Does Heat Impact Panel Production?
A friend asked me this question. I felt compelled, as a result, to expound upon it here!
The short answer is: yes it does. The more important questions:
- By how much?
- Are more efficient panels worth it?
A Tale of Two Panels
As I was doing my research, I wanted to know between the two candidates just how their panels would perform over the hypothetical long-run. Actual production is affected by many factors:
- Overall panel efficiency
- Heat losses
- Age losses
- Soiling
- Obstructions
Let's talk briefly about these.
Overall Panel Efficiency
The panels convert solar energy into electrical energy, but not at a 1-to-1 rate. The photons striking the panel have to interact with electrically-complex junctions, and not all wavelengths are used evenly. Beyond being a way to compare two panels for scientific purposes, this number is - to the consumer - little more than a curiosity.
Most panels operate around 19% efficiency. Some panels tout a 20% or 21% efficiency. What does this mean to you, dear buyer? Smaller panels, usually with a larger price tag. A bigger 19% panel will generate just as much power as a smaller 21% panel. Unless you're a solar farm and installing a cool million of them, those two percentage points probably won't be worth much.
Do we want more efficient panels? Yeah, someday, when they're maybe closer to 40% or more, but only so long as they are still affordable.
Heat Losses
Panels lose efficiency as they get hot. Conversely, they perform better as they get colder. All panels are tested at a standard reference temperature - usually 25 C (77 F). Most roofs during the day get hot, and that heat will belong to your panels after they're installed. Some panels will advertise better resiliency against heat (lower losses). Are they worth the superior price?
Age Losses
Panels also degrade over time. This should be a no-brainer (Kansas nods slowly). But the rate is usually guaranteed not to exceed some amount of loss over some amount of time. A common standard is around 15% over 20 years. The loss rate is usually given as a percentage of degradation per year, something like 0.8% per year.
Soiling and Obstructions
These two have no representation on the spec sheets, because the panels can't control where they're installed. However, for the purposes of generating power effectively, these two factors must be borne in mind. Panels need occasional cleaning (I'm told maybe once a year). Obstructions, such as trees, clouds, and adjacent buildings, will affect their performance.
Evaluating
The last two losses - soiling and obstructions - we'll take as constants because no matter what panels we put on the roof, they're all going to be suffering the same fate. In other words, there is nothing for us to calculate there. We'll assume that all panels receive awesome, direct light, and focus on heat and age.
I punched in all the data from the data sheets, if for no other reason to see them side-by-side. Data sheets, by their nature, do not make such a comparison otherwise easy.
Considering Heat Losses
We're given that the SunSparks lose slightly more power (Temperature Coefficient of Pmax) for each degree C than the RECs. How does this look for daily, monthly, and yearly production? I present a somewhat unrealistic calculation below, since over the course of the year production varies with the sun's position and duration in the daytime sky. I also hold constant the temperature through the "day", the "month", and the "year".
Is this a legitimate way to compare? Well, panel temperatures are probably going to vary the same - or so we'll tell ourselves - no matter what panel we put up there. Some might even be hotter from being placed directly on the roof shingles, but we'll assume the mounting is essentially identical. So we can scratch hourly variation off the list.
If a panel is constructed in such a way as to not be as badly affected by the heat, that should be represented by the coefficient - and so there it is.
Second, panels are going to receive the same amount of incoming light no matter what panel is in place. The sun is the sun, clouds are clouds, and a panel can't make power out of nothing. Even the most "efficient" panel won't convert darkness into power. It'll just produce more in any light than a less efficient panel would. For sake of comparison then, it seems fair to keep the incoming sun and the overall module temperature the same.
We see int he above that at evaluation temperature (25 C), our yearly production would provide 43,006 and 36,385 kWh for SunSpark and REC respectively. Bear in mind there are two different panel populations, with two different costs and overall production targets. But both arrays are intended to deliver 100% power replacement for my needs.
At 45 C (113 F), panel efficiency is predicted to drop by the given rates. REC has better rates, slightly, but you can be sure a salesman will make sure you know it! Our yearly production totals? 40,597 and 34,638 kWh, respectively. SunSpark lost roughly 2400 kWh and REC lost 1740 kWh.
Considering Age Losses
The 5 year age loss estimate is listed in the table above, but for sake of reference here is the complete table:
At the toasty-hot temperature of 60 C (140 F), the arrays might produce 38,791 and 33,328 kWh for the year, respectively. Our losses on the SunSpark are greater, but we're still producing quite a lot. What's more, it hasn't dipped below the RECs, and of course it shouldn't given the specs. But the aging effects are more considerable.
The SunSparks are expected to lose 0.8% per year, whereas the RECs should only lose 0.25% per year. That will definitely be in the advertising slick. But over twenty years, what are the results? At the temperature we just considered (60 C), this comes to 32,585 and 31,662 kWh expected production for the year, SunSpark and REC respectively.
The Cost of Production
Ultimately, the question we find ourselves asking is: "Is it worth it?" Over the course of twenty years, the slightly more-expensive array - at first glance - will still be out-producing the smaller, less-expensive array.
There are few things we could do here. We could up-size the smaller array, albeit at a much more expensive price-point (a 22.4 kWh REC system, which is how large the SunSpark array is spec'd to be, would have cost around $69,753.60). We could down-size the larger array (down to around $54,256.80 in case you're interested - remember these are rough estimates though). We could even split the difference.
However, one thing to bear in mind is that panels that produce more watts are also usually larger. Even with the slightly higher "efficiency," a 370 watt panel is going to be heftier than a 320 watt panel, which means potentially fewer panels on your roof. Whether this is good or bad depends on your goals. If you want to squeeze as much power as you can get out of your available space, larger panels might compromise that desire by being too large to fully utilize the available space - think having only a single row instead of a double row, or losing a few huge panels because of vent stacks, chimneys, roof joints, etc.
Another way to look at it is: are the cheaper panels a worse deal? I think that is an easier to answer question, and the answer here is that they're not. Yes, there are more of them. Yes, they will degrade faster over time. But they are not unreasonably priced and thanks to the larger overall system size, the long-term degradation is somewhat nullified. That is to say, my overall needs will still be met, ideally, twenty years down the road. Our goal is then to make sure we've received a good return on our investment, so as long as the system covers our power needs for the foreseeable future, that return will be solid.
Conclusion
This is obviously not a "you should buy this" article, and it wasn't meant to be. It was only meant to explore and demonstrate some of the complexities when evaluating prospective panels and arrays, and to understand a few of the gimmicks that are used and based off of honey-colored statistics. Overall, the simplest comparison for the price-focused consumer is simply the cost-per-watt, which is simple and direct to calculate. If nothing else, hopefully the above shows that a panel can tout absolute awesomeness over the competition, and yes in the end it might not really amount to much.
Maybe you're aesthetic-focused and can't bear the thought of panels on the front of your house: larger panels in the back might be superior for you. Maybe a crazy roof is not conducing to large panels, and you need a more flexible layout. Maybe you are driven primarily by price-point. Maybe you want to check every efficiency box you can. Those factors are going to drive your decisions more than the spec slicks probably will.
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