Solar Panel Efficiency

[ArtJoyce]

If your panels are pointed directly ot the sum and at the sun angle and a full sun day you should get the rating of the panels. I get slightly over the rating of my portable panel when it perfectly setup.

But my panels flat on the roof in the summer I get 50 to 60 on a good sun day. When it comes to winter where I live with a sun angle on December 21 of 50 degrees, I see maybe 30 %

[hclarkx]

Originally Posted by SteveJ.

On two different occasions I have actually seen output more than the 240 watts of power that the panels are rated for. Both times in the upper 240's when the sun came out after a cold rain shower. Once warmed up a bit the output went down a bit. Measurements were according to the Victron SmartSolar 100/30 MPPT controller. The 60 watt panels are setup as 2S2P.

This setup is portable so % efficiency runs quite a bit higher than rooftop, usually 80-90% depending on the temperature.

I did as well. In my case it was the "hole in the cloud" effect. We were at 7400 feet on a cloudy day. The clouds were fairly thin and a hole in the clouds allowed direct sun on all panels .... as well as diffuse light from clouds across the whole of the sky. I had 600 Watts of solar at the time and saw power hit just under 650 Watts for a while.

[oatmeal]

Originally Posted by slabman

Can anyone claim having over, say, 70% of rated power delivered to their controllers? Maybe very few actually keep track of such things?

I don't have flex panels, but I do have a lot of data.

I have six 175-watt solar panels (manufactured by Sharp) flat mounted on my roof, for a total of 1,050 watts. The first four were installed in 2005, and I added two more in 2006. I've kept a spreadsheet of solar output since Day One.

The original controller logged the daily solar production and float time, and if you looked at it before it went to sleep for the day, it showed the peak watts for the day.

I used to check the controller frequently because I was curious, plus I wanted to know when the batteries had reached float mode, so I could turn on the water heater. I remember seeing impressive peak outputs--pushing 100% of the panels' nominal watts. However, that number didn't get recorded in the controller's "last 60 days" data that I put in the spreadsheet, so I'm just working from memory.

But the new controller, installed in 2016 (Midnite Solar Classic 150), does record and save the peak watts for the day, and it supports my memory of these panels producing high peak outputs. Here are the five highest peaks recorded by the new controller, with the location and month and day, for my nominal 1,050 watts of panels, from January 2016 to through April 2019 (3-1/2 years):

1,200 watts in Crested Butte, Colorado, July 31 (114% of nominal)
1,162 watts east of Los Angeles, May 13
1,086 watts in Grand Junction, Colorado, June 17
1,086 watts in Lake Elsinore, California, May 21
1,063 watts in Oxford, Mississippi (50 miles south of Memphis), May 3

Since you mentioned a benchmark of 70%--I had a peak of 773 watts in Revelstoke, BC (the same latitude as Calgary), on September 13, and 631 watts in Vancouver, BC, on October 1. That's 74% and 60% of nominal, respectively, in Canada in the Fall.

There were about 40 days during that time span where I saw a peak of 90% of nominal (945 watts), about 100 days with a peak of 80% of nominal (840 watts), and 200 days with a peak of 70% of nominal (735 watts).

Also, all the data was collected just living normally. I'm sure there were days where the possible peak was never reached because the batteries went into absorb or float state before peak conditions occurred.

So when people say they never see a peak of more than 60% of nominal, and blame it on flat-mounted panels or non-ideal conditions, I have to wonder, since I get much better numbers with my flat-mounted panels in non-ideal conditions. It could very well be their batteries are in absorb mode before the peak conditions for the day happened, or it could be that they their controller doesn't record data for later retrieval and they don't happen to look at it at the right time. But it could also be that there's something wrong with their installation, and I know that if I never saw more than 60%, I'd think something was up.

In fact, that actually happened. I was seeing numbers lower than I expected, and sure enough, a couple of the panels had come unplugged.

[Tomahawk]

Originally Posted by oatmeal

Since you mentioned a benchmark of 70%--I had a peak of 773 watts in Revelstoke, BC (the same latitude as Calgary), on September 13, and 631 watts in Vancouver, BC, on October 1. That's 74% and 60% of nominal, respectively, in Canada in the Fall.

Good data points. Your numbers make sense. I used an app that shows on 10/1 in Vancouver the max sun elevation is about 42 degree. The sine of 42 degrees is about .67. So the peak power output on 10/1 at suns highest elevation should be close to 67% of nominal. You got a little less than that which makes sense. I did the same thing for Lake Elsinore and got a max sun elevation 80 degrees, which translated to 98.4% of nominal. The fact that you get days at or over nominal make me wonder if Sharp is under selling the panel wattage.

The real benefit to tilting is to raise the efficiency in the winter to compensate for the shorter days (fewer hours of sunlight). In the summer, when there are more hours of sunlight, the efficiency can be lower and generate the same total amount of power (watt hours). An optimized residential installation will set the angle for the panels to be most efficient near the winter solstice.

[Bobby F]

Originally Posted by Tomahawk

The real benefit to tilting is to raise the efficiency in the winter to compensate for the shorter days (fewer hours of sunlight).

Maybe it's just me, but it's so much easier and less complex to simply buy an extra 15%-20% of your panels and then just mount everything flat. (Only if you have room, obviously.) Cheaper mounts, easier to install, never have to worry about which way to face when parking, and no climbing the roof to adjust angles. That many more panels help compensate for the loss due to sun angle. And did I mention no need for deployments or adjustments or put-away up on the roof?

[757driver]

Originally Posted by Bobby F

Maybe it's just me, but it's so much easier and less complex to simply buy an extra 15%-20% of your panels and then just mount everything flat. (Only if you have room, obviously.) Cheaper mounts, easier to install, never have to worry about which way to face when parking, and no climbing the roof to adjust angles. That many more panels help compensate for the loss due to sun angle. And did I mention no need for deployments or adjustments or put-away up on the roof?

Your math is not right. The difference in Arizona in the winter with tilting solar panels is a 50% gain over leaving the solar panels flat so adding 15-20% more panels would not do it.

[Bobby F]

Originally Posted by 757driver

Your math is not right. The difference in Arizona in the winter with tilting solar panels is a 50% gain over leaving the solar panels flat so adding 15-20% more panels would not do it.

Well, first, that's why I said it would help compensate.

Second, I think I'd have to question your 50% figure. Here at Quartzsite, with my 600 watts of panels mounted flat, on 12/21 - the day of the winter solstice - my Victron history shows P-max at 445w. That's about 75%.

(Granted, that was an instantaneous maximum - maybe due to light funneling from the partially-cloudy day - but I've been consistently hitting 400w+ P-max each day since then, also on semi-cloudy days.)

And, to my basic point above, if your 50% figure IS correct, I still think I'd rather add panels than mess around on the roof with tilt brackets and aligning the RV to the sun track. The less time I spend on my roof, the happier I am, and panels are cheap!

[757driver]

Originally Posted by Bobby F

Well, first, that's why I said it would help compensate.

Second, I think I'd have to question your 50% figure. Here at Quartzsite, with my 600 watts of panels mounted flat, on 12/21 - the day of the winter solstice - my Victron history shows P-max at 445w. That's about 75%.

(Granted, that was an instantaneous maximum - maybe due to light funneling from the partially-cloudy day - but I've been consistently hitting 400w+ P-max each day since then, also on semi-cloudy days.)

And, to my basic point above, if your 50% figure IS correct, I still think I'd rather add panels than mess around on the roof with tilt brackets and aligning the RV to the sun track. The less time I spend on my roof, the happier I am, and panels are cheap!

When I post information on this forum it is not passing on some internet myth or incorrect rule of thumb but is based on research and/or practical firsthand experience.

The 50% increase in watts IS correct because I see the increase every time I raise the panels from flat. Question it if you like that is up to you. A data logger showing a daily P max is of little use for a comparison of tilting vs flat as there is no indication if this max was momentary or a sustained value. My guess would be a momentary value. A true comparison would be daily kwh harvested flat vs tilted for a similar sized solar array.

Whether or not you tilt your panels is a personal choice and was not being called into question. To each their own. Adding extra panels is not always practical or possible for available roof space or maintenance functions.

My only reason for posting to this thread is to correct inaccurate information and help others make an informed decision when it comes to the choice of tilting vs flat mount.

[hclarkx]

Originally Posted by 757driver

When I post information on this forum it is not passing on some internet myth or incorrect rule of thumb but is based on research and/or practical firsthand experience.

The 50% increase in watts IS correct because I see the increase every time I raise the panels from flat. Question it if you like that is up to you. A data logger showing a daily P max is of little use for a comparison of tilting vs flat as there is no indication if this max was momentary or a sustained value. My guess would be a momentary value. A true comparison would be daily kwh harvested flat vs tilted for a similar sized solar array.

Whether or not you tilt your panels is a personal choice and was not being called into question. To each their own. Adding extra panels is not always practical or possible for available roof space or maintenance functions.

My only reason for posting to this thread is to correct inaccurate information and help others make an informed decision when it comes to the choice of tilting vs flat mount.

Believe it or not, in Quartzite, for the month of December, the potential of 1000W of flat panels is about 2 kWh per day on average (57 kWh for the month). Tilt those panels up at about 60 degrees (to the south) and the production increases to about 4 kWh per day on average (119 kWh for the month). So, one can potentially double the output of a solar array in Quartzite in December by tilting the panels.

Above derived via PVWatts. https://pvwatts.nrel.gov/pvwatts.php

Though my 81 year-old 6-3 200# body also does not like being more than a foot off the ground so my panels are fixed. My panels are on the sloping part of my roof and more often than not I can point the tail south. Angle is far less than optimum, but helpful.

[757driver]

Originally Posted by hclarkx

Believe it or not, in Quartzite, for the month of December, the potential of 1000W of flat panels is about 2 kWh per day on average (57 kWh for the month). Tilt those panels up at about 60 degrees (to the south) and the production increases to about 4 kWh per day on average (119 kWh for the month). So, one can potentially double the output of a solar array in Quartzite in December by tilting the panels.

Above derived via PVWatts. https://pvwatts.nrel.gov/pvwatts.php

Though my 81 year-old 6-3 200# body also does not like being more than a foot off the ground so my panels are fixed. My panels are on the sloping part of my roof and more often than not I can point the tail south. Angle is far less than optimum, but helpful.

Interesting calculator to experiment with. Thanks for the link. When plugging in my array size and values I find that the calculator is reasonably accurate but the estimate for flat is slightly below what I routinely get and the estimate for tilted is less than I routinely get. For example the calculator shows 6.24 KWh per day for November for Quartzsite for a 1.3 kw array at 50° tilt. I was regularly getting 7+ KWh. Also, the default losses seem rather extreme so I cut that value back to 1%.

Interestingly for my system the calculator showed a 68% increase for tilted vs flat. Perhaps a little too optimistic but the difference is significant.

[hclarkx]

Originally Posted by 757driver

Interesting calculator to experiment with. Thanks for the link. When plugging in my array size and values I find that the calculator is reasonably accurate but the estimate for flat is slightly below what I routinely get and the estimate for tilted is less than I routinely get. For example the calculator shows 6.24 KWh per day for November for Quartzsite for a 1.3 kw array at 50° tilt. I was regularly getting 7+ KWh. Also, the default losses seem rather extreme so I cut that value back to 1%.

Interestingly for my system the calculator showed a 68% increase for tilted vs flat. Perhaps a little too optimistic but the difference is significant.

The calculator was first introduced over 20 years ago and has undergone overhauls every five years or so. Accuracy has gotten really good.

Do keep in mind that the calculator is based on about 50 years of data depending on which data base you select. So it represents an average performance over many years. Individual years can vary to some extent. Monthly variations will be larger, annual variations will be smaller since the monthly performance tend to "average out."

The model has been improved not just from weather station data, but from millions of home solar installations across the country, some dating back 20 years.

Additionally, the numbers I posed are averages for the month. Some days will be much better and some much worse. Day to day variations will be very wide. As such, it's hard to compare one's own numbers with the calculator unless one collects data for a month. That data has to include total daily potential energy which is hard to get since we cannot keep the panels fully loaded over all of the daylight hours.

I'll download and plot hourly variations for a representative December in Quartzite soon and upload that here. Hopefully this evening.

EDIT: Ack, I just realized that I mis-spelled quartzsite and I think the location I was looking at was in Kansas. The optimum angle in Quartzsite is closer to 50 degrees and the improvement over flat is about 90%, not 100%. And the average daily generation numbers are much higher. I'll correct in the plots.

[hclarkx]

Okay, fixed. Here are representative days in December in Quartzsite for flat and 60 degree panels. I can do hourly if there's interest. Production can really bounce around from hour to hour on cloudy days.

Abscissa says "day of the year" but it's day of the month (I extracted the remainder of the year).

For flat 1K of panels .........


For 60 degree 1K of panels .........


Large variations from day to day. You can see that one would have to record daily a full month to get a calibration on average daily production.

The data is "statistically representative" in that in a usual December one will see not the same pattern, but will experience vary nearly the same extremes, multi-day patterns, etc.

Monthly totals are about 82 kWH for flat panels and 151 kWH for 60 degree panels.

[hclarkx]

Originally Posted by 757driver

Interesting calculator to experiment with. Thanks for the link. When plugging in my array size and values I find that the calculator is reasonably accurate but the estimate for flat is slightly below what I routinely get and the estimate for tilted is less than I routinely get. For example the calculator shows 6.24 KWh per day for November for Quartzsite for a 1.3 kw array at 50° tilt. I was regularly getting 7+ KWh. Also, the default losses seem rather extreme so I cut that value back to 1%.

Interestingly for my system the calculator showed a 68% increase for tilted vs flat. Perhaps a little too optimistic but the difference is significant.

I agree on the losses. 4% for the solar controller is built-in. The other losses include maintenance and outages and such that are not relevant for us. I used 8% for the miscellaneous losses above, though actual is surely closer to 1%.

[hclarkx]

For those that want to do a deep dive, very deep, on their RV energy use and solar production and energy storage and risk of having to run a generator and such, here is a link. I use full hourly data from PVWatts in this tool. With this tool one can even plan on solar and battery planning to include mini split A/C.

https://diysolarforum.com/resources/...ta.154/updates