We use Sen software called Soldraft, it's free and looks relatively professional. They are a German company and getting registration took a little while but the site is great once you get on it. Great roof kit too! www.sen.eu I think
I don't think its a good software. 1st of all its for location germany, 2nd its has only option to select there module.
What you want to draw actually @ earyzhe ?
You need whole PV system design?
If you do a bit of research you can match wind load and snow load areas with the UK. Google translate does a fair job of translating and the help manual is printed in English. As long as the panels are the same size you can use the software to plan the roof and you'd use the inverter software to match you're chosen panels and inverter. Total cost = 0. Not perfect but when you're struggling with cashflow a very viable short term solution. I've had several email conversations with SEN, they're bringing out a UK version later in the year but sorted out a couple of queries I had very quickly and in English. Just as well the Germans aren't buying of us - not sure that we'd be able to converse as well in German!
We use Google Sketchup, 'tis excellent and free (there is a pro version too but we haven't found the need). You can model the roof, chimneys etc, and then define its exact position in Google Earth to run a shadow analysis - it's very nice to watch how the shadows pass by over the year. And easy to use. There are some quite good plugins too, one I quite like draws the Sun's path through the sky for that location so you can see when those lines drop behind various shadow-making objects..
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