How to figure monthly optimum array angles?

showme · April 2022

Now that I've installed the two MT Solar adjustable pole mounts, I'm finding it hard to get an idea of how to figure the numbers for where I'm at. The problem is, I've found 3 different formulas, so I'm not sure what is what. Surely there is a simple, correct formula for establishing monthly angles. I've found plenty for adjusting them for winter/summer and even a few that throw in fall and spring. But even on those, different people/books have different formulas. But since I've invested in these two arrays that are easily adjustable, I plan on zeroing them in every month.

The formula that seems to come up the most, after the "easy" way is explained, is (Latitude * [degrees] x 0.9) + 29* for winter, and (Latitude* x 0.9) - 23.5* for summer. These would put me at 62.8* mid-winter and 10.3* mid-summer. This seems about right, but it doesn't give me monthlies. Where can I find a page or formula to help me figure out the necessary angles. BTW, my arrays are 37.58N - 90.40W. Thanks for any help on this. Lee

BB. · April 2022

Part of the issue is your local weather... In some tropical areas, you have afternoon clouds/thunderstorms. And in some marine climates, you have morning overcast.

Anyway... A couple of quick ways to experiment with angles and directions:

http://www.solarelectricityhandbook.com/solar-irradiance.html (simple--may not be adjusted for local weather)
https://pvwatts.nrel.gov/pvwatts.php (complicated, adjusted for local weather conditions--Can eve download hour by hour spread sheet of daily harvest)

More or less, if your within 5% of various comparisons/output predictions--Probably "close enough" for solar work.

And, for off grid solar, if you can measure to a 10% or better accuracy of harvest--You are doing well.

For off grid systems, you want the "average" harvest to supply around 50% to 65% of your daily loads--Allow for bad weather and minimize genset usage (i.e., 50% of predicted harvest or 2x your daily energy needs).

-Bill

Chris11 · April 2022

If you have a smart phone, go to your app store and find an app. I have an android and use one called "Solar Tilt." I probably was told about it here years ago and would like to give credit but I can't remember who recommended it.. But download it ... it uses your location and gives you the accurate tilt for your array on whatever day.

mike95490 · April 2022

I leave mine set for winter, which helps the wash water run off in the summer

Photowhit · April 2022

BB. said:
For off grid systems, you want the "average" harvest to supply around 50% to 65% of your daily loads--Allow for bad weather and minimize genset usage (i.e., 50% of predicted harvest or 2x your daily energy needs).

Bill, I think this is quite a bit off. But perhaps I don't understand what you are saying. Average harvest per day? to supply only 50-65% of daily loads? But then you add "(i.e., 50% of predicted harvest or 2x your daily energy needs)"

Also Off grid solar is wildly situational. I have heat with sun in the summer, and a battery bank of only 1 days autonomy, but it works great sa there is abundent sun when it's hot and I use more than I can store in a day, if I had an empty battery, running 2 air conditioners.

Then in the fall when we can have more than a week without direct sunlight, it's nice to have an oversized array even if daily loads fall to 2 kWhs, as you are getting some minimal charging everyday.

Maybe I'll understand this better in the morning....

Horsefly · April 2022

Years ago I found this web site: http://www.solarpaneltilt.com/ The guy is a bit pedantic, but... It certainly addresses the question!

showme · April 2022

Thanks very much for all the info, guys! I do have the Solar Electricity Handbook, and although it has a lot of informational graphs, the closest to us is St. Louis (an hour north). There is a graph that shows tilt monthly, though. But when I read his formula for calculating my site, it didn't seem to match with any others I'd seen.

Per the apps, I also saw in the above book (which is now a little dated for some information) two suggestions for Apple and Android. Being the geezer that I am, I don't have a smart phone for daily use, but I've got an android that's used for traveling directions and for apps that I can use at our property. Unfortunately, the app listed in that book is defunct. So I will definitely try one of these apps. Also, with the degree angle indicator that I got for with the MT Solar mounts, the measurements will not be super precise. But it's pretty handy, I think.

I'll try these apps today. Hopefully they'll line up with the formula I mentioned earlier.

Also, per the comment about the expected daily loads by Bill, we're going to be off grid and are hoping (betting) our 8k array, the secondary 960w array and the Bergey XL.1 wind genny will produce 100% of our power. I hope to only have to use the back up genny in extreme situations. Since I'm having a hard time trying to account for all the new loads that will be in the earth home we build, I went the easy route and am just using our present, 110 year old house's utility bills for our needs estimates, which include electric dryer, water heater, forced air gas furnace (the fans) and air conditioning, etc. Way over what we'll use down there. But this will include a shop that I'll tinker in till they bury me.

I'm heading down to the property this morning to put the metal roof on the power shed, so I'll use these android apps and see what I can find, then I'll report back. Thanks again, folks! Lee

BB. · April 2022

Photowhit, let me try to clarify.

Regarding 50% or 65% of predicted harvest... As always the details matter.

For example, a load that runs 24x7 such as a refrigerator... I would suggest that the system produce more power than needed on a statistically "average" day:

1,500 WH per day fridge * 1/0.65 system "fudge factor" = 2,308 Watt system "average power" design (winter for most people)
1,500 WH per day fridge * 1/0.50 system "fudge factor" = 3,000 Watt system "average power" design

This is what I call "designing" for base loads--those 24x7 loads that cannot be shut down in poor weather, etc... Over-sizing an off grid solar power system is pretty much a requirement.

For other loads, such as water pumping (for irrigation), clothes washer, A/C, home computer usage (variable usage) vs computer used for business (5 days a week, etc.)--Those loads you can cut back on during bad weather/may not need during winter--Or start the genset when those loads are needed during poor weather.

I try to avoid piling on more and more fudge factors without better understanding of the members actual power needs/usage.

It is very easy to end up with a costly system and 10x "margin of safety" when it is not really needed. That 10x factor is a common Engineering Design Margin of Safety for most "standard" materials (wood, concrete, etc.) where you are given materials in their near raw states (wood with knots, concrete that did not mix/poor correctly). Unlike highly engineered systems (aircraft, rockets, office towers, etc.) where structural elements have high quality control and in process inspection procedures with traceability.

So, for the 3,300 WH per day system minimum I suggest people first look at for cabins and small/very efficient homes works out to around 1,500 Watts for fridge, 100 WH for LED Lighting, 300 WH for small laptop computer, a few hundred WH for a RV style water pump... Fridge and lights may be "base loads", the rest (and the occasional clothes washer of 1,000 WH per day, vacuum cleaner, etc.) are reserved for good weather/possible genset when needed. So the 50%/65% suggested fudge factor could apply to the Fridge+Lighting+water pump--And the rest do not... That gives you as an example 1,500WH+100WH (fridge+LED) * 1/0.50 = 3,200 Watt system design for base loads--And 3,300 WH for "average" daily loads in sunny weather.

For batteries--Most of them are killed from under charging (over discharging, etc.). Having "excess" solar energy available is a good way to keep those batteries "happy" and save on genset fuel (something like $1+ per kWH) + wear and tear.

-Bill

Photowhit · April 2022

BB. said:

Photowhit, let me try to clarify.

Regarding 50% or 65% of predicted harvest... As always the details matter.

For example, a load that runs 24x7 such as a refrigerator... I would suggest that the system produce more power than needed on a statistically "average" day:
1,500 WH per day fridge * 1/0.65 system "fudge factor" = 2,308 Watt system "average power" design (winter for most people)
1,500 WH per day fridge * 1/0.50 system "fudge factor" = 3,000 Watt system "average power" design
This is what I call "designing" for base loads--those 24x7 loads that cannot be shut down in poor weather, etc... Over-sizing an off grid solar power system is pretty much a requirement.

For other loads, such as water pumping (for irrigation), clothes washer, A/C, home computer usage (variable usage) vs computer used for business (5 days a week, etc.)--Those loads you can cut back on during bad weather/may not need during winter--Or start the genset when those loads are needed during poor weather.

I try to avoid piling on more and more fudge factors without better understanding of the members actual power needs/usage.

It is very easy to end up with a costly system and 10x "margin of safety" when it is not really needed. That 10x factor is a common Engineering Design Margin of Safety for most "standard" materials (wood, concrete, etc.) where you are given materials in their near raw states (wood with knots, concrete that did not mix/poor correctly). Unlike highly engineered systems (aircraft, rockets, office towers, etc.) where structural elements have high quality control and in process inspection procedures with traceability.

So, for the 3,300 WH per day system minimum I suggest people first look at for cabins and small/very efficient homes works out to around 1,500 Watts for fridge, 100 WH for LED Lighting, 300 WH for small laptop computer, a few hundred WH for a RV style water pump... Fridge and lights may be "base loads", the rest (and the occasional clothes washer of 1,000 WH per day, vacuum cleaner, etc.) are reserved for good weather/possible genset when needed. So the 50%/65% suggested fudge factor could apply to the Fridge+Lighting+water pump--And the rest do not... That gives you as an example 1,500WH+100WH (fridge+LED) * 1/0.50 = 3,200 Watt system design for base loads--And 3,300 WH for "average" daily loads in sunny weather.

For batteries--Most of them are killed from under charging (over discharging, etc.). Having "excess" solar energy available is a good way to keep those batteries "happy" and save on genset fuel (something like $1+ per kWH) + wear and tear.

-Bill

Looks to me that you are switching "harvest" and "system size".

A grid tied 'net zero' home would have a harvest equal to it's loads., An off grid home, depending on weather will need 3x the size array in areas rarely going more than a couple days with poor weather or a liberal use of a generator. This has been my experience living without a generator in an area which will often go 4-6 days with little direct sunshine.

littleharbor2 · April 2022

Chris11 said:

If you have a smart phone, go to your app store and find an app. I have an android and use one called "Solar Tilt." I probably was told about it here years ago and would like to give credit but I can't remember who recommended it.. But download it ... it uses your location and gives you the accurate tilt for your array on whatever day.

I may have been the person who recommended the Solar Tilt app. Have a few times here over the years.

showme · April 2022

Well, I downloaded the solar tilt app before I went down to the property, and it works great. Thanks for telling me about it. Haven't compared it to the formulas I've come across, but it looks like it's pretty dead on in it's sun alignment at noon. All I have to do now is mark the calendar for the first of each month. Thanks!

How to figure monthly optimum array angles?

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