Thread: Battery Box
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Old 12-09-2013, 01:06 PM
DaveInDenver's Avatar
DaveInDenver DaveInDenver is offline
Hard Core 4+
Join Date: Jun 2006
Location: Larimer County
Posts: 8,443

Yeah, panels may be better now. Mine's a Sharp 24V, 240W from 2011. I don't get close to 240W from it unless I get the incident light is perfect. I don't have a tracker or anything, on the old house it was just bolted on a south facing roof on the garage and here I just use a 2x4 frame and set it out in the backyard when I need it.

Since I had it flat mounted on the roof I was limited to the slope, which was pretty steep at 8-in-12, roughly 34 degrees and not too bad on tilt. The way I do it now, with the frame I put it at 40 degrees in the back yard.

Anyway, point is that for every 10 degrees of elevation or azimuth misalignment my panel output drops 5%. If I leave it set up directly due solar south all day I might get on average about 135 W-hr and get full output for about an hour or so total (which incidentally isn't noon but about 9~10AM and 2~3PM and above 80% efficiency for around 3 hours total. If go out and rotate it every hour or two it does better (this would be 1-axis rotation), about 165 W-hr, but my tilt is non-ideal really. I tore it all apart this fall and plan to make once that I can adjust the tilt and dispense with the cinder blocks and guessing.

I have not done year-round detailed measurements since I don't leave the panel set up all the time. But what I have done indicates to me that 75% is the best you can do and to get it you'd need to do 2-axis alignment and even then the 240W rating is during ideal temperature, too. In the summer my panel gets really hot during the day and efficiency drops a ton. The output rating is for 25*C and Sharp gives a temperature coefficient of -0.51%, which means for every degree over 25*C the panel output drops 0.51%. So if my panel gets to 40*C the output will have dropped 7.65%. That's not all that hot, 104*F, and in full sunlight during the summer the panel will get much hotter than that, 50 or 55*C isn't unusual. You can give up 10% or 15% efficiency there alone.

That the reason why with a fixed panel I was getting less than 50% capacity and why I suggested the 2x capacity. Just the amount of sun missing it took ~20% easily and then the heat zapped a bunch more. OTOH in the winter the temperature coefficient works to your favor and efficiency goes up as it gets cooler. Which is good because you start to loose a lot because the lower sun angle and more energy is absorbed and refracted by the atmosphere.

Also sizing the panel much larger allows the load to be comfortably accommodated simultaneous with charging at the same time in all weather. There are a lot of places to lose solar energy and panel capacity itself is really a small part of the equation.
'91 Pickup - Imelda
'08 Tacoma TRD - Donna

"The Only Thing That Is Constant Is Change." -- Heraclitus
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