Number crunching

The tilt angle of solar panels should be changed for best efficiency at different times of the year.
In winter, the Sun does not rise high up and solar panels receive more energy at an angle of 80-90 degrees.
That is, by installing the panels vertically, you will receive more energy and snow will not accumulate.

solar panels by MAX2 posted Jan 15, 2025 at 9:28 AM

In summer, the tilt angle of solar panels should be reduced to 30 degrees.
Solar Angle Calculator | Solar Panel Angle Calculator

 

What the heck is that white stuff?
Sand????

 

While direction and azimuth are important for capturing max amount of seasonal sun rays, it is largely impractical to try chasing the sun for most home users. The commercially installed sun tracking system is expensive. And both commercial units and DIY frame like shown on the photo by @MAX2 are not suitable for large number of PV panels. I wonder what the cost would be to install dual axis Solar tracker for our system. The costs of tracking system hardware, location preparation, and installation have to be considered.

By far, for rooftop installation, the direction and azimuth of the panels are unchangeable. Our roof does not face perfect South. And the pitch of the roof is not adjustable for season. The installer took those factors into estimation for the system performance. But I had a suspicion that most installers underestimated the annual production kWh to maximize the solar system size to profit more.

The estimated kWh production by plugging in numbers on NREL site (PVWatts Calculator) gave me 17,927kWh annual production for the size and spec of the 15kW system I decided to install. Yet our installer gave an estimated annual production of 13,382kWh from the 15kW system they installed using their estimator. That's over 30% "underestimated" from NREL number. Since, I started a year 2025 with the system in operation, it will be very interesting to see what the system would end up generating for the entire year. Would it be closer to 13,382kWh? Or more to 17,927kWh?

While I want the fastest payback period. More electricity generated by the system than I can use does not necessary shorten the payback. For our state Net-Meter billing, there is no buy back of surplus, only credit for future usage that expires after 12 months in rolling base. So, having huge surplus of credit does not contribute to the ROI. Historically, we use ~14,000kWh of electricity annually or ~1,166kWh/mo average. Monthly fluctuation is minimal in our household. If the installer estimated annual 13,382kWh production is distributed evenly throughout the year, the monthly production would be 1,115kWh/mo. This is THE PERFECT production rate for us. I would end up paying for monthly electric bill for 51kWh (1166-1115=51) which would be ~$15 at our current rate ($0.30/kWh). But if the solar production exceed the monthly usage so that credit covers total kWh used, then we actually ends up paying $21 for 0kWh electricity for the minimal service fees imposed on the account.

I will be keeping a log of all those numbers and will have to do the end-of-the-year ROI assessment. I need more data!

 

The Smartflower is one of those nifty cool products out there.


It is a self contained and self cleaning suntracking unit. The company claims the 2.5kW system is effectively 4kW compared to typical fix mounted systems. At $27k though. it is about $10k more than my 5.2kW roof system. Plus, I don't have the yard space for it. You can take it with you in the event of having to move.

There are options to include 6kWh to 13kWh of battery storage. Where it would make sense for it seems limited to me, but the company also sells a unit for directly charging an EV.

The Solar Flower You Need to Experience — SmartFlower
SmartFlower Solar Review: The True Cost of a Solar Flower | Solar.com

On other news, I got a letter from the township last week. Electric rates are going up 16% this year, and might increase further. Sucks, but my pay off sped up.

 

Aye. Can only see it being worth it if space for more traditional systems is lacking. Maybe for a business that wants an art piece to draw eyes. For that case, the EV model's cost might not be bad when compared to tearing up pavement and sidewalk.

 

Some > None.

 

Half of a month since my solar started generating, today was the coldest day this season. Day time high barely hit positive integer (in F). But the total generated was 30 kWh. That's almost our average daily kWh usage from the grid. A new record high so far. Apparently, as long as sun is shining, cold does not affect the generation. However, our PHEV barely made 24 miles on 11 kWh full charge (usually 37 miles). I must think better way to use the electron than feeding the PHEV in cold. LoL

 

It's the hot days that will curb production.

 

Is there any documented data on the PV efficiency decrease on hot days? I may have to Google it, but I wonder how much of a "hit" are we talking about at what temperature? And why?

Edit to add: cursory search found plenty of references on this topic.
Summary by AI.
Key points about PV efficiency and temperature:
Temperature coefficient:
This is a key metric used to measure how much a solar panel's power output decreases with increasing temperature; a lower temperature coefficient indicates better performance in hot environments.
Mechanism behind the decrease:
As the temperature increases, the electrons within the solar cell gain more energy, making it harder for them to be collected and converted into electricity, resulting in power loss.

 

This is due to the increase in resistance in the semiconductor materials of the panels.
On average, the efficiency of the panels decreases by 0.5-0.7% for every degree above the optimum temperature.

 

Thanks. Yes, I found many articles on this topic. I don't know the temperature coefficient spec on my panels, but they are rated 21.8% efficiency. The temp affecting the efficiency must be at the panel internal not the ambient air temp. I wonder how hot the panel gets under full sun, say when temp at 0F vs 70F on a calm day. From the reading, the optimum temp is usually set at 25C (77F). Does the efficiency drops from the optimum at lower temperature? We rarely get ambient temp above 80F even in mid summer, so heat is not likely to be the worst of enemy for the PV in our climate... But I don't know.

 

Unfortunately, the temperature of the panels is always higher than the ambient temperature.
The accumulating effect for heat accumulation is provided by the protective glass.
The dark color at the base of the solar cells also provides more opportunities for heating relative to the reflective light object.
Of course, this is better assessed with a thermometer in the warm season.
Low temperatures in winter do not create problems.

 

On days that are 95-100F I see about 10% less total production for the days compared to around 70-80F which seems to be the peak.
Very rough numbers

Mike

 

Thanks for the data. I had a crush course on temperature effect on solar panel by reading a few articles. I learned my PV panels has temperature coefficient of PMax: -0.3%, Voc: -0.24%. This means every degree in C from the optimal temp the panel loses ~0.3% efficiency.

So in your real-world performance, approx 20F temperature increase from 80F to 100F which is about 10C change from 27C to 37C, you lost 10%. That means your panel temperature coefficient is about -1.0%. But your empirical number might be higher than the spec on your panel. If I go by the spec sheet, I would only lose ~3% of efficiency from 80f to 100f.

But my biggest question is not so much about loss of efficiency in high temperature. We don't get temp much hotter than 80F even in mid summer day time. My question is what happens to the efficiency when temperature is lower than the optimal temperature? Most references I read defines the optimal temperature at 25C (77F). If that is an optimal, then it means the temperature lower than that would also decreases the efficiency. If so, by how much?

 

I found the answer here: Most efficient solar panels 2024 - Clean Energy Reviews

STC = Standard test conditions - 25°C (77°F) is not the "optimal" temperature. It is just a standard testing temperature manufacturer uses. From the chart below, it is clear that PV panels efficiency increases linear to lower temperature. So, all things being equal, colder the better. At -20C temp we just had, my solar panel was producing 16.5% (55 x 0.3 or 55C degree less than STC at 0.3% temperature coefficient) more electricity than the spec sheet number. That's fabulous!

 

As it turns out......it WASN'T SAND!!!!

https://www.axios.com/2025/01/22/winter-storm-southern-states-snow-photos

One of the only advantages of being a D**M Yankee is that I really DO know what snow and ice is all about.
That's WHY I'm a D**M Yankee!!!!

Fortunately I now have a Jeeplet (Compass) as my company car.
It may only get 30MPG (real world) but it has 'for real' 4WD and nearly adult-sized (17") tires.

Adult driven, and with all weather tires it can manage quite well with 6-8" of snow and icy bridges!!!

 

Also, keep in mind that by roughly 10% drop when it is hot is the average over the whole day.
I'm not doing any instantaneous measurements. So at 10am when it isn't real hot yet those watts are closer to optimum, but at a lower sun angle.

Mike