The model A+ RPi uses significantly less power than a Model B/B+/2 due to not having the power-hungry ethernet chip.
See for example: http://raspi.tv/2014/raspberry-pi-a-how ... es-it-need
Model B recording 1080p video takes 480 mA @ 5V or about 2.5 watts.
Model A+ recording 1080p video takes 230 mA @5V or about 1.15 watts
If you have a constant load of 2.5 watts over 24 hours, that is 60 watt-hours.
sealed lead-acid battery charge efficiency (0.75) * 12V->5V power supply efficiency (0.9) * panel charger loss (0.8) = 0.54
So as a guess, your system needs to capture 60 / 0.54 = 111 watt-hours per day.
according to http://www.pveducation.org/pvcdrom/prop ... -radiation
Stuttgart averages between 3 and 6 kWh per day per m^2, depending on the month of the year.
If your solar panel is 15% efficient, that means 450 Wh per day per m^2 in winter, and twice that in summer.
If you need 111 Wh/day input, that's a 0.25 square meter (active area) panel *on average*, not counting more-cloudy-than average days. You need only half of that if you use a RPi A+ instead of a RPi B. Size can be a few percent less if you get a premium more-efficient panel.
> I want to run the pi 24/7
The key question then is how many peak-sun-equivalent hours per day you get. It seems to me that you could have several days where it is cloudy most or all of the time, is it acceptable for the Pi to be down during bad weather? If not, you are going to be looking at a pretty large battery to hold over during the darker days, and similarly more solar panel to be able to recharge that large battery plus maintain the Pi, when it is sunny.
You can also refer to sites like http://photovoltaic-software.com/PV-sol ... lation.php
but again remember that's a yearly average, not accounting for short-term weather
For a 12V battery, 111 Wh is 9.25 Ah, a reasonable size. I have an 8 Ah 12V SLA battery in front of me which measures 15 x 9 x 6.5 cm (6 x 4 x 2.5 inches). But that's if the battery capacity is 100% used in 24 hours. If you get 6 useful hours of sun a day, you're still discharging the battery 18 hours of the day. Remember the deeper the discharge each night, the shorter the overall lifetime of the battery is. Maybe you should plan for double that for longer battery life.
I assumed lead-acid battery because it's by far the cheapest per stored watt-hour but are those even still used, over there in the land of RoHS ?