Max reported output vs system size

[J.P.M.]

Ok, so what should my "ideal" tilt have been, and why? Isnt "the flatter, the better"? (Max surface area, at peak).

As The Solar Queen wrote, tilt = latitude is a good first approximation. The lower the angle of incidence of solar radiation on an array, the more intense will be the solar radiation on the array. The lowest annual sum of the hourly angles of incidence on an array is approximately when the array is facing south and tilted at the local latitude. That will result in what's pretty close to the max. amount of solar radiation striking an array. That's what you want and that's the reasoning.

[Amy@altE]

Exactly. For most grid tie situations, flush on the roof is best structurally, so you don't have to deal with tilting and inter-row shading from the panels in the front row shading the back panels. My point was just that you should expect to see lower output than if you were at 42 degrees, because as you know, the sun isn't directly above us, it's at an angle.

For off-grid systems, where every kWh matters, many people will put their system on an adjustable tilt rack. In the winter latitude minus 15 degrees is best, summer is latitude plus 15 degrees, spring and fall latitude. So for best year round performance, if you aren't tilting throughout the year, latitude is best. But again, for grid tie, on a home roof, the angle of the roof is usually the best for other reasons. Just set expectations for output accordingly.

[J.P.M.]

Exactly. For most grid tie situations, flush on the roof is best structurally, so you don't have to deal with tilting and inter-row shading from the panels in the front row shading the back panels. My point was just that you should expect to see lower output than if you were at 42 degrees, because as you know, the sun isn't directly above us, it's at an angle.

For off-grid systems, where every kWh matters, many people will put their system on an adjustable tilt rack. In the winter latitude minus 15 degrees is best, summer is latitude plus 15 degrees, spring and fall latitude. So for best year round performance, if you aren't tilting throughout the year, latitude is best. But again, for grid tie, on a home roof, the angle of the roof is usually the best for other reasons. Just set expectations for output accordingly.

I probably would have been better off using different wording for the last sentence of my response to the OP's last question, or at least qualified it more.

I appreciate the idea that mounts parallel to a mounting surface such as a roof are usually easier to design, usually less expensive and thus probably more cost effective to erect, and that those design considerations will be part of any "ideal" tilt decisions.

But, from the context of the posts immediately prior to the OP's question that seemed to be pretty much discussing orientation only, and not a discussion of what criteria make an array "ideal", led to what I (perhaps incorrectly) assumed be the OP's question about "ideal" orientation. I read the OP's question to be about array orientation vs. array production, while using or taking the word "ideal" in the context of the array orientation that will have the highest probability of the most annual system production, and not necessarily in relation to cost effectiveness, or actual site conditions or limitations.

I thought the OP's questions I was focusing my response to were about orientation only.

[Amy@altE]

I agree. That's why I added in the bit about tilt, since it wasn't being discussed. Just wanted to make sure he knew what he didn't know, as they say.

[farokhboston]

Exactly. For most grid tie situations, flush on the roof is best structurally, so you don't have to deal with tilting and inter-row shading from the panels in the front row shading the back panels. My point was just that you should expect to see lower output than if you were at 42 degrees, because as you know, the sun isn't directly above us, it's at an angle.

For off-grid systems, where every kWh matters, many people will put their system on an adjustable tilt rack. In the winter latitude minus 15 degrees is best, summer is latitude plus 15 degrees, spring and fall latitude. So for best year round performance, if you aren't tilting throughout the year, latitude is best. But again, for grid tie, on a home roof, the angle of the roof is usually the best for other reasons. Just set expectations for output accordingly.

Thanks for the lesson. Mine is set to "roof angle" as its a home/grid tied system. So what should my expectation be, for output? 10% less than optimal?

[J.P.M.]

I agree. That's why I added in the bit about tilt, since it wasn't being discussed. Just wanted to make sure he knew what he didn't know, as they say.

Understood. Thank you.

[J.P.M.]

Thanks for the lesson. Mine is set to "roof angle" as its a home/grid tied system. So what should my expectation be, for output? 10% less than optimal?

Run PVWatts for your array with two orientations, one with tilt = latitude, and 180 deg. azimuth, the other at the actual orientation of the array. Subtract one estimated output from the other and divide the result by the latitude tilt, 180 deg. azimuth orientation. That will be a very approximate "off ideal penalty " for what you have as a % of an "ideal orientation" long term average annual production.