Are standoffs justified?

DaylanDarby · December 2012

I couldn't find an answer in the forums (or the net) to this question, please direct me if I missed it.

How do I determine the cost benefits of standoffs? I'll give you my designed system below but I'd like a generic answer so others can find and use this thread.
It appears that an L-foot plus a rail provides 3-4"s of clearance between the bottom of the panel frame and the top of the roof. Is this normally sufficient?
Is distance (IronRidge's 1/4" compared to Unirac's 1" mid-clamp width) between modules much of a factor?
Is butting (or not) the short ends of panels in portrait layout a factor? [Side Note: Is butting better or worse for snow?]
Is panel size (65" vs. 77" long) or wattage a big deal?
At what height are standoffs 'too much'? (wind? roof pitch?)
Has anyone used different (i.e., 6" on top, 3" on bottom) heights standoffs to provide increased panel tilt? How did that turn out?
Has anyone regretted installing (or not installing) standoffs?

Average summer high is 99 degrees (max is 111), snow load is 30 in suburban Salt Lake City.
I've got a 40' x 13' spot on a 6/12 pitch southern shade-free shingled roof that I'm considering placing 2 rows of 12 columns 77.01 × 39.06 (Canadian Solar).
According to the IronRidge calculator I need 40 feet (I want them every 4 feet), adding $400-$500 (about 5%) to my cost.

Thanks in advance for your thoughts,
Daylan Darby

inetdog · December 2012

Re: Are standoffs justified?

DaylanDarby wrote: »

I couldn't find an answer in the forums (or the net) to this question, please direct me if I missed it.

Thanks in advance for your thoughts,
Daylan Darby

I don't have specific numbers, but it appears that around 6" is the point at which you get diminishing returns from adding more ventilation space under the panels. The difference between 3" and 6" in terms of panel temperature is large, but depending on your panel to inverter ratio you may not get a significant increase of power output (if your GTI is already clipping, then increasing the summer peak hour production from the panels will not net you any more power.)
If the roof space under the panels contributes significantly to your air conditioning load in summer, the additional spacing will get you lower roof temps as well as panel temps.

Vic · December 2012

Re: Are standoffs justified?

Hi DD,

Think that it will be very difficult for you to do a cost/benefit analysis, without running the experiment yourself.

Agree with i.netdog, that there is some data suggesting that 6" space beneth the PV modules is about the point of diminishing returns as far as operating temps of the PVs. What beneift this might be to you is better left for you to decide.

You speak of needing 40 feet, and guess that this is 40 L-feet (?)... The UniRac type approach of stand-offs, flashing for each, L-feet and rails is expensive for the hardware and somewhat for labor. It seems to offer (perhaps) better resistance to water penetration than simple L-feet and mastic goo for water resistance.

There are roof mount adjustable tilt racks that allow a greater up-tilt of the PVs and so on. ALL commercial racking solutions seem very expensive to me. BUT, if one needs the system to be inspected, these systems, in general, can smooth the permitting/inspection process, for a price.

On a recent installation, considered trying to get a bit cute with differing height stand-offs for the rails of each run of PVs, but seemed a small gain, and some risk of binding some of the hardware, if it was not designed for this angular difference.

In the end, you could look at the NREL on-line PV power calculator -- PVwatts, or PVwatts2 to try to see how small the benefit of the added tilt that one short, one taller mount for L-feet might make in your situation.

Regarding wind effects on roof mounts, setting back the PVs from the edges is important, and the rack solution calculators help you see this. YMMV, Good Luck, Vic

jaggedben · December 2012

Re: Are standoffs justified?

DaylanDarby wrote: »

I couldn't find an answer in the forums (or the net) to this question, please direct me if I missed it. How do I determine the cost benefits of standoffs

I think the only simple answer is what people have said: Once you reach 6" there is no benefit to going higher.

For what it's worth, California uses four different standoff heights when adjusting estimated energy production for its rebate program: 0", 0-3", 3-6", and >6". From their point of view, that level of precision accounts for the differences in production.

It appears that an L-foot plus a rail provides 3-4"s of clearance between the bottom of the panel frame and the top of the roof. Is this normally sufficient?

Maybe you just didn't mention it, but you should be using a flashing such as PV Quickmount, and this should add another inch to the height, and should get you to about 5". The production advantage of adding a standoff on top of this is near zero. And BTW, the cost effectiveness of flashing your attachments is much, much more important. Especially in snow areas where your roof won't last as long.

Is distance (IronRidge's 1/4" compared to Unirac's 1" mid-clamp width) between modules much of a factor?

I doubt it, and I doubt anyone knows.

Is butting (or not) the short ends of panels in portrait layout a factor? [Side Note: Is butting better or worse for snow?]

On the snow issue, you should do one or both of two things:
a) leave a small space between rows
b) adjust the upper row a bit higher off the roof so that the lower row does not create a shelf that encourages snow collection

Is panel size (65" vs. 77" long) or wattage a big deal?

No.

Has anyone used different (i.e., 6" on top, 3" on bottom) heights standoffs to provide increased panel tilt? How did that turn out?

On the racking systems that I'm familiar with, doing this would create a clamping problem because the panel frame must be perpendicular to the rail. So you would need additional hardware to angle the rails. I'd say it is only worth doing for changing the angle by 10 degrees or more.

Are standoffs justified?

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