Need some help determining how panel positioning would impact my % efficiency

[thirtywest]

I have a roof that faces just about 250 deg SW (true). I'm in Houston so let's call it 30 deg latitude. Figuring the tilt isn't difficult. And, from what I've been reading, a fixed tilt (like my solar exhaust fan up there) gives about 71% efficiency.

What I'm looking for is: what is the efficiency if they are NOT tilted? Everyone assumes you 'can' tilt, but just for argument's sake I'm trying to see the difference if left flat on the roof, pointing 250 deg on a standard 8/12 roof.

[ButchDeal]

The efficiency doesn't change with the tilt, it is a fixed efficiency for the module.
What you will get is different production, at different times of day etc. Most pitched roof installs are not tilted but installed in situ (parallel to the roof).

You say yours are SW but they are closer to West than SW. or more formally WSW, but thats just the name, 250 degrees is the azimuth and pitch is 33.7 degrees.
If there are no shadows particularly to the SW,
Your production will drop off by less than 10%

[thirtywest]

The efficiency doesn't change with the tilt, it is a fixed efficiency for the module.
What you will get is different production, at different times of day etc. Most pitched roof installs are not tilted but installed in situ (parallel to the roof).

You say yours are SW but they are closer to West than SW. or more formally WSW, but thats just the name, 250 degrees is the azimuth and pitch is 33.7 degrees.
If there are no shadows particularly to the SW,
Your production will drop off by less than 10%

Ok, you got me. WSW. Now I could go all nautical on you and start using a 32-point compass but i won't

during the best parts of the day, no---few shadows.

And yes, I didn't mean panel efficiency, I just meant to call 100% as meaning "everything the panel needs to meet the claim on the box".

So, how did you come to 10%?

I'm asking because I'm looking at building some 1-axis (maybe 2-axis if I'm really feeling ambitious) ground-based panels. My goal isn't whole-house. I'm looking at covering the typical kitchen routines that occur twice daily. I don't have the funds to aspire for HVAC and fridge operations. What I can do is cover things like: toaster, coffee, juicer, stove for breakfast, about 1.5 hrs of LED light fixtures, etc.

I know what I can produce on the ground as I've been crunching numbers with an excel sheet I put together (poor-man's rapid fire development platform).

What I couldn't do was compare it to in situ panels 'up there'.

And, thanks for being quick to reply.

[thirtywest]

well, i just wrote a lengthy reply and it looks like it evaporated under the auspices of "waiting for approval". I'll give it a bit before trying to craft it again.

[thirtywest]

The efficiency doesn't change with the tilt, it is a fixed efficiency for the module.
What you will get is different production, at different times of day etc. Most pitched roof installs are not tilted but installed in situ (parallel to the roof).

You say yours are SW but they are closer to West than SW. or more formally WSW, but thats just the name, 250 degrees is the azimuth and pitch is 33.7 degrees.
If there are no shadows particularly to the SW,
Your production will drop off by less than 10%

How did you come by 10%?

[ButchDeal]

How did you come by 10%?

just experience from years of modeling. you can check your actuals by going to pvwatts and running your own simulation. Usually shadows cause more of an issue with residential anyway.

http://pvwatts.nrel.gov/

[SunEagle]

well, i just wrote a lengthy reply and it looks like it evaporated under the auspices of "waiting for approval". I'll give it a bit before trying to craft it again.

Your post had the word "k" i t c h e "n" in it which has been used by spammers and therefor is now a "bad" word to use.

[thirtywest]

Your post had the word "k" i t c h e "n" in it which has been used by spammers and therefor is now a "bad" word to use.

khichen? (see what I did there?). seriously though? THAT's a flagged word? Oh well.

I'll check out that link too.

[NEOH]

Solar Panel Tilt

http://www.solarpaneltilt.com/

[SunEagle]

khichen? (see what I did there?). seriously though? THAT's a flagged word? Oh well.

I'll check out that link too.

You have no idea on the hundreds of spams we got daily advertising that room. So the word was put on the approval list by the admin.

[ButchDeal]

khichen? (see what I did there?). seriously though? THAT's a flagged word? Oh well.

I'll check out that link too.

also note that if you tilt them then you will be able to fit far less on the roof due to them shadowing each other.

[azdave]

Probably not an issue to you in Texas but in Arizona, the latest grid-tie agreements are favoring solar output later in the afternoon. West facing panels can be the best dollar value due to the mandatory TOU plans for RE customers.

[thirtywest]

Probably not an issue to you in Texas but in Arizona, the latest grid-tie agreements are favoring solar output later in the afternoon. West facing panels can be the best dollar value due to the mandatory TOU plans for RE customers.

that has something to do with overloading the grid during the peak hours? essentially, smoothing out 'their' end of it?

[J.P.M.]

I have a roof that faces just about 250 deg SW (true). I'm in Houston so let's call it 30 deg latitude. Figuring the tilt isn't difficult. And, from what I've been reading, a fixed tilt (like my solar exhaust fan up there) gives about 71% efficiency.

What I'm looking for is: what is the efficiency if they are NOT tilted? Everyone assumes you 'can' tilt, but just for argument's sake I'm trying to see the difference if left flat on the roof, pointing 250 deg on a standard 8/12 roof.

Suggestion: Start with a (free) download of "Solar Power Your Home for Dummies". An hour or two will impart a lot of knowledge that will make the answers to your questions you get here and elsewhere a lot easier to understand.

The answer(s) to your questions will be answered better and more thoroughly by a read.

Side bar: Don't use a horizontal orientation. Performance will suffer for lots of reasons, one of the bigger ones being a horizontal array will wind up functioning something like a sand box, get fouled up faster, and get dirtier as mud cakes up with rain/dry cycles. That will kill performance if not cleaned a lot more often.

Lots more stuff you need to consider that you'll find in the book. Another factoid: Unless you're talking something like MW size arrays, tracking arrays cost more in $$ and maint. than fixed arrays when the value of the added output is figured against the added cost.

[J.P.M.]

also note that if you tilt them then you will be able to fit far less on the roof due to them shadowing each other.

To a first approx., for sawtooth arrays and for most panel tilts once self or adjacent panel shading is accounted for by adjusting the row pitch, the total annual insolation on a horizontal roof (or a horizontal array of the same area) is about the same as the total annual P.O.A. insolation that the max. size sawtooth array that can be fit on the same horizontal roof will see. Not perfect because of parapet shading, etc., but not too far off.

Kind of a no brainer if you think about it. Took me a long time to see it. Not sure what that says.