# DIY 5v Solar Panels

In a nutshell : What is a good way to power a 5V project with one-two 0.5 V solar cells?

The motivation is as follows:

My project needs about 2.5-5 Watts from the solar panel. Also, this project involves several dozen units so cost effectiveness is important. In terms of power these cells would probably suffice:

Power: 2.8w
Efficiency: 17.6-17.8%
vmp: 0.523V
Imp: 5.215A
voc: 0.629V
Isc: 5.585A

and are much cheaper per Watt than buying 5V solar panels. I though about using 0.5V cells with a dc converter but after a long search i could not find a converter that can output 500mA @ 5V from an 0.5V source, so maybe a boost converter is not the way to go.

Clarification, I require 2.5-5W after taking into account the fact that solar panels never output 100% except perfect conditions and in fact my actual project requires less power and is also able to save power by doing less work when the battery is too low. Bruce Abbott's answer does not really work for me as I prefer not to have to "pay" with extra cost and space (the project is small) jut for the sake of voltage conversion. (this is my first question so i am sorry if it is not clear enough)

Two cells gives you 5.6W in full sunlight, which is enough power but not enough voltage. You need more cells, but they don't have to cost a lot more because they can be smaller. A panel with two 5x5 cells may cost less than $4, but at this price the cost of the converter could be a significant factor. It may be better to pay a little more for the cells, rather than having to use a more expensive converter that works on lower voltage. Most boost converters require at least 3V to produce 500mA at 5V. To get 5W at 3V you need 6 cells, with each cell delivering at least 0.8W. A good 2x6 cell can generate 1W, and the cost for a whole panel's worth could be under$5 (eg. 40-2x6 Solar Cells for DIY solar panel).

Alternatively you could make a panel with 12 1x6 cells to provide 6V, then just use a cheap 5V LDO linear regulator.

• Please read my edit above, I clarified some of the assumptions – Sorin Sep 5 '14 at 17:48
• So you actually only need 2.5W (500mA@5V), and you have a battery which powers the circuit when there is insufficient sunlight? Is the battery charged from the 5V supply? – Bruce Abbott Sep 5 '14 at 18:49
• yes yes exactly . – Sorin Sep 6 '14 at 21:15
• Did you read my revised answer? Is it acceptable? – Bruce Abbott Sep 6 '14 at 22:01

It's very hard to convert such low voltages, because diodes (used in typical DC/DC converter) have relatively high voltage drop (0.3-0.7V).

My suggestions:

• Use 12-15 panels for one Raspberry Pi (with battery buffer). If you don't live near equator or in middle of desert - 30-40W in solar panels is not overkill if you want your Raspberry Pi to run 365 days per year without external supply. Note, that you need energy at night, when it's raining etc.

• Just use higher voltage solar panel (like Bruce suggested)

• Before you build your system - calculate your power demands. Do some research about average number sunny days in year where you live etc. And don't focus on watts and volts. Think about watt-hours first.

• I think I generally agree with what you are saying. In my case, however, i am not using a Pi 365x24x7 but rather turning it on briefly and back off. So most of the time I am only using 5-10 mA (for the arduino pro mini i am using to turn on and off the pi. SO i think 30-40W is overkill. THe fact that i am using the Pi for only about an hour-two every day made me hope that i can avoid huge panels and 12v panels. (btw thanks for the edit :) ) – Sorin Oct 7 '14 at 7:28