[RE-wrenches] Multiple angle conundrum

[Hilton Dier III]

Wrenches,

I am talking with a potential client about a roof mounted PV array. The 
client has a reasonably large south facing roof, but it is broken up by 
a large (18' W x 16' H) shed dormer. The roof is 10:12 and the dormer is 
6:12. I designed a layout that gave the dormer some room to avoid 
shading. The client was disappointed by how much wattage I could get on 
his roof. I pointed out that installing modules in the shade of the 
dormer was a bad idea.

He is also talking to another installer. The other installer proposed 
installing part of the array on the 10:12 roof and part on the 6:12 
dormer. The client had already asked me about that and I gave him a 
rudimentary explanation of maximum power point tracking and the 
inefficiency of such an arrangement. He asked them about that and their 
guy said "No big deal." He is still considering the two-angle scenario.

So, how much efficiency will he lose by going with two angles? It seems 
like the ultimate MPPT design error to me, but I don't have a number for 
him to make my case. I suppose it all comes down to cents per delivered 
kWh in the end, but two different roof angles seems the the wrong way to 
go. I'd like to be able to give him a definitive idea of what he would 
be sacrificing with that scenario.

Side note: The other installer is using the new Sunpower E20 modules. 
They seem to be available only from the manufacturer. Anybody have a 
source? Has anybody used them yet? Do they live up to the hype?

Many thanks,

Hilton

-- 
Hilton Dier III
Renewable Energy Design
Partner, Solar Gain LLC
453 East Hill Rd.
Middlesex, VT 05602

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