To Tilt? Or Not To Tilt?

[quantum`]

That is the question.

I read conflicting advice here about whether it's worth it. Yet every commercial install I see is tilted.

I'm building a housing development and all houses will be certified Energy Star NextGen Homes and DoE Net Zero Homes. The south-facing roof is large enough for seven 660w panels in 3 rows. So I have to put the remaining three panels on the west wall to catch afternoon Sun (and impress the neighbors). Roof slope is 22.3*. My latitude is 47.887.

I figure if I mount the roof panels landscape they'll be less likely to shade the ones behind them, worst-case in Winter 1/3 shaded, affecting one (bypassed) string. And if I remember Code correctly you can only raise an edge 10". I can't move the rows further apart. So I don't know how much angle this would add for 3.72' wide panels, but the addition of the back gather should help something.

I should mention that these panels are Longi HI-Mo 9, which are bi-facial. If I tilt I'll put down a layer of mirrored mylar on the standing-seam roof to reflect incident light to the back. The cost for mounting hardware is about the same for tilt or non-tilt. If tilt I'll put a rail along the top and bottom edge and bolt it through the roof, anchored to the concrete beams in the roof. (It gets windy 3 storeys up)

 

[quantum`]

To tilt it is. It's the only realistic answer for bifacial.

And with mirrored mylar, diamond-imprinted, adhered to the roof with exterior paint. This diffuses light, eliminating hot-spots.

 

[scoharri]

Tilt

 

[Gaijin2]

You can calculate how much difference it makes, using PVWatts.

I found some place in Washington (Monroe) with a latitude similar to yours, and ran a computation for a 4K array at 22.3 degrees to match your roof slope. PVWatts projects production of 4,842 kWh per year.

Then I changed the angle to the "optimum" angle for latitude 47.887, which is 34.7 degrees. PVWatts says that same array at this new angle would produce 4949 watts per year.

So the hassle of angling will theoretically net you a whole 2% more power harvesting. Plus it'll avoid any panels shading any others. But maybe you'll see a little bit of bifacial gain, although to get any substantial bifacial gain you have to get the panels seriously up off the roof. If the base of the panel is attached to the roof and you're tilting the panel up about 12.4 degrees to bring the total angle to 34.7, then I would guess that the bifacial gains are going to be minimal.

Unless I'm doing something wrong and someone wants to point out what it is; I'm always up for a learning opportunity.

 

[Madcodger]

Bifacials are overrated. There, I said it. Almost half our panels are bifacials, and yes, on a cloudy day, especially a cloudy day after it has snowed, we get perhaps a 15% gain from bifacial panels. If I can buy bifacials at about the same price as monos, I'm fine with that. But for a wall? YOU DON'T NEED THEM. One of our four strings is mounted at 90 degrees. In winter, it produces an amazing and welcome amount of power. It will perform less well as we move into spring, of course, but by then we produce more power than we can use, as is common for a system that produces about enough in winter. The gain one gets from bifacials vs monos mounted to a wall, tilt or no tilt, is not terribly meaningful and a whole lot more potential trouble.

Also, people get obsessed with tilt. It matters, but less than many think. At this time of year, my vertical (90 degree) panels perform almost as well as my panels mounted at 67 degrees, which is the exact angle of the sun on Dec 21. And being mounted to the wall, there's much less chance of the verticals catching snow, having things like leaf debris end up behind the panels, etc.

Don't OVERthink this, folks. It's simpler than you'd think. Just mount the panels vertically, and enjoy the carefree production.

 

[quantum`]

Mounted to the wall, mounted to the wall, I know this of course, Madcodger. But it is more of a marketing decision than a technical one, as I've said. I am building and intend to sell a housing development. My panels are bifacial, period, and as to their value we agree to disagree.

You presume no reflectance to capture incident light, particularly what I have in mind. I am 70yo, but I tell you, come in to the 21st Century.

P.S. The fad of mounting them vertically sacrifices too much, when you consider the overall picture.

 

[Madcodger]

Mounted to the wall, mounted to the wall, I know this of course, Madcodger. But it is more of a marketing decision than a technical one, as I've said. I am building and intend to sell a housing development. My panels are bifacial, period, and as to their value we agree to disagree.

You presume no reflectance to capture incident light, particularly what I have in mind. I am 70yo, but I tell you, come in to the 21st Century.

P.S. The fad of mounting them vertically sacrifices too much, when you consider the overall picture.

We can agree to disagree, and good luck to you. If you want to do something just "to impress the neighbors" which I interpret as "sell these houses", that's up to you. Not my bag. But I can assure you that we are very much in the 21st century. I think we just have very different goals.

 

[Gaijin2]

Also, people get obsessed with tilt. It matters, but less than many think.

I'm pretty new to solar, but I must admit I was surprised if not shocked at how little actual difference it seems to make, at least way down south here near the Gulf, at least according to PVWatts.

I thought adjusting the tilt throughout the year (quarterly) was a big deal and the best way to optimize output. Then I started running PVWatts and it really opened my eyes. Either it's dead wrong, or Madcodger's assessment is pretty spot on.

For example: my latitude's optimal angle is 26.7 degrees. I got the Brightmount adjustable version, with 25-, 35-, and 45-degree angle. I ran simulations for a 4K array for all those scenarios, and here's what I found:

Facing south, tilt 25 degrees, optimal scenario: 5,671 kWh annual production
-- tilt 35 degrees, 5,584 kWh annually.
-- tilt 45 degrees, 5,353 kWh annually.

That means the worst tilt angle was still producing 94.4% of the best tilt angle I could possibly get. It amounts to about $30 per year of savings, so for the one-time effort of putting the Brightmount at the 25 degree angle I'll do it. But if I had to buy and install different mounts on the roof to capture that slight difference, I doubt it's worth it. At that rate I'd just throw an 11th panel up there, even at the 45 degree angle the array would be way outproducing the 10-panel 25-degree array (5,889 vs 5,671).

The crazy thing was panels facing south at the optimal angle, vs. just laying them on the ground. The ones laying flat still produced 5,192 per year! That's 91.6% of what the "optimal angle" south-facing panels did! Based on that I've decided that avoiding shadows is much more important than worrying about tilt. Seems like a shade-free east, west, or north panel will outproduce an optimally-angled south panel that gets partial shade occasionally.

 

[mciholas]

I'm pretty new to solar, but I must admit I was surprised if not shocked at how little actual difference it seems to make

This is true.

You are better off putting your money into more fixed panels than trying to tilt panels.

The important mathematical fact is that cosine of 20 degrees is 94%, so being off 20 degrees is still pretty good.

Mike C.

 

[Madcodger]

I started running PVWatts and it really opened my eyes. Either it's dead wrong, or Madcodger's assessment is pretty spot on.

There's nothing quite like necessity to sharpen one's focus. We could have easily connected to the grid (and I built the house to make that reasonably easy in the future) but we deliberately elected to forego that, in part to force us to make decisions that optimize both saving power and generating it via solar. So, we installed four strings at three different tilt angles (45/67/90). It allows us to observe the performance of each string every day, at differing times and under differing weather conditions. You don't need to be very smart to learn in this way. You just learn a lot by doing, day after day. If it doesn't kill you, you gain valuable experience. Much of the theory one reads is a good place to start, but personally I learn more by doing. What I find funny are those folks who argue, having never actually done it. PVWatts, by the way, is one of my favorite tools. It's a great way to get a solid idea about what's going to happen. I've found it to be reasonably predictive of reality.

 

[42OhmsPA]

Tilt.

 

[OffGridForGood]

OP shows Seattle as location - a search tells us Seattle generally doesn't see a lot of snow, maybe a few days per winter, occationally longer stretch of snowy weather but rare.
My own experience is snowy areas need the tilt not so much for better solar collection per se as it is to assist with snow clearing.
If the OP can and is prepared to clear snow during a few brief snow-days per winter, then no tilt is required.
PV watts doesn't show a lot of kWh difference between tilting or not tilting, since winter is generally not great in both cases; I plugged in 47-degree tilt for a 1kW PV array and Dec shows 40kWh. Changing to 65-degrees for the same 1kW PV gives 41kWh ie nearly the same.
If you don't have snow to deal with, don't worry about tilting is my advice, in your region.

 

[42OhmsPA]

I see significantly better output from my arrays that are tilted for the time of year.
I'm ~41°N.

Graphs are from today.


How the panels on inverter 2 are angled for said graphs.

Drone shot and data from 12-2.

 

[OffGridForGood]

I see significantly better output from my arrays that are tilted for the time of year.
I'm ~41°N.

but not in Seattle, and appear to get a bit more than Seattle snowfall.

In PV Watts using Seattle as the location, input 1kW array, tilt to anything you think is appropriate, and check output for December.
A few examples I tried:
roof slope say 30-degrees PVWatts says 37kWh for the month of Dec.
latitude angle 47-degrees PV Watts says 40kWh for the month of Dec.
Latitude plus 23-degrees = 70 degrees PV Watts says 41kWh for the month of Dec.

 

[42OhmsPA]

but not in Seattle, and appear to get a bit more than Seattle snowfall.

In PV Watts using Seattle as the location, input 1kW array, tilt to anything you think is appropriate, and check output for December.
A few examples I tried:
roof slope say 30-degrees PVWatts says 37kWh for the month of Dec.
latitude angle 47-degrees PV Watts says 40kWh for the month of Dec.
Latitude plus 23-degrees = 70 degrees PV Watts says 41kWh for the month of Dec.

I'm guessing that it's due to the cloud cover.
Old data set but ... https://www.currentresults.com/Weather/US/cloud-fog-city-annual.php

 

[OffGridForGood]

I'm guessing that it's due to the cloud cover.
Old data set but ... https://www.currentresults.com/Weather/US/cloud-fog-city-annual.php

226 days of cloud !! Well I will shut up and stop complaining about my weather now LOL.
Anyway the data show that tiltiing in Seattle may not make a lot of difference.
The OP is in Seattle, and asking if tilting is recommended - from what I see - unlikely to help much.

 

[42OhmsPA]

226 days of cloud !! Well I will shut up and stop complaining about my weather now LOL.
Anyway the data show that tiltiing in Seattle may not make a lot of difference.
The OP is in Seattle, and asking if tilting is recommended - from what I see - unlikely to help much.

After seeing the PVWatts data you compiled I agree.
I still like tilts

 

[OffGridForGood]

I had to double check your spelling on that last comment...Like em too, with or without the L.
But you and I deal with snow, and tilting makes it sooooo much easier.
For the member in Seattle, where snow may not be the factor we are used to, we need to focus on the cost of tilting racks, vs benefit of them.

Maybe better to look at it from a cost benefit angle: (see what I did there)
Looking up Seattle utility rates a seach says 13.9c/kWh lets call it 14cents.
Typical home using 30kWh per day or 900kWh per month approx.
December is the worst month - If they sized the arrays for the typical 80-90kWh per kw of array size they will see most of the year, then they could get most months energy covered with 900/80 = 11kW PV array.

This array if it tilts up to 70 degrees for December will bring in 41kWh x 11 = 451kWh
If the same array is roof mounted at only 30-degrees, the it brings in 37kWh x 11 = 407kWh for Decmber.
a loss of 43kWh

This loss is going to need to be made up from the grid or a generator (even more expensive) so using the 14 cents per kWh x 43kWh lost
the value of tiltin for 11kW array in Seattle is 43 x $0.14 = $6.02 for the month.

Building a tiltiing array is quite a bit more work and expense than a non-tilting one - or at least mine were, so at $6.02 difference per month, the OP can decide if it will be worth it for them.

 

[Daferris]

well pasting the cells did not work here is a screenshot of the sheet.

 

[OffGridForGood]

well pasting the cells did not work here is a screenshot of the sheet.
View attachment 272964

adding up all the 'best' angle values for each month I get 21,747
but just using 30-degrees gives 21,041
a difference of 706 kwh for the whole year, or 3%
Using utility power rate to "value" those 706kWh at $0.14 each = $98.84 / year if these can all be used.

Some of the differences will be during summer months - ie the difference may not be meaningful if it represents additional lost kWh - un-used kWh.
Will you be selling back to the grid?

 

[skyking1]

For ground mounts, I'd optimize it for sure. For roofs not so much. The wind loads, the hassles vs flat mounting are not worth the troiuble. If you can squeeze in another flat mounted panel somewhere it negates the advantage, and panels are relatively cheap.

 

[doityourself2021]

I had to double check your spelling on that last comment...Like em too, with or without the L.
But you and I deal with snow, and tilting makes it sooooo much easier.
For the member in Seattle, where snow may not be the factor we are used to, we need to focus on the cost of tilting racks, vs benefit of them.

Maybe better to look at it from a cost benefit angle: (see what I did there)
Looking up Seattle utility rates a seach says 13.9c/kWh lets call it 14cents.
Typical home using 30kWh per day or 900kWh per month approx.
December is the worst month - If they sized the arrays for the typical 80-90kWh per kw of array size they will see most of the year, then they could get most months energy covered with 900/80 = 11kW PV array.

This array if it tilts up to 70 degrees for December will bring in 41kWh x 11 = 451kWh
If the same array is roof mounted at only 30-degrees, the it brings in 37kWh x 11 = 407kWh for Decmber.
a loss of 43kWh

This loss is going to need to be made up from the grid or a generator (even more expensive) so using the 14 cents per kWh x 43kWh lost
the value of tiltin for 11kW array in Seattle is 43 x $0.14 = $6.02 for the month.

Building a tiltiing array is quite a bit more work and expense than a non-tilting one - or at least mine were, so at $6.02 difference per month, the OP can decide if it will be worth it for them.

Fixed Tilting has some, and I repeat some value..not the same at Tracking Tilt single axis. cloud density.. keep in mind, diffusion... of light by water droplets/moisture in the cloud.. you can have a cloud depth of 500 feet thick, or 100' thick or 2000' thick, the darkness of a cloud pends on how much moisture and debris(dirt /dust) is in it.. then you have to take the water droplets, some clouds you can go thru and be 2000' thick and hardly a bit of moisture in or a 500' thick cloud and wet very wet, raining inside.. get below it, won't be a drop of moisture.

Clouds play a big part, take a high cirrus form wispy cloud at 20-40k feet above.. faint in thickness and can reduce the PV UV 40-60%..vs. clear blue.. and add or subtract that from Tilt angle it really can affect production.

Where one really gets to appreciate production, with 2 axis tracking of the UV Light, clouds & sun.. Pending where you live and the type of weather patterns and climate all play with us on the PV production..

Finding the Best Fixed Tilt for say using two of the 4 season... can in a 12 month period can produce a better production vs. one fix position, especially if its not the best angle (tilt).

What I have learned in 4 years of two axis tracking, that for the particular time of year, tilting most productive if one can just follow the Sunrise to the Sunset, produces higher results vs. Azimuth.. However there are about 1/4 to 1/2 of the 12 month cycle that being able both track E to W and Azimuth can really benefit.

I understand most do not want or able to use tracking, since panels cost vs the tracking system demands just makes it cheaper and easier to add panels.. If you live in a rural area with lots of roof or acres.. then come up with a simple tilt system.. E to W angle..

If interested in simple tilting or both, tilt and azimuth there are a lot of designs now out and very low cost.. under $500 for material. And if your a DIY you can beat that easily.
Tracking controllers for just tilt angle or both tilt and azimuth, all under $200. including actuators for the mechancial aspect.. If you want some automation.. If not.. just go out and change the tilt, Sunrise 11am, and 2 pm vs. fixed you will see a 5-10% increase.

Just for the know, I have been running two axis auto tracking for 4 years. As an experiment I have stopped the tracking at a Fixed positon for a day to watch the effect and production. Summary, my production was reduced between 25-30%... For myself that reflected a change from normal average of 17kw per day production on 5.5-6hr to 12.5kw production with a Fixed tilt no tracking, aprox 4.5kw loss.

So tracking has its place. Years ago, a lot more tracking was done because of the cost of PV panels, using early 2000's year a Solar Panel cost about $5-10/watt so a 200w pv per panel cost you $800-$1000 on average, so tracking was much more economical.. today 2024, you can buy a Bi-facial 550-700 wat PV panel for under $100 per..

So people buy Pallets of PV panels.. that's ok if you have the roof, or area to use all that.. But keep this in mind and I will use my own experience.. with two axis.. I have 8 panels 460w 51.4 v 11.6 / with a 5 hour average day, 18kw per day because of tracking, now with same if I was fixed to a preset best tilt and azimuth, as I have tested more than once with mostly sunny days I would see 13.2kw, again about a 5kw loss,, you add that up on a 30 day timeline, keep in mind if we lived in a perfect world of sun each day 18k x30= 540kw vs 13.2 x 30= 396kw
about 144kw per month .. Now for myself and significant other we have all the modern conviences but our average load consumption is 13-14kw.. for another it may be 30-75kw.. to each his own. Propert tilt and azimuth does add up..

so you would buy 12 more panels to not track to make up the difference of what I am able to do.. all well and good, but that would be another 336 sq feet of area you have to take up on a roof or ground mount.. lets see that would be aprox 24' wide x 14' = 336 sq' height foot print ground or roof to do the same thing I am doing with only 8 panels, taking 32' w x 7' h=224 sq' vs. my 224 plus 336 = 560 sq' to do the same as my 224 sq ' because of tracking.. Using up more than double foot print to do same as my tracking.

All I say, Tilt Fixed, Tilt Track and Two Axis have their results. You have to decide what works best for you.. I also realize some if not many want to provide 30-90kw per day to run the whole home.. and some of us can't or not willing to trade off some of the high demands. So you need a lot of panels and batteries... But even with a lot of PV panels and if you look at some of these large PV farms they use tilt Sunrise to Sunset tracking.. lots more production.. Two-three more hours of production in the same time line between 4.5-6.5 hour makes a difference.
R
Something to consider if you see a place..

 

[mciholas]

For the cost (and future maintenance and downtime) of that 2 axis tilting mechanism, they could have had 3 times the panels in a fixed position and produced more total energy.

Panels have gotten so cheap they make tilting not economical any more.

Utility scale systems sometimes use tilting for load leveling reasons, making the power generated more level during the day, because they don't have enough (or any) storage. That's why you see north south axis tilting, to get early and late sun to fill in the power curve. They sacrifice midday production (panels are flat then) to get good early and late day production (tilted east and west).

For individual use, with some storage, the goal is the most energy per day, not the most level energy during the day, so a fixed tilt to the equator (south for most of us) is best. A single house demand is so intermittent that we all need storage or net metering to make it work, so making the most power per dollar of system cost is the goal.

Mike C.

 

[OffGridForGood]

Panels have gotten so cheap they make tilting not economical any more.

I recently got an offer of 9-cents per watt for JA 710W panels - based on 40-foot seacan minimum unit of purchase.
710W panel x 9-cents per W is only $63 per PV panel (plus the shipping, yes). Lets say $83 including shipping.
most regions can count on 1 kW per year from each watt of PV = each JA 710 would produce 710kWh per year.

Say utility power is 15-cents per kWh, then each JA 710 brings in $0.15 x 710kWh = $106.50 per year...x 20 years ! oh, but loosing efficiency over that 20 years. True, lets' say average of 90% (100 % year one, and 80% year 20) the panel can be expected to collect $1917 worth of electricity...but utility rates will increase over that same 20-years...how much? IDK.
If the panels can collect $2,000 over a 20-year timeframe but cost $83, plus racking, inverters, connections - looks like a good gig.

For the big utility scale PV contracts you can see prices far lower than 9-cents per W.

But the "real" benefit from solar energy is not based on utility rates - at least for me - the real benefit is the substitution of gasoline with electrical power, where the payout I get is 71-cents per kWh of "not-gas" transportation.
Now I wave my hand as I drive past every gas station ... sometimes I use all my fingers...

 

[jjww]

Optimize tilt for Winter-when you need the power the most.
little if any summer A/C use in Seattle.