavier or slower, you might decide it's easier just to bring a larger battery and keep recharging it at the resupply points.

This brings us to one other problem with USB batteries. They have to be recharged. This can take a long time. Also, charging your phone can take a long time. If you are using the standard charger that came with your iPhone, that delivers 1 Amp. So it can take up to 3 hours to charge the phone. If you have a 15,000mAh USB battery, you might be sitting around all day for that to charge. Do you want to wait all day at MTR for your USB battery to charge, and then your phone?


There are a few of ways to avoid waiting around for things to charge.:

1. Ditch the 1 Amp charger that came with your iPhone and bring a 2 Amp charger. 

2. Use a USB battery that can be charged up at 2 Amps (check the fine print - many are only 1A)

3. Send a fully-charged USB battery in your resupply

4. Bring a solar panel so you can keep everything topped up.



Solar Panels are better if you will be out for a long time between resupplies. The break point is about 7 days. Over 7 days, a solar panel will give you about 7,000mAh of energy if you set it up for an hour at lunchtime, plus the potential for more if you really need it and are prepared to stop and set it up. Don't waste time trying to charge your phone while hiking, it's too flakey. For hikes shorter than 7 days, you can get the same amount of power from a "15,000mAh" USB battery of about the same weight, and you won't need to fuss with it.



Speed hiker: 10 day JMT.

Start hike with no solar panel and no USB battery. Carry a 2A charger. Start hike with phone fully charged.

At Tuolumne, top up phone at store or permit office.

At Red's, pick up a small USB battery (eg 3,300mAh) in your resupply and charge phone at restaurant. If you are really speedy, you might not need this battery, since your phone will probably last 2 days and you can get to MTR by then)

At MTR, ditch the small USB battery (put it in the battery bucket in a bag with your charger, cable, and a note saying it all works) and pick up a large USB battery (eg 15,000mAh) in your resupply bucket. Charge phone if there is time, but if there isn't time don't worry about it.

Average hiker: 15-20 day JMT. No solar panel. 15,000 - 20,000mAh USB battery and 2A charger. You have plenty of time to keep it all charged up at MTR, VVR, Red's, Tuolumne. 

Slow hiker: 20+ days JMT. Solar panel. No battery. You are going to be stopping for lunch anyway so why not harvest some sunshine? No point carrying a heavy battery if you don't have to. You would have to carry a huge 20,000mAh battery to match the energy you will get from the solar panel, and those weigh 20oz or so, more than twice the weight of a solar panel.