Solar panels on a flat roof - tilt or no tilt?

[dusterbuster]

I just received the proposed site plan from our solar installer (11kw system in Santa Monica), and noted that they propose flush mounted panels on our flat roof (no tilt). Our house is oriented about 45 degrees off axis, such that if the panels were mounted and lined up along the roof joists and tilted, they would either have to be tilted southeast or southwest. We get a ton of sun exposure (no shade issues) here in Santa Monica. It would seem that tilting the panels SE or SW would mean increased efficiency for half the day, and decreased efficiency for the other half. I'm wondering if this might be a reason the installer proposed a flush mount, or whether they were just trying to minimize installation costs/time.

Also, even if it's true that a flush mount would be the appropriate layout for our situation, I'm wondering if the panels should at least have a slight incline (5 degrees maybe?) to help reduce dust accumulation on the panels when it rains (which admittedly is very rare in these parts lately).

Any thoughts/recommendations would be great appreciated. Thanks!

UPDATE:

Hey guys, the installer came back with a proposed plan that incorporates a 10-degree SW tilt. The designer said he didn't want to go too high because we have a short parapet around the roof of our 1 story home (so hopefully the panels remain unseen from ground level) and to reduce spacing between the panels. Do you think a 10-degree tilt will be sufficient to help keep the panels clean?

Also, to provide the tilt, the installer has switched the mounts to Ironridge Tilt Leg and Foot w/Chemlink E-Curb (rather than Quick Mount PV QMNC Q Base Comp for a flush mount). In comparing the diagram of the tilt versus flush mount, it looks like there is a bit more waterproofing required with the tilt leg mount (although maybe that is just manufacturer specific to Ironridge). Should I have any concerns that a tilt leg mount may increase the likelihood of water intrusion?

[emartin00]

I would strongly suggest a SW tilt. Southwest orientation will give you more power in the afternoon, when you would be using it. There is actually a significant push to start installing West facing arrays, because they produce power when people come home and turn on the AC.
As you said, the tilt will also help water shedding, and keep them much cleaner.

[Mike90250]

Agree with SW for increased peak hours harvest

[solarix]

Whatever you do, don't have them installed flat - unless you want to be up there cleaning them all the time. Tilted panels will keep themselves pretty clean just with rain washing them, but on a flat orientation the dirt will just build up and up. Installers like to orient the arrays to the roof framing of course, but there is no reason (with significant extra hassle) they can't go diagonally across the framing. Generally though, if you are within 30 degrees of south, go with it.

[SunEagle]

While I agree a SW tilt is best depending on the amount of roof space, tilting the panels will create shade issues if there isn't enough spacing between rows. That might limit the total installed wattage.

[ncs55]

Yes SW, agreed. Tilting does limit the total wattage installed, however it will increase the amount of harvest. So tilting can actually shave a few modules off of the system and still have the production needed to meet the demand. The total wattage installed can be drastically different from the total harvest and system production, especially when tilting is concerned. It is not about the total wattage installed, unless you are a sales person on commission. It is about how can you get the most harvest in the space / orientation allotted for your installation. It really depends on the location and what you have to work with. It is a good sign to see that at least the one company bidding is willing to use tilts.

[dusterbuster]

Thanks everyone for the helpful comments. I've told the installer that I would like the panels mounted at the appropriate tilt, so they are going to see what they can come up with.

[J.P.M.]

As others have pointed out horizontal orientation has drawbacks, the most practical being a much more frequent cleaning schedule if reasonable performance is to be maintained. A SW orientation of around 30 deg. or maybe a few degrees more would be a reasonable estimate for a decent orientation. Avoid horizontal orientation.

[DanKegel]

SW tilt!

(Also consider bifacial panels; with tilt mounts, they could get a fair bit of light on the back side.
Probably not competitive, but ask just in case.)

[dusterbuster]

Hey guys, the installer came back with a proposed plan that incorporates a 10-degree SW tilt. The designer said he didn't want to go too high because we have a short parapet around the roof of our 1 story home (so hopefully the panels remain unseen from ground level) and to reduce spacing between the panels. Do you think a 10-degree tilt will be sufficient to help keep the panels clean?

Also, to provide the tilt, the installer has switched the mounts to Ironridge Tilt Leg and Foot w/Chemlink E-Curb (rather than Quick Mount PV QMNC Q Base Comp for a flush mount). In comparing the diagram of the tilt versus flush mount, it looks like there is a bit more waterproofing required with the tilt leg mount (although maybe that is just manufacturer specific to Ironridge). Should I have any concerns that a tilt leg mount may increase the likelihood of water intrusion?

[ncs55]

If you have a composition type of roof make sure they used approved flashings at every standoff. If you have a tile roof make sure they double flash each standoff. Also make sure that they are not mix matching the racking system. IE say using one manufacturers standoff with another manufacturers rails. Mix matching these components voids any warranty from either manufacturer.

[Ian S]

Hey guys, the installer came back with a proposed plan that incorporates a 10-degree SW tilt. The designer said he didn't want to go too high because we have a short parapet around the roof of our 1 story home (so hopefully the panels remain unseen from ground level) and to reduce spacing between the panels. Do you think a 10-degree tilt will be sufficient to help keep the panels clean?

Also, to provide the tilt, the installer has switched the mounts to Ironridge Tilt Leg and Foot w/Chemlink E-Curb (rather than Quick Mount PV QMNC Q Base Comp for a flush mount). In comparing the diagram of the tilt versus flush mount, it looks like there is a bit more waterproofing required with the tilt leg mount (although maybe that is just manufacturer specific to Ironridge). Should I have any concerns that a tilt leg mount may increase the likelihood of water intrusion?

My foam flat roof solar installation here in Phoenix has a 15 degree tilt. The 15 degree tilt results in slightly reduced annual production but that's offset by the higher production during the expensive summer months. In the future, you can expect many utilities to follow the lead of SRP in Arizona to make late afternoon production far more valuable than production at other times. A southwest orientation works best for that.

A flat roof installation that's tilted looks OK from the front but the back view leaves a lot to be desired IMHO but I would not have wanted a flat install due to the dirt issue. Make sure you have your roof inspected before installation so that any flaws can be corrected. There should be no issue of water leaks if your roof is sound and the installation is done properly.

[jflorey2]

I would strongly suggest a SW tilt. Southwest orientation will give you more power in the afternoon, when you would be using it.

In addition, "June gloom" is a big factor in Socal installations, and often there isn't much sun before 11am in May/June. A recent study here determined that the best azimuth for coastal installations near LA and San Diego was about 195 degrees to account for the loss of sun during prime months due to morning clouds. (And that doesn't even take into account afternoon load peaks.)

[inetdog]

I... (And that doesn't even take into account afternoon load peaks.)

Nor does it consider potential Time Of Use (TOU) rate advantages for any overproduction.

[J.P.M.]

Hey guys, the installer came back with a proposed plan that incorporates a 10-degree SW tilt. The designer said he didn't want to go too high because we have a short parapet around the roof of our 1 story home (so hopefully the panels remain unseen from ground level) and to reduce spacing between the panels. Do you think a 10-degree tilt will be sufficient to help keep the panels clean?

Also, to provide the tilt, the installer has switched the mounts to Ironridge Tilt Leg and Foot w/Chemlink E-Curb (rather than Quick Mount PV QMNC Q Base Comp for a flush mount). In comparing the diagram of the tilt versus flush mount, it looks like there is a bit more waterproofing required with the tilt leg mount (although maybe that is just manufacturer specific to Ironridge). Should I have any concerns that a tilt leg mount may increase the likelihood of water intrusion?

Often, solar is a trade off of many requirements, but if one (aesthetics) impairs performance in one or more ways, like lousy tilt angle, that's when compromises may be warranted. Simply put, horizontal orientation of solar arrays almost always impairs performance due to less than optimum tilt/azimuth and more frequent cleaning, and there is not much to be done about it in a practical sense. If you are willing to accept those limitations and the penalties imposed, so be it.

Tilted racking has been around as long as flat plate collector technology ( > 100 years or more) and is pretty well developed. Well designed tilted systems can easily be as leak tight as arrays parallel to a mounting surface if done in a competent and professional manner. That's one more reason why an established vendor is worth a (slight) premium.

[dusterbuster]

Hi guys, just another update. I went back to the installer and asked if he could incorporate a 15 degree tilt and relocate a few panels, which they did. I think all things considered for our situation, this is a decent compromise. Thanks everyone for your helpful input.

[dominic213]

I would also suggest an SW tilt as it will give you more power in the afternoon. The usage will be very high on the afternoon and so SW tilt will be ideal. What I did was, I changed the orientation of my flat roof by hiring a commercial roofing contractor of Empire Roofing Corporation and made an ideal slope for the roof which will help in water shedding and keep them much safe and clean.

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