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Fixed Tilt and Azimuth Angles - it doesn't really affect yearly power generated numbers

LiFer

Hobbyist
Joined
Mar 16, 2020
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60
Location
Boston
TIL. Just sharing. I think I worried too much about the importance of the panel angle to the sun. I'm stuck at a fixed 20 degree angle with worst-case azimuth 225 (south-east).
The DoE calculator enabled me make the financial case using real-world yearly data.

US Department of Energy PV Watts calculator: https://pvwatts.nrel.gov/pvwatts.php
Article explaining the PV Watts calculator: https://www.thesolarnerd.com/blog/optimal-solar-panel-tilt-most-calculators-are-wrong/

So... in ~43 degree latitude Boston azimuth 225 the yearly difference in generated power seems to be negligible between 20 and 45 degrees.
20 degrees, yearly kWh = 14,341 Dec 635kWh Jul 1579 kWh
45 degrees, yearly kWh = 14,447 Dec 781kWh Jul 1441 kWh

Azimuth 180 (South) only makes a 5% - 7% yearly difference:
20 degrees: 15,028 kWh ~= 5% better
45 degrees: 15,605 kWh ~= 7% better

Better tilt angles would be nice, but the main thing is that putting solar above my patio seems financially feasible.
I can actually build something with a 20 degree slope over my patio!

Anyone come to a different conclusion?
 
Short answer: True, but Netflix....

Long Answer:
Different tilt/azimuth will give different yearly values and the size of the array magnifies the difference. There is an optimum for your location; but how much of a difference does it make?

Let's say there was a 600 kWh/yr difference between actual and optimum and that you're paying between 10 to 20 cents/kWh; that 600 kWh/yr difference between the angles amounts to between $60 to $120/year (at your 100 kWh the delta is only $10 to $20). Enough to pay for your NetFlix subscription? Over a lifetime of 20 years that's $1200 to $2400 (not including the 4% inflation of the cost of electricity).
 
I agree the angles make a difference when you look at the daily/monthly numbers. The data I provided shows this makes a difference too.
I was really trying to look at the _yearly_ financial case for panels at a fixed angle/azimuth.

For example, if we take Thunder Bay, ON PVWatts gives me about a 5% difference in _yearly_ power generated:
13,840kW yearly at 20 degrees
14,601kW yearly at 45 degrees

Why am I looking at the yearly number? It simply seems like the best/fairest way to build the cost/benefit case. Maybe I needed to see it this way to see the positive benefits more clearly.

It was demoralizing when I looked at my situation in smaller slices of time:
o It is impossible for me to build a useful array close to the recommended angle to the sun.
o Some folks were accurately/truthfully showing very large drops in power generation when dropping to a 20 degree angle.
o Some folks were showing big losses for the wrong azimuth, shade, clouds, etc.

Ultimately I needed to see some real data to see if my panels were going to be worth it, and PVWatts seems to be the best (only?) way to get that data.

Thanks!
 
Thanks for the SAM info, and for providing real data on a *flat* roof!
Not quite flat... it's flush to the roof (HVHZ was my primary consideration) and the roof is at 6° . Also not quite east/west (83°/263°). Finally about 25.08° N, 80.45° W. No free NetFlix for me, but on the upside my roof shouldn't cave in during a hurricane.
 

As Steve_S's solarelectricityhandbook.com/solar-angle-calculator.html shows. An angle of 0 is panel facing horizontal and 90 would be flat, pointing up to the sky. I think I've seen it differently elsewhere. Are there different standards for angles or am I mistaken about what I tink I've seen?​

 

As Steve_S's solarelectricityhandbook.com/solar-angle-calculator.html shows. An angle of 0 is panel facing horizontal and 90 would be flat, pointing up to the sky. I think I've seen it differently elsewhere. Are there different standards for angles or am I mistaken about what I tink I've seen?​

Yeah, I've seen it the other way with 90 as horizontal and 0 as flat. It still works. Just need to know where to start measuring the angle. A common rule is to take your degrees of latitude and then add 15 degrees for the winter and subtract 15 degrees for the summer. Spring and Fall the angle would be your latitude.
 
Conventional wisdom says just set it at your latitude year-round if you don't want to move it.
So, in my case, due south at 45 deg tilt. Optimum for spring and fall. Less so in winter and summer as an acceptable compromise.
 
A few comments:

0. Trigonometry for the win.
Sin(90ᵒ) = 1.000
Sin(80ᵒ) = 0.985
Sin(70ᵒ) = 0.940.
IOW being 20ᵒ away from "optimal" only costs ~6%.

1. Solar panels are so cheap that you can often just add a panel or two to make up for any loss of not having an optimal tilt or azimuth.

2. The variances in production between tilts/azimuths also depend on the seasonal weather conditions. For instance, some places experience a lot of morning cloud at certain times of year. This might make a more west facing array actually better than a south facing array (in nthn hemisphere). Storm season is late Summer for us and avg daily production in February is often well down on months with much shorter days.

3. Also one has to assess the cost involved in getting panels at an "optimal" angle. A rooftop provides the supporting structure already for no cost, so there's no need to spend money to build an entire dedicated structure. That's a big cost saving quite probably worth a lot more than some production losses from non-optimal orientation. Just put a couple more panels up, way cheaper.

4. A half way house is using special racking to tilt rooftop arrays to face in a more optimal direction. That can be OK however it usually means more cost (the tilts are more expensive) but more importantly it also often means fewer panels can cover the same roof area because enough space has to be left between panels to ensure they are not shaded by neighbouring panels. In many instances you'll get better production filling the roof with the tilt and orientation it has rather than attempting to put tilted arrays up.

5. What is optimal also depends on when you need the energy and the energy tariff regime you have. If you get the same $ value for exporting energy to the grid that you pay to import it, then load up on panels and who gives a toss about when the energy is produced/consumed. The grid is your (financial) battery. But if you get much less credit for exports than you pay in imports, then when you produce your solar energy matters more - it's better to align it with when you need the energy (and vice versa - load shift to solar production hours where possible).

In hot/warm climates like ours, this means west facing arrays are good as they can power air conditioning much later into the day when it's needed. In an off-grid scenario you might prefer to have split azimuths for lengthening the daily production window and slightly reducing the cycling of battery storage. This can be better than an array which produces the most output overall but is more peaky with its daily production curve.

Sometimes one needs to weight Winter production as more value than overall annual production. Sacrificing Summer production for better Winter output might better suit a some premises.

You have the roof you have. Load it up, start producing and don't let perfect be the enemy of good (or great).
 
Thanks for replies. To me, latitude makes sense for year around fixed setup. Somewhere I've seen something like lat -9 or 10 deg (for fixed). The reply saying + or -15 for summer / winter also seems to me to not be correct unless we are rounding from 23.5. But then, what do I know?
 
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