I got the current pricelist from a solar supply company a couple of days ago.
Was pretty surprised to see brand new Talesun 345W panels at .59C/W. Trina's are .57c/W for 270's and .62C/w for 290's.
The days of $1 per watt are LONG gone. Thankfully.
Dunno how much cheaper they will get. They will level out and I think they are near that Plateau now. One things for sure, your roof is going to be capable of supplying a LOT more power than it could just a few years ago.
The problem is that panels are all over the place with efficiency, number of cells, voltage, and current ratings.
You simply cannot hook up an odd selection of mixed panels together, either in series or parallel if they are all different.
You can hook up similar Voltage panels in strings but if you hook up dissimilar WATTAGES, each panel will only push the lowest wattage of the group. IE, If you have say 10 panels in a string and 9 of them are 250s and 1 is a 100W, you wont get 2350W out of the string, You'll get 1000W. Maybe. It's like having an irrigation pipe of 2" with a piece of 1/2" somewhere along it. 1/2" flow is all you are going to get because that's the bottle neck.
Hooking up in parallel is a bit better but you'd still want similar voltages although the wattage wouldn't matter because they will work independent on that front.
Another thing I am of the opinion of rather than knowing for fact is with low wattage panels, the efficiency of the inverter may be worse for Higher voltage strings. The afore mentioned example of 10 panels may only be 1Kw but if they are running at 360V or so, the inverter has to work harder to convert that to mains voltage. I'm pretty surprised the heat my inverter generates with under a kw going through it and the string voltage being within 20-30v of the output. If you had double Voltage, I imagine the heat losses and inefficiency in conversion would be a lot more and exponential for the amount of power produced. I imagine with say 250W panels at the same Voltage, the conversion loss would be the same which would be a much smaller percentage of the actual amperage delivered.
Another thing I have found is the 3x 260W new panels I have on my "play" system generate a disturbingly close amount of power at the end of the day to my 1500W of 175w used panels. The difference in the daily output is more like 20% rather than the 50% as would be expected by the panel wattage differences. The irony is I'm getting good power totals for the used panels, the new ones just seem to over deliver on their rating.
The used panels were cheap and are repaying themselves well but as soon as I find some 250s again I'll replace them with those.