I have a basic PV setup for the cabin...it runs a few CFL lights and that's about it. I bought all my stuff at canadian tire (gasp!) which includes
- watt solar panel
- Ah AGM battery with 400W inverter
- charge controller
I've been thinking about building two panels (about 100W) so I think I can still use my 7A charge controller,
7A * 12V = 84W. You want to keep at least a 20% buffer there (solar panels can deliver 10-15% more than their rated amperage on a cold, sunny day), so the maximum your 7A controller should be connected to is 7A * 12V * 0.8 = 67W (call it 65W).
If you want to use 100W then you want a controller capable of 100W / 0.8 / 12V = 10.5A (round to 10A).
Also, if you're planning to run a 100W panel then you'll be wanting a bigger battery bank for it to be worthwhile (otherwise your controller will rapidly bring the battery up to full charge in the morning, then will keep the panel switched off most of the rest of the day). For example, I have 540W of solar panels feeding into a 1000Ah 24V battery bank (equivalent to a 2000Ah 12V bank). I'd suggest around 350-400Ah as suitable for a 100W panel.
After all, you wouldn't fill a 1-gallon bucket with a 3" fire hose, would you? ;-)
but I have no idea how to combine them. Do I need to build or buy some kind of combiner box? Can I build something simple myself? The way I could imagine is some kind of terminal box where you attach the leads and tighten down the screws...something like 4 'ins' and 2 'outs'...then you attach the controller in-line after the current has been combined. Is this right? And if one panel is shaded, won't the current run from one panel to the other (backwards!), possibly causing damage to the cells?
Larger solar panels have junction boxes on the back like this:
http://www.geocities.com/za2bb/solar/index.html/index.html
(See the second-last picture). In this installation, I have 2 sets of 2 panels connected in parallel, with a 10mm2 cable pair from each set going down through conduit into the shed. This cable is too thick to fit 2 strands into the connectors on the controller, so about 3" from it they're spliced into a single one. The maximum current from the panels is 20A at 24V, and the controller is rated at 40A.
Also I have no fusing in my system right now which didn't seem like that big of a deal with a 15W panel but I'd like to be safe (and legal). What kind of setup should I buy (or build)? Any links or comments would be greatly appreciated.
Fuses are cheap, fires are not. Also, the fusing is to protect you from your battery, which can deliver quite a bit more than 15W. For your current setup just buy an inline 3AG fuseholder and connect it to one terminal of your battery (usually the +ve one, as I've done here):
http://www.fieldlines.com/story/2004/9/19/16327/4685
