When out hiking or trekking there are several ways to ensure the power supply of navigation, camera, smartphone, lamp and so on. On the one hand it is possible to take spare rechargeable batteries for each device. On the other hand there are also models of all these devices that can be charged via USB. In this case the supply on tour would only need to be secured from one power outlet as a charging option to the next. With a power bank the required time gap to the power grid can be extended. This is the most frequently chosen method for trekking and especially for ultralight trekking. Nevertheless solar panels offer another option independent of the power grid.
I have had several experiments with solar power on tour behind me which were more or less crowned with success. In 2015 in Sweden the type of power supply for four people with one solar panel worked great while in 2016 in Madeira with two people it turned out to be a disaster. The following are the solar panels I have tested recently. The XTPower panel got a new cover made of lighter SilPoly fabric and thus saved a lot of weight. The China A5 panel was trimmed a bit with scissors and got two additional holes for mounting with a Dremel.
A1 - Indicates the output in amperes with an aligned solar panel A2 - Indicates the output in amperes with a non-aligned solar panel A3 - Indicates the output in amperes in diffuse sunlight
Solar panels have a decisive disadvantage compared to charging via a power outlet. The power supply during charging is anything but constant. Due to clouds/foliage and similar external influences the output drops. Apple smartphones stop charging completely after this happens and only start charging again after the solar panel is replugged. Therefore for Apple devices it is recommended to first charge a power bank via the panel and then charge the phone using it. In contrast to Apple phones most Android phones do start charging again directly but here too there are pitfalls. As a rule the settings on smartphones with regard to charging with solar panels are chosen suboptimally. On the one hand there is the setting “Wake on plug”. This setting means that when the phone is being charged the display should light up as a signal. But displays are one of the biggest power guzzlers on current devices. Thus every time a leaf in the forest casts a shadow on the solar panel and reduces the output the phone is woken from standby and the display is turned on. On my LG G4 a switched-on display requires around 600 mA of power which if it doesn’t come from the solar panel is drawn from the battery. As a result it often happens that the phone consumes more power during solar charging than it generates through the solar panel. The next function that should be turned off is the “Daydream” function. Through it a certain animation is displayed on the display while charging. Thus the same problems arise because the display is switched on while charging and a lot of power is consumed unnecessarily.
Variables
A solar panel's performance depends on various variables. In the following I describe those that have turned out to be the most important for me. Variables with only a minor influence on performance I have excluded from consideration for the sake of clarity.
Output of the aligned panel
Solar panels should be aligned so that the sun’s rays hit the panel at a right angle. This way the maximum possible yield of energy from the sun’s rays is converted into electricity. In further calculations I assume that this value is achieved for one hour with sufficient sunlight during breaks.
Output of the non-aligned panel
This value indicates what output of the solar panel is possible when it is not directly aligned with the sun. This is the case for example when the panel is mounted on the backpack.
Output in diffuse light
This value indicates the output under cloud cover or in fog. This value is only a fraction of that of the aligned panel. The amount of electricity generated is unfortunately insufficient. This became fatal for my girlfriend and me in Madeira. To store the low energy yield a power bank that can be charged with low currents is recommended.
Hours of sunshine
The number of hours of sunshine is decisive for the energy generated. In Germany it is an average of 5.3 hours of sunshine per day. For further calculations I used the sunshine hours values from a two-week period of bad weather from the Brocken in the Harz mountains.
Power bank
To bridge a period of several days with little sun it is necessary to buffer the electricity gained. The larger the power bank is the longer the period of bad weather that can be bridged.
Consumption
Consumption is and remains essential. Someone who consumes 500 mAh of power per day needs a smaller panel and a smaller power bank than someone who consumes 2000 mAh of power per day. I have my consumption by turning off unnecessary functions.
Model
I poured the power generation with the various variables into an Excel model. This model is not perfect but rather serves as an estimation aid as to whether a trip is feasible via solar. Currently the model is limited to 14 days. The primary question in the model is whether it is possible to bridge a period X with given variables and a solar panel. The model still has some minor errors that are not taken into account. For example more power can be stored in the battery than capacity is available and a battery can also have negative capacity. However this is sufficient for the question at hand. The model is also available for download as a Microsoft Excel document.
Have fun testing a bit with the model. The rows highlighted in blue are the variables. For me the setup on solo trips of three or more days looks as follows in the future: Spring and autumn: 10W panel + MYOG power bank High summer: attempt with the 5W panel + MYOG power bank
My options for being on tour with solar energy are now workable. With my variables it is worth taking the panel along from a period of 5-6 days between power outlets. It was a bit fascinating for me that between 5W-10W is already enough for me. Equally exciting is that a larger solar panel does not become lighter per watt in terms of weight. Thus it is hardly worth taking a large panel for several people. Weight-wise you are better off with small isolated solutions for several people. Alongside the disadvantages of weather dependence you also have a great advantage. After all with such a method of power generation in reasonably good weather you can significantly extend the time between civilization.
The China A5 solar panel surprised me the most. Not only because it plays in a different league in terms of weight and price compared to the larger panels but also because in good weather in December it fully charged my LG G4 smartphone within four hours. Having a panel aligned for several hours a day while trekking is in my opinion a bit unrealistic; a quarter of the time however is already much more likely. Thus it should work wonderfully with a daily power consumption of under 1000 mAh. This is however far outside the normal range for me with the current smartphone; I would need a very power-efficient smartphone (with a still good camera) for that. Maybe such a device will be on the market in the future, let’s see!
