Has anyone successfully set up their Pi to run on solar power? If so, what would be the cheapest way to sensibly and reliably achieve this, using a combination of solar cells / batteries / voltage regulators?
Is it a case of just building panels that give the required current, regulating the voltage and supplying a battery backup, or is there something else I should be aware of?
There are a few things you need to be aware of if you want to build your own solar power supply.
The main thing is that the voltage output of solar cells can vary wildly based on the incident sunlight (how sunny it is).
Simple unregulated solar panels sold as 'battery chargers' are often designed for deep-cycle 12v 'leisure' batteries, but can be measured at 18v or more open circuit in bright sunlight (even in the UK *8'). The internal resistance of the battery keeps that voltage down to the battery level, but without one the higher than nominal voltage could do damage to directly connected electronics if it isn't regulated.
As such, you should be looking at either regulating the solar panel or moderating it's output with a battery, and ideally both. Using a battery also allows you to accumulate excess energy production and supply power when power from the solar panel dips, which means that you may be able to get away with a solar panel sized for your average power needs, rather than one sized for your maximum power needs.
If you want to keep the size of solar panel and battery to a minimum, you may want to calculate the total expected power load, average power generation throughout the year and battery capacity to get you through the nights and the winter months.
There is some excellent advice about solar charging in answers to my question over on electronics stack exchange, including information on how to calculate whether sunshine in your area is sufficient for your application for a given solar panel and battery combination, using location specific information from gaisma.
There are many 5V solar chargers commercially available.
This answer has previously linked to several commercial solar chargers. The links frequently broke, but as time passed, solar chargers have become rather ubiquitous
You should avoid exposing your Raspberry Pi to direct sunlight as this can cause issues (such as overheating or the degradation of certain types of components such as capacitors). UV radiation is not beneficial to electronics. Apart from that, I think you should be fine if you have battery backup and a quality regulator.
Note that you can easily shield an electronic device from UV radiation by putting it in an opaque container, a simple cardboard box would do, or you could wrap it in construction paper. Even a transparent glass or plastic case would filter out most UV light. You could then run the wiring from the solar panel to the Raspberry Pi through a hole in your case.
Here is my setup: 30 watt solar panel with a 12 volt charger hooked up to a 33 amp hour, 12 volt battery. Off of the battery I have a 12 volt to 5 volt battery eliminator that drops the voltage safely to 5 volts that I spliced into a micro USB cord.
It's been running successfully for two days now. I would like to experiment with a smaller battery and panel but based off my math, my setup has extra padding on both the battery and panel. I am open to improvements though.
At a minimum you need: a solar panel, a charge controller, a deep cycle battery, a DC-DC buck converter (to reduce battery voltage to 5V), hardware to mount the panel, something to house all the components.
The trick is in sizing the solar components, and that depends on how much sunlight you get and how much reserve power you want. However, even under the best circumstances, I believe it would be challenging to power a Pi 24/7/365 in most places with anything less than a 30W panel and 12V 5AH AGM SLA battery.
The reason is that the panel has to provide enough power to not only keep the Pi running during the day, but also charge up the battery to keep the Pi running at night. The Pi consumes approximately 1.5 watts. For 24 hours of operation, that's 1.5 watts x 24 hours = 36 watt-hours. A 12V 5AH AGM SLA battery can theoretically supply 60 watt-hours of power. But you never want to drain lead acid more than 50%. Also there are always inefficiencies. So you'll only get perhaps 23 watt-hours from the battery. That's enough to get a Pi through the night—but only if the battery was fully charged to begin with. And that's where the panel wattage becomes important.
A 30W panel can theoretically provide 30 watts per hour at "peak sunlight". But realistically you're only going to get perhaps 23 watts from the panel. So you'll need 1.6 hours of peak sunlight (1.6 hours x 23 watts = 36.8 watt-hours) to power a Pi that uses 36 watts a day. You can check online to see how many hours of peak sunlight your area gets. You want an average for the winter.
Seattle, Washington, for example, only gets 1.6 on average during the winter. So the 30W panel would just make it. But that still assumes you'll get at least 1.6 peak hours of sunlight per day. In reality, you may go days with no sunlight whatsoever. That's why you want to upsize your panel + battery for reserve power.
It would therefore not be unusual to power a Pi with a 150W panel and a 75AH battery. Many would say that's overkill. But it's not if you want reserve power for cloudy days.
I use a slightly different approach... I use a full size solar kit from Harbor Freight. I charge a regular 12 volt battery deep cycle type, and feed 12 volts to a 12 volt to 120 volt AC inverter.
This provides 120 volts to power the pi supply and an HDMI monitor all in one shot. Due to the square wave feed to the Pi supply I use a 5 amp wall wort supply to take care of the voltage drop. The monitor runs fine on its own from the 120 volt AC inverter.
There is a company that does this things for raspberry Pi, they are called PiJuice comes with portable battery and solar panels can be added as extras. :
https://www.kickstarter.com/projects/pijuice/pijuice-a-portable-project-platform-for-every-rasp
