# KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

Solar Expert Posts: 75 ✭✭✭✭
I just need some people to confirm my results below, and confirm the formula below will give a good ball part estimate or recommend a formula that will. Simply put, I just want to find the best tilt that you should mount a fixed solar panel system to receive the best results year round. I know you will have to sacrifice some power by not adjusting the angle every month or so.

So I just want to take this 1 step at a time until I'm decent enough to figure this out for any location.

1. First, I went to this website, and typed in my zip code to find my latitude: http://www.findlatitudeandlongitude.com/
It gave me the following info:

Latitude:N 36° 9' 16.5262"
or
Latitude:36.15459060668945°

2. Next I read a solar book from Amazon that gave me this formula for figuring out the best tilt: 90° - your latitude - 15.6° = optimum winter setting

So based on my latitude, would that formula turn into: 90° - 36° - 15.6° = 38.4°
So I would need to mount my solar panel with a 38.4° tilt.

So does that appear to be right, or a good ball park estimate for my location?

• Solar Expert Posts: 4,822 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

do you get a lot of sun and snow? if so a (near) vertical position may be better as it will shed the snow and get more reflection offf the snow.

hth

ps the formula is probably ok

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge, Hughes1100 Sat Modem
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Solar Expert Posts: 75 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
westbranch wrote: »
do you get a lot of sun and snow? if so a (near) vertical position may be better as it will shed the snow and get more reflection offf the snow.

hth

ps the formula is probably ok

Nope, this year (Jan&Feb) we didn't get any snow almost. But when we do some years, it is not that much. No more than an inch.
• Solar Expert Posts: 10,311 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

i think there may be a discrepancy as it may depend on what a pv laying flat would be determined to be. a flat pv we deem to be 0 degrees. we usually say lat + 15 degrees from the flat. they may be using a vertical pv as at 0 degrees and that might be why they would put 90- on there. the extra .6 degrees won't hurt and i don't deem that as all that important as i doubt you will mount it to that degree accuracy anyway.
• Solar Expert Posts: 75 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
niel wrote: »
i think there may be a discrepancy as it may depend on what a pv laying flat would be determined to be. a flat pv we deem to be 0 degrees. we usually say lat + 15 degrees from the flat. they may be using a vertical pv as at 0 degrees and that might be why they would put 90- on there. the extra .6 degrees won't hurt and i don't deem that as all that important as i doubt you will mount it to that degree accuracy anyway.

_ represents the solar panel's position

-So if the PV's flat position was at 0 degrees horizontally => _ then the equation would be: my latitude + 15 degrees = best fixed position for year round results

-And if the PV's flat position was at 0 degrees vertically => | then the equation would be: 90 - my latitude - 15.6 = best fixed position for year round results

I may have to make a quick sketch of the panel tilt as when I do the calculations on the vertical part, it just seems like the angle should be larger, but maybe I'm just not use to seeing panels with a small tilt.
• Solar Expert Posts: 10,311 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

you may or may not be understanding me for a vertical pv can't be laying flat. i am still not sure of the meaning the book may have portrayed and i am doing a bit of assuming here on that. rather than worry about it just add 15 degrees to your latitude. this is the same as a pv laying flat on the ground and then lifted up on one end until the angle where the pv meets the ground is at that angle equivalent to the latitude + 15 degrees. now if you think you can make the 15.6 degree addition with accuracy then go ahead and add 15.6 degrees to your latitude. there's no need to make it complicated as being a degree off is not going to make much of a noticeable difference.
• Solar Expert Posts: 3,009 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

I agree with Niel, and would even take it a bit further, saying that being a few degrees off won't make any real noticeable difference. I live in the snow belt, further north than you, and in Winter have my panels tilted 90* vertical. That's further up than the formula's recommend, but taking into consideration that no snow what-so-ever collects on them, + the fact that I get sun reflected off the snow covered ground, my overall output is actually considerably higher than if my panels were tilted to the exact "proper" angle. I re-angle in Spring, and again in Fall. That's it. For me, any gain from tilting more often isn't worth the bother.
• Solar Expert Posts: 1,973 ✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
_ represents the solar panel's position

-So if the PV's flat position was at 0 degrees horizontally => _ then the equation would be: my latitude + 15 degrees = best fixed position for year round results

-And if the PV's flat position was at 0 degrees vertically => | then the equation would be: 90 - my latitude - 15.6 = best fixed position for year round results

I may have to make a quick sketch of the panel tilt as when I do the calculations on the vertical part, it just seems like the angle should be larger, but maybe I'm just not use to seeing panels with a small tilt.

Tilt is measured as degrees from pointing straight up. At the equator, the best tilt angle for year round production is 0 degrees, discounting climatic variations (more cloudy in summer than winter, etc.). That's the latitude. Similarly, the best tilt at the north pole would be 90 degrees.

Wherever you are, if you tilt your modules at your latitude, the sun spends as much time higher than straight on at solar noon as lower in the course of a year, which would be the optimum tilt for year round production if nothing else were considered. But the lower the sun is in the sky, the less solar radiation reaches the earth where you are because it has to travel through more air to get to you. Also, it may indeed be more cloudy in summer where you are.

All that is taken into account in the database at http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/sum2/state.html. Thirty years of solar irradiance data is referenced by location and you can see what the monthly and yearly average incident sunlight would be on a surface at zero degree tilt, 90 degree tilt, latitude tilt, and plus/minus 15 degrees from latitude tilt. It also has data for single and dual axis trackers. You can even drill down into it and get hourly data for every day of the year if you want to carry it to that extreme (for large commercial installations you might want that). This is the data that PVWatts uses for its projections.

I would advise looking at this data for the nearest collection point to you rather than using any formula you can come up with. Even easier would be to run a bunch of test cases through PVWatts.
• Solar Expert Posts: 10,311 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

as ggunn pointed out there is data that is more site specific out there. even that data can be wrong for your particular install location as influencing factors can change even over just a few miles. what i referred to and that book was a generalization. do know that one size does not fit all and all are compromises unless tracking is done. what is the best compromise is sometimes hard to say and that can vary to a point with other influencing factors that aren't always consistent. my big point was that if you do go a few degrees one way or another that it will not make that big of a difference in general. adding 15 degrees or adding 15.6 degrees you simply will not see a difference on the output and even if you did it is liable to change some due to other variable factors so you can draw a wrong conclusion as well as a right conclusion from one day, week, month, season, or even year to the next. it's not that critical to worry about.
• Solar Expert Posts: 1,366 ✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

A question I didn't see asked yet - is this for a grid-tied system or an off-grid system? For grid-tied I would use the angle that gets you the best output year-round. For off-grid use the best angle for winter production.
4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
• Solar Expert Posts: 1,973 ✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
techntrek wrote: »
A question I didn't see asked yet - is this for a grid-tied system or an off-grid system? For grid-tied I would use the angle that gets you the best output year-round. For off-grid use the best angle for winter production.
I guess many offgridders will use more power in the winter, but I wouldn't think that's universally true.
• Solar Expert Posts: 3,738 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
Simply put, I just want to find the best tilt that you should mount a fixed solar panel system to receive the best results year round.
The optimal angle (as techntrek pointed out) may depend on whether you are on or off grid.

Another consideration is that the optimal direction for you may not be due south. This may be because of local conditions such as fog in the morning or shading in the afternoon, for example. If you are doing a roof mount, and your roof is not facing due south, the calculations for optimal tilt are different.

Still another consideration (affecting both tilt and direction) may be the timing of your loads. This is true if you are off grid and may be true on grid depending on what net metering arrangements or time-of-use you have. With panels as cheap as they are now it may be cost effective to have split your panels into two arrays (with two charge controllers) with different tilts and directions. If you are using microinverters you might even have more than two tilts and directions.

ggunn wrote: »
I guess many offgridders will use more power in the winter, but I wouldn't think that's universally true.
I use less in the winter because a large portion of my energy budget is a freezer in a detached (very cool in winter) outbuilding. If I ever get air conditioning or dehumidifier my summer energy use will be still greater.
--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 3,009 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
vtmaps wrote: »
I use less in the winter because a large portion of my energy budget is a freezer in a detached (very cool in winter) outbuilding.

That's also my experience. The colder the weather the less that freezer runs, thus the less energy used. Come summer my consumption goes way up because the freezer really has to work.
• Solar Expert Posts: 157 ✭✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
ggunn wrote: »
I guess many offgridders will use more power in the winter, but I wouldn't think that's universally true.

But the optimum winter angle will often produce just as much if not MORE power in the summer anyway, even at the less-optimal angle. For on-gridders this doesn't matter, since they can use higher summer production to offset lower winter production, but off-gridders are basically sizing their systems based on the months of lowest productivity, and so it makes sense to optimize to this condition.

Example: PV watts says that 35 degrees is the optimal angle for year-round production at my location. For a 5 kW system, standard derate, that means a total of 6158 kWh, with 328 in December and 600 in June.

If I tilt that same array up to 60 degrees, I produce only 5679 kWh per year, but now I produce 365 in December and 459 in June. Tilting the array even more loses production in both summer and winter, unless the snow-glare effect is prevalent.

It's not a huge difference, but that way I can squeeze a few more kWh into the winter months, when the sun is scarce, and I still generate excess power in the summer months, so who cares about kWh I can't use anyway?
• Solar Expert Posts: 407 ✭✭✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

Here is a great resource. (sorry, US only)

This page calculates the altitude and azimuth of the Sun (or Moon) at multiple times during any day between 1700 and 2100. Simply specify the object, date, tabular interval, and place:

http://aa.usno.navy.mil/data/docs/AltAz.php

Our tax dolars at work.
• Solar Expert Posts: 1,366 ✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
jagec wrote: »
It's not a huge difference, but that way I can squeeze a few more kWh into the winter months, when the sun is scarce, and I still generate excess power in the summer months, so who cares about kWh I can't use anyway?

This is what I was alluding to in my last post, thanks.
4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
• Solar Expert Posts: 121 ✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

I found this link here on the forum I think. I think it is quite useful.
http://www.macslab.com/optsolar.html
• Solar Expert Posts: 108 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

If you didn't know shirt from shin-ola you can set the angle by
1.) know "solar noon" at your local
2.) use a 12" stake with a true, square 90* base
and place it on the panels, adjust said panels until there is no shadow.

Way too simple...
• Solar Expert Posts: 1,973 ✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
plongson wrote: »
If you didn't know shirt from shin-ola you can set the angle by
1.) know "solar noon" at your local
2.) use a 12" stake with a true, square 90* base
and place it on the panels, adjust said panels until there is no shadow.

Way too simple...

But on what day? Summer solstice? Winter solstice? Equinox? On the equinox would tell you the optimum tilt for yearly average production if no other factors are in play. Of course, other factors are always in play.
• Solar Expert Posts: 1,973 ✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
vtmaps wrote: »

Another consideration is that the optimal direction for you may not be due south. This may be because of local conditions such as fog in the morning or shading in the afternoon, for example. If you are doing a roof mount, and your roof is not facing due south, the calculations for optimal tilt are different.

Of course, most residential roof mounts are on slanted roofs and are for all practical purposes restricted to the azimuth and tilt of the roof. If you installed racking that would mount the modules at a different tilt than the roof, the extra expense incurred would likely far exceed the benefit gained.
• Solar Expert Posts: 10,311 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
ggunn wrote: »
But on what day? Summer solstice? Winter solstice? Equinox? On the equinox would tell you the optimum tilt for yearly average production if no other factors are in play. Of course, other factors are always in play.

to add to what ggunn said here, i would say when you find the angle on the equinox for solar noon that aiming for solar noon is not going to give you optimum power harvested because that is only the peak angle and not the average angle around that time. at all other times that day the sun is lower in the sky as well as a bit farther east or west. aiming below the equinox peak sun position will still reap good power at noon with little degradation and bring the focal point closer to the sun positions at other times just off of solar noon. this is why the extra 15 degrees in tilt is often added. this makes it optimum for that time of the year, barring other factors, and does not make it optimal for the winter btw. this is the best average production reaped on the day of the equinox which is the average of all seasons. a winter peak will be latitude + 23.5 degrees as long as that answer is 90 degrees or less and being it is a peak the sun will sit lower in the sky than that angle for all other times.
• Solar Expert Posts: 106 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

Thats a good point Niel, thats the way a pivital tone arm on a record player is set. Instead of intersecting the groove at 90 degrees at the center of the arc it is set to intersect it twice at 90 degrees, once at the rise and once at the fall of the arc. This allows for overall less tracking error particularly at the extremes of the arc.
However I suspect that even though setting an array at solar noon on the equinox would not provide the best output for that time of year, it would be a good compromise annually.
The suns angle at solar noon on the equinox I believe is the same angle as your latitude.
• Solar Expert Posts: 1,973 ✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
niel wrote: »
to add to what ggunn said here, i would say when you find the angle on the equinox for solar noon that aiming for solar noon is not going to give you optimum power harvested because that is only the peak angle and not the average angle around that time. at all other times that day the sun is lower in the sky as well as a bit farther east or west. aiming below the equinox peak sun position will still reap good power at noon with little degradation and bring the focal point closer to the sun positions at other times just off of solar noon. this is why the extra 15 degrees in tilt is often added. this makes it optimum for that time of the year, barring other factors, and does not make it optimal for the winter btw. this is the best average production reaped on the day of the equinox which is the average of all seasons. a winter peak will be latitude + 23.5 degrees as long as that answer is 90 degrees or less and being it is a peak the sun will sit lower in the sky than that angle for all other times.

Consider the numbers for Austin, TX (from http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/sum2/state.html):

"SOLAR RADIATION FOR FLAT-PLATE COLLECTORS FACING SOUTH AT A FIXED-TILT (kWh/m2/day), Percentage Uncertainty = 9"
"Tilt(deg)"," ","Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec","Year"

"Lat - 15 ","Average", 3.7, 4.4, 5.2, 5.6, 5.8, 6.4, 6.7, 6.5, 5.7, 5.0, 4.1, 3.5, 5.2
" ","Minimum", 2.9, 3.5, 4.2, 4.8, 4.5, 5.6, 5.5, 5.6, 4.8, 3.9, 2.8, 2.9, 5.1
" ","Maximum", 4.5, 5.3, 6.1, 6.9, 6.6, 7.1, 7.3, 7.2, 6.6, 5.7, 5.0, 4.6, 5.5

"Lat ","Average", 4.2, 4.8, 5.4, 5.5, 5.5, 5.9, 6.2, 6.3, 5.8, 5.4, 4.6, 4.0, 5.3
" ","Minimum", 3.2, 3.7, 4.3, 4.7, 4.3, 5.2, 5.2, 5.4, 4.8, 4.1, 3.0, 3.2, 5.1
" ","Maximum", 5.2, 5.9, 6.4, 6.8, 6.2, 6.5, 6.8, 6.9, 6.8, 6.2, 5.8, 5.4, 5.6

"Lat + 15 ","Average", 4.4, 5.0, 5.3, 5.1, 4.9, 5.1, 5.4, 5.7, 5.5, 5.5, 4.8, 4.3, 5.1
" ","Minimum", 3.3, 3.7, 4.1, 4.3, 3.8, 4.5, 4.6, 4.9, 4.6, 4.1, 3.1, 3.4, 4.9
" ","Maximum", 5.7, 6.2, 6.3, 6.4, 5.5, 5.6, 5.9, 6.3, 6.6, 6.3, 6.2, 6.0, 5.4

The yearly averages for latitude, latitude + 15, and Latitude - 15 are bolded. Latitude tilt is the highest production, but it's only a smidge greater than -15, which is a little greater than +15. Also note that the uncertainty is 9%, which is much larger than the differences. It's a wash for here, anyway.
• Banned Posts: 17,615 ✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

To state the obvious, the further you are from the Equator the more drastic the difference between Winter & Summer insolation and thus the more important it is to have the right panel angle.
• Solar Expert Posts: 10,311 ✭✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel

ggunn,
i keep forgetting summer intensities are much stronger than winter barring other influences such as snow or water reflections. figures from sites like that are quite interesting and even looking at my area it is strange to note that even for january that it varies little from 0 up to 90 degrees.

here's the kwh/m^2 for january
0
1.7
lat -15- 2.4
lat
2.6
lat +15- 2.7
90
2.5

to optimize winter what i said is good, but there's no getting around the higher figures of summer production. off grid should go to 2 angles minimum as there wouldn't be just 1 viable angle. it would benefit gt as well going 2 angles, but one angle can work for them as they bank power during the summer.
• Solar Expert Posts: 121 ✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
niel wrote: »
ggunn,
but there's no getting around the higher figures of summer production. off grid should go to 2 angles minimum as there wouldn't be just 1 viable angle. it would benefit gt as well going 2 angles, but one angle can work for them as they bank power during the summer.

2 angles is just what I did after looking at the graphs on the page I linked. I am at 23°S and I am using around 20° during spring/summer/fall and close to 42° during the winter. The angles are not exactly as recommended but it was a practical thing when building my frame.
• Solar Expert Posts: 1,973 ✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
To state the obvious, the further you are from the Equator the more drastic the difference between Winter & Summer insolation and thus the more important it is to have the right panel angle.
Of course. Up at the north pole where you live there IS no insolation in winter. ;^)
• Banned Posts: 17,615 ✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
ggunn wrote: »
Of course. Up at the north pole where you live there IS no insolation in winter. ;^)

Yes, but in Summer it's 24 hour sunshine!
• Solar Expert Posts: 1,973 ✭✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
Yes, but in Summer it's 24 hour sunshine!
Yeah, but you have to use a single axis tracker with a vertical axis to capture it.
• Banned Posts: 17,615 ✭✭
Re: KISS: Find Optimum Winter Tilt Angle or Best Year Round Tilt for Fixed Solar Panel
ggunn wrote: »
Yeah, but you have to use a single axis tracker with a vertical axis to capture it.

Nah, we put the panels on a turntable and spin 'em 'round!
Storage is the big problem: the self-discharge rate even on AGM's over six months ...