Yep, I finally bought a solar panel. But let's start from the beginning.
The first try
A year ago (sry, it's a german post on my old blod) I tried to charge my power bank with solar energy only. I used a 14 Watt mobile solar panel which I just hang outside of my window.
Unfortunately it wasn't very successful.
The problem is that this mobile solar panel splits the power through two USB-Outputs which reduces the already-not-that-much output of energy into half. That would make a maximum of 7 watts per hour under perfect conditions (blue sky, sunny day, direct sunlight insolation). You could power your smartphone with that setup, but in my case I had to already give up the plan after the second cloudy day. It just wasn't efficient enough and I believe that the headline "14 watts" on that product is just the super duper very perfect value which you'll never reach.
The other problem was the mounting. As you can see in the picture I just hang the panel on a carabiner with a kite rope. The panel rotated when it was windy. That's neither efficient nor safe.
I was frustrated, since I bought the power bank and the mobile solar panel for about 100 euros and it didn't went as expected. So I postponed that idea for a while.
The second try
Now, one year later, I wanted to try it again. But this time with a good setting. With a real solid solar panel.
Buying a solar panel, a battery and a charge controller isn't that big of a deal, but I had to solve the mounting problem. Since I'm only living in a flat inside of a multiple family dwelling in the city, I'm unfortunately not the proud owner of a garden or at least a balcony, ... yet.
So I would've had to mount the solar panel on my window, which is on the south side of the house (perfect!). But, what I've learned from my first try was, that the most efficient place would be outside of the window. Put a window between a solar panel and the sun and you'll only receive half of the energy.
And there was the other problem. I couldn't just drill holes into the outside wall of the house (not because I couldn't do it physically, but because I'm not allowed to). And also I couldn't drill holes into the window frame. So what to do?
Luckily my windowsill is a bit wider than usual, so I have a surface of about 35 centimeters deep where I can put things on. My first drafts where inspired by mountings for small television satellite dishes.
But I decided that this mounting would be too unstable for heavy wind. The pressure of the wind pushing from the side on the panel would be even more powerful because of the lever act which would eventually rip the clasp out of the window frame.
Since I was too much thinking in this direction of how the mounting could look like, it took me some days to get to my final, very simple result.
I bought the solar panel, sized the mounting and bought the aluminium parts and everything I needed to built that thing. Turned out that a prototype can be very expensive.
I just jammed the two solar panel cables between the window gasket. So I didn't had to cut the gasket or, as mentioned, drill any holes.
The rest was really easy. I plugged in everything and it just worked out of the box. Boom, solar energy!
I have a 30 watts monocrystalline solar panel, 9 ampere hours 12 volts lead-gel accumulator, a solar charger with display and an automobile 12 volts controller which has three 12 volts car female jacks and two USB outputs where I can put any other adapters and devices on.
I additionally have two power banks, one with about 8 and one with 12 ampere hours which I use to charge the smartphones and USB devices in our household.
After one week I come to the following conclusion:
Full success! I'm not only able to charge both power banks, I'm also able to load many more things. I didn't exhausted the possibilities yet, but after two sunny days I've had far more energy stored then all of our USB devices could drain in one day. So I'm just at the very beginning of experiencing what it could feel being autarkic from energy suppliers.
But, one side note, it's not a question of cost efficiency. I spend a total amount of about 250 euros for all that (the mounting prototype had cost me about 70 euros in material costs). With that amount of money, I could pay my electricity bill for about 5 months, which includes powering everything in my flat. So it's, not yet, a thing to save money. It's more the first step into future and being autarkic. As of many other things, I just wanted to dive into a topic which interested me.
Nevertheless, I'm very sure that there will be an update in some weeks from me about using solar energy. So stay tuned!
Ping me on Twitter if you have any questions. I'm happy about every feedback!