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.