Can you use a car battery for in a solar panel system or do you need a Deep Cycle Battery?

You shouldn't, Deep cycles are used for things that need more power and longer life (forklifts, battery car)

If you want your batteries to work for more than a few weeks, there is a big difference. If you are just doing this for a test, with donated batteries, or something temporary, a regular SLI battery (like the one in your car) will work for a few days to a few weeks. A deep cycle battery would last for a period of years. WHY? SLI Batteries are for Starting Lighting and Ignition. These lead acid batteries are designed to give their power in 20 seconds to start the vehicle which is a very high amp load in a short period of time. To do this the plates inside are very thin, to maximize surface area, increasing chemical reaction speed and amperage output. When you deep discharge them, they break down REALLY fast. Deep Cycle Batteries on the other hand are designed with really thick lead plates in them. The thicker lead plates in the deep cycle battery are then able to discharge for longer periods of time (albeit at lower amperage) for many years without breaking down under these harsh operating conditions. To ease your journey through sizing, I included a link to a couple sizing calculators. AC - DC Battery Bank Sizing Calculator http://www.bdbatteries.com/acdcsolar.php DC - DC Battery Bank Sizing Calculator http://www.bdbatteries.com/dcdcsolar.php