Impact on output if I can't mount panels level to the ground

[tandrup]

I have a situation where I am considering doing a ground mount system, but the ground isn't flat. I can orient the panel due South and give them a tilt angle like any normal situation. But the ground slopes East - West so the leading edge of the panel (the one facing due south) will be tilted so the panel edge is not horizontal. Hard to describe - I hope you guys get what I am trying to describe.

Will a ground mount like that have any negative effect on output? I have tried to search for websites or posts where I can specify that angle as well, but cannot find anything. Any thoughts?

Thanks,
Lars
Santa Rosa, CA

[sensij]

If you know the east-west slope and the amount of tilt, the effective azimuth / tilt for the array can be calculated. If you don't know how to do that, post the numbers here for help. The proposed layout can be modeled in PVWatts and compared to what a "level" south facing array would produce. If you install as described, it will either be somewhat SE or SW, depending on which direction is uphill. The impact on production will not be much, and could be beneficial if you end up with a slightly SW orientation and are considering a TOU rate plan.

[diogenes]

tilt angle (at top) 90- latitude

[ButchDeal]

I have a situation where I am considering doing a ground mount system, but the ground isn't flat. I can orient the panel due South and give them a tilt angle like any normal situation. But the ground slopes East - West so the leading edge of the panel (the one facing due south) will be tilted so the panel edge is not horizontal.

Or you could make the down hill legs longer so the array is level...

[skipro3]

I think you are saying g you can't get the panels a true landscape or portrait lay due to an east slope. No problem. The panels so t care. It's the azimuth and bearing that matters. Not if they are leveled to the horizon or not.

[Naptown]

The important thing is to make sure the wires come out on the low end to reduce resistance from the electricity having to go up hill. 😳

[lkruper]

The important thing is to make sure the wires come out on the low end to reduce resistance from the electricity having to go up hill. 

That's only true in Australia where the toilets flush counter clockwise.

[tandrup]

I think you are saying g you can't get the panels a true landscape or portrait lay due to an east slope. No problem. The panels so t care. It's the azimuth and bearing that matters. Not if they are leveled to the horizon or not.

I think that would only be true if I had a tilt angle of 90 degrees. Imagine the extreme situation with a tilt of 0 where you then drop one of the south facing corners down the east west slope. Ultimately you would end up with either an East or West facing panel (depending on whether east or west is uphill). It sounds like It does matter and that I have to figure out what the slope is so I can calculate the effective azimuth and maybe compensate for it. Exactly like the first response suggests.

[tandrup]

If you know the east-west slope and the amount of tilt, the effective azimuth / tilt for the array can be calculated. If you don't know how to do that, post the numbers here for help. The proposed layout can be modeled in PVWatts and compared to what a "level" south facing array would produce. If you install as described, it will either be somewhat SE or SW, depending on which direction is uphill. The impact on production will not be much, and could be beneficial if you end up with a slightly SW orientation and are considering a TOU rate plan.

Hi Sensij,
Here are my data points:
- Rack tilt: 32 degrees
- Rack Azimuth: 180 degrees
- East-West Slope with West being lower than East: Approx, 10% or 5.71 degrees. I do have a little freedom here to play around with making the downhill support a little higher off the ground.

If you could help show me how you calculate the effective azimuth and tilt I would be very grateful.

Thanks,
Lars, Santa Rosa, CA

[foo1bar]

Hi Sensij,
Here are my data points:
- Rack tilt: 32 degrees
- Rack Azimuth: 180 degrees
- East-West Slope with West being lower than East: Approx, 10% or 5.71 degrees. I do have a little freedom here to play around with making the downhill support a little higher off the ground.

If you could help show me how you calculate the effective azimuth and tilt I would be very grateful.

Thanks,
Lars, Santa Rosa, CA

I'm assuming this is a pre-built ballasted rack that's at 32 degrees?

For a complex set of angles I'd probably not try to calculate beforehand. I'd measure the panel's tilt using a plumb bob, a ruler, and a calculator with a tangent function.

If you just need a rough estimate before starting construction, you can guarantee that for a rack with a 32-degree tilt sitting on a 5 degree slope that the final tilt will be between 27 degrees (facing straight uphill) and 37 degrees (facing straight downhill)
Since it's a shallow slope (5 degrees) and it's mostly perpendicular to your main angle (slope is east-west, main angle is south) you're going to be very close to 32 degrees. +/- a degree I would guesstimate, depending on exact slope. So for calculations of what you'll get for production IMO 32 will be good enough.

[tandrup]

I'm assuming this is a pre-built ballasted rack that's at 32 degrees?

For a complex set of angles I'd probably not try to calculate beforehand. I'd measure the panel's tilt using a plumb bob, a ruler, and a calculator with a tangent function.

If you just need a rough estimate before starting construction, you can guarantee that for a rack with a 32-degree tilt sitting on a 5 degree slope that the final tilt will be between 27 degrees (facing straight uphill) and 37 degrees (facing straight downhill)
Since it's a shallow slope (5 degrees) and it's mostly perpendicular to your main angle (slope is east-west, main angle is south) you're going to be very close to 32 degrees. +/- a degree I would guesstimate, depending on exact slope. So for calculations of what you'll get for production IMO 32 will be good enough.

Hi there, thanks for the input. My system is a standard ground mount and I am trying to figure out what the effective azimuth and tilt angles will be so that I can experiment with different setups before I install the mounts - to see if I should compensate. Since I have a tilt on the panel in the S/N direction and an E/W slope, I know that my effective azimuth will be >180 (i.e. the effective azimuth will be more towards the SW) - and I am trying to calculate how much. I just can't remember my geometry and vectors well enough to calculate it...

[foo1bar]

I am trying to calculate how much. I just can't remember my geometry and vectors well enough to calculate it...

I think it's going to be fairly minimal effect.

You can do a paper/cardboard mock-up to see the effect.
Or mock it up within a CAD program. (there's a lot of them that are free for home use or free for trial period. Even old high-cost Autocad has a free trial offer - but I think there are better options than that.)

I am sure there's a geometry/math way of doing it too - but I don't know the formulas and for a one-off project like this I wouldn't spend the time learning them.

[foo1bar]

My system is a standard ground mount

What do you mean when you say "standard ground mount"?

Because with no other context, I would think "poured footings with poles coming up out of the footings nearly perfectly vertical" And the racking is level - which means the footings may vary slightly as the ground goes up/down across the length of it.
IOW it doesn't matter what the ground's slope is because you effectively make it a flat spot where you're doing the mounts.

[inetdog]

Hi there, thanks for the input. My system is a standard ground mount and I am trying to figure out what the effective azimuth and tilt angles will be so that I can experiment with different setups before I install the mounts - to see if I should compensate. Since I have a tilt on the panel in the S/N direction and an E/W slope, I know that my effective azimuth will be >180 (i.e. the effective azimuth will be more towards the SW) - and I am trying to calculate how much. I just can't remember my geometry and vectors well enough to calculate it...

If you cannot keep all of the panels in one plane (need to stair step the panels along the E/W slope) be sure that the panels are not going to be shading each others' edges when the sun is low in the E or the W. (Although the total output may not be worth worrying about at those extremes. Just make sure you space them far enough apart that there is no shading within a couple of hours each side of solar noon.

[tandrup]

If you cannot keep all of the panels in one plane (need to stair step the panels along the E/W slope) be sure that the panels are not going to be shading each others' edges when the sun is low in the E or the W. (Although the total output may not be worth worrying about at those extremes. Just make sure you space them far enough apart that there is no shading within a couple of hours each side of solar noon.

Agreed - I am looking at doing that as an option too. I actually went out and measure the slope of the ground - it sits at about 27%-30%. Stepping might be what I have to do. And I will definitely space them apart to make sure I don't get too much shading from the uphill panels (the Eastern panels) in the morning. Another alternative I have to consider is to locate the panels on the lot somewhere else. I have an area that is further away from the house (approx. 300 feet compared to the original location I was thinking that is 100 feet away) - I don't know if that causes significant losses with a longer run from the panels. The other location has a natural South facing slope and is level in the East/West orientation - so almost ideal. Only hiccup is that it is outside of our building envelope and per my conversations with the building department in Santa Rosa, the panels are treated as a "structure" and therefore need to be located within the building envelope. I might be able to get an approval to extend the building envelope. Does anyone have experience with that in Sonoma County?

It will have to be either a "stepped" system or a system that is parallel to the ground - if I can figure out what the impact of mounting it that way is (i.e. azimuth = 180, rack tilt = 32 and a sloped rack that follows the ground).