Do I need to clean my solar panels?

Residential Solar Power Question

I have had some solar panels fitted to my new house for about 3 months now and they have started to get dirty, some leaves have blown onto them and they look dirty.

I am wondering whether being dirty will make them less effective, and if so will the lose be enough to make a difference to my system.

If they need to be cleaned, do I need to use any special type of cleaner, or can I just get the hose pipe and wash them down with clean water?