Renewal battery calculator

A couple of friends have been asking me about what batteries they require to run a 12volt sound system.  I spoke to them about it a bit, recalling what I'd learnt from folks such as Raymundo and Clive Magrec.  Then I had a bit of a read up on good old Wikipedia and thought I'd put a summary of it together for them; I got a bit carried away and wrote a JavaScript based calculator for working it out.  

Check it out online at this wonderful link.