I want to make my Pi be 100% solar powered. I'm no electrician so bear with me on this.
Ive read that the Pi uses about 48Wh a day, which equates to about 9.6Amps at 5 volts. With the minimum requirement for the pi being about 700-1000mA, would I be able to power a Pi off using a 12W panel @ 6 volts and a ~10,000mA battery? What about an 18W 6V panel?
More specifically something like this:
I know solar needs to be ideally placed and doesn't account for cloudy days, but I'm looking for the ideal minimum to run a solar powered Pi. From there I can make adjustments, but a baseline minimum will help me understand things better. The Pi I plan to run will only be running my personal web server, so there is nothing else attached, save for the Ethernet cable.
here is my guess:
12W solar panel charging a 12V battery.
the panel has sunshine 8 hours each day
on average, the power output is 1/3 of the nominal 12W, which is an optimistic guess in California were we seem to have blue sky every day.
12W/3*8h/24h=1.3W average power over 24h
it seems to me that you need at least twice as big panel to run the Pi under the assumed conditions.
regarding the size of the battery I think that you need to figure out how many cloudy days you can expect in a row and how many watt hours will have to be drawn from the battery during those cloudy days. this gives you a number for the watt hours of battery energy you need. (if you want to calculate in Ampere hours you divide those watt hours by the battery nominal voltage 12V. batteries are usually sold by the ampere hour).
then you have to consider that if you want any life time from a lead acid battery you should not remove more than 20-30% of the charge before recharging. so your battery will have to be quite large if you want to run your Pi on a solar panel continuously.
a suitable battery could be a 12V deep discharge battery. not an ordinary car battery, they will not last. you will need some kind of charge controller between the panel and the battery.
I found this graph on the web, it shows the hours of sunshine in Fresno, California each month of the year. in December you would need a 40W panel to avoid sucking your battery down, on average.
There is some information on the web on amount of sunshine (assuming no clouds) http://www1.solmetric.com/cgi/insolatio ... 0INTL%20AP
where I live, near San Jose, the annual incoming sunshine is 2066kWh/m2. assuming that the panel is fixed and mounted with 32 degree tilt and towards South. this means 235W per m2. assume panel efficiency is 14% and there is never any cloudcover or dirt on the panel, or shade, you would need about 2 square meter panel area to deliver 2.5W continuously. that would be a 300W panel? perhaps someone knows how to calculate this??
I found another map of kWh/year that appears to take cloud cover into account. it shows 1800kWh/year for my area which makes sense. http://www.solar-estimate.org/images/kwh_kwyear_map.gif
it still seems like the panel size would need to be 340W to power a tiny Pi. that seems like a large panel for a small load. where did I go wrong?
using online calculators for domestic solar panel dimensioning I come to the conclusion that a 32W panel is needed during the month of February at my location, to run the Pi - if a very large battery is used to handle cloudy days.