Gee whiz... solar panels only work during the day! What are you supposed to do if you need power at night?
The answer is to store the solar power in a special storage battery. This allows you to capture & store power from the panel all day, and plug into the battery to access the power whenever you need it. Day or night.
What are these special storage batteries? There are a few options for batteries...
Lithium (ion & polymer types)
The lightest & most efficient storage option, but they are expensive, not easily replaced worldwide, and will not drive an AC inverter for household AC power (in most cases... there are some minor exceptions).
Sealed LEad Acid (SLA)
Heavy, but cheap, easy to use, and available wordwide. These are capable of driving AC power inverters for portable household AC power.
NiMh rechargeable
We don't see these much anymore in the 12V range, but do see little AA battery packs for the USB-based solar panels & smaller solar chargers.
The best storage batteries will provide lots of power storage (up to 3x the storage of a laptop battery), and will provide multiple output voltages & connectors to help you connect to your equipment. The best of these are the Solo models from Brunton, along with the Generator battery from Voltaic Systems. Brunton is also releasing a new line of power packs in 2010: the Brunton Impel, Brunton Sustain, Brunton Inspire, and Brunton Sync. Brunton's largest battery packs are able to drive a small 30 Watt AC inverter. These are all lithium-based battery packs.
When it comes to SLA battery packs, you typically see custom-built systems using back-up batteries in the 7-26 Amp-Hour range (5-20 lbs in weight!). Check to see if the lithium will meet your needs before deciding to go with the heavier SLA.
Please feel free to contact me with any questions regarding your solar project.