How Does Heat Affect Solar Panel Efficiencies?

Excessive heat can significantly reduce the output of a PV system. This article gives some pointers on avoiding unexpected energy loss in an array.

How Heat Affects Solar Panel Efficiency

 

It may seem counter-intuitive, but solar panel efficiency is affected negatively by temperature increases.  Photovoltaic modules are tested at a temperature of 25 degrees C (STC) – about 77 degrees F., and depending on their installed location, heat can reduce output efficiency by 10-25%.  As the temperature of the solar panel increases, its output current increases exponentially, while the voltage output is reduced linearly. In fact, the voltage reduction is so predictable, that it can be used to accurately measure temperature.  

As a result, heat can severely reduce the solar panel’s production of power. In the built environment, there are a number of ways to deal with this phenomenon.  

Different module designs and different semiconductor compounds all react to temperature – here’s a brief intro into what to expect.

 

Determining Your Efficiency

 

The best way to determine your panel’s tolerance to heat is by looking at the manufacturer’s data sheet. There, you’ll see a term called the “temperature coefficient (Pmax.)”  This is the maximum power temperature coefficient.  It tells you how much power the panel will lose when the temperature rises by 1°C above 25°C. @ STC (STC is the Standard Test Condition temperature where the module’s nameplate power is determined).  

For example, the temperature coefficient of a Panasonic VBHN330SA16 solar panel is -.258% per 1 degree Celsius.  So, for every degree above 25°C, the maximum power of the Panasonic solar panel falls by .258%, for every degree below, it increases by .258%.   What this means no matter where you are, your panel may be affected by seasonal variations. However, the temperature coefficient also tells you that efficiency increases in temperatures lower than 25°C.  So, in most climates, the efficiency will balance out over the long run.  

For a geographic region where temperatures higher than 25 degrees C. are the norm, one can consider alternatives to Mono or Polycrystalline modules, which have the highest efficiency (At 1:1 concentration), but also the highest temperature coefficient at PMAX. Project designers may want to consider a thin film or CdTe module – or in the case of a very large project, High Concentration PV, which is designed for hot climates, but not applicable for small projects.

 

How to Reduce the Effects of Heat

 

After the module technology is selected for an installation, there are several ways to minimize the negative effects of high temperatures: 

 

• Install panels a few inches above the roof to allow convective air flow to cool the panels down.

• Ensure that panels are constructed with light-colored materials, to reduce heat absorption.

• Move components like inverters and combiners into the shaded area behind the array.

 

Further information about temperature coefficients can be found in this research paper, produced by Sandia National Laboratories: "Temperature Coefficients for PV Modules and Arrays" David L. King, Jay A. Kratochvil, and William E. Boyson" (PDF)

 

Comments

Really great article, thanks.   It is amazing how much high ambient temperature reduces the efficiency of Mono and Polycrystalline solar panels.   It further explains why panels in high altitude locations perform so well.  

Use the "view lifetime" of my array.Look at how heat debilitates the 10 panels that are mounted above asphalt shingles by ~ 10% beginning in mid-May thru August (temp about 49C 120F---space clearance for cooling is important!  We know there is much more sun in July than the highest output time in the merry, cool month of May.  What a waste.  

I live in Mexico and because of the higher average temperature spent the money and got Sanyo Hip panels, they actually are the only ones we found in our research that perform better in high temperatures. We are Loving our pannels and so far are not seeing any loss at all due to heat.

I believe in the future of renewable energy. Our house has solar panels to save money on our bills (which are extremely high). I suggest anyone with a bit of property, a business or a roof to see if you can put solar, it's really worth it. Quick tip: we used www.mysolarinstaller.com to get tailored quotes for our home. They got us the top solar installers in our area to send free quotes...it took us less than a minute with their process and we got to compare apples with apples.

Hi Rob. Do you have an actual install or are you just promoting MSI? We allow postings with outside links if the information is related to the article / question. Please post details of your project and how they relate to heat and solar arrays.

A typo..

"So, for every degree above 25°C, the maximum power of the Sharp solar panel falls by .485%, for every degree above, it increases by .485%."

This should read "for every degree below, it increases by .485%."

Great article though! Thanks..

Great article. Acknowledges what we have known, and worked around for years here at Falmouth Solar, LLC on Cape Cod, MA. What really would be helpful would be some hard numbers, tested in my area. What output loss is experianced at what height / and roof temps for example. WE don't like the trend of the local installers here to put the array within a couple of inches of the roof, then put a sheet metal cover along the bottom row. How can heat convect? It's a nice look from the ground, but stupid from an engineers view. We don't install railess racking because we generally will even provide 1.5" between rows to further let the excess heat evaluate. This, as far as I know is an original thought, and I am unaware of any other installers doing it. Testing is needed to validate the effects of side skirts, row spacing, and height above roof for validation of our efforts, but a potential loss of 10-25% is HUGE in this industry, and generally ignored (except by the wise few reading this article!) 

My Sunpower E19 panels have a power coefficent of 0.38% so even at 115 degrees Fahrernheit the output is reduced less than 7% and only for those hours when the ambient temperature is that high. Where I live the hottest it ever gets is 83 degrees and that results in output of "only" 95% of the panel's rating.

Hours of daylight and the height of the sun are far more of a factor and I wanted maximum production throughout the year and selected the panel angle to accomplish this and I also added an extra two panels as doing was not going to have a big impact on the overall cost of the installation but helped maintain output during the winter months.

I used racking to get the optimum angle for the panels and to allow for cool air flow around them. Attaching panels directly to the roof is not going to provide for an optimum angle in most situations.

I don't suppose it is worth it to water cool the solar panels somehow.. Just use that money to install more panels if you have the surface area with an ideal angle. 

System too efficient, get to high temperature:
Hello. I have two solar panel of 150 litters each. The panels are connected to an inside geyser tank of 200 litters.
There is a pump that moves the cold water from the tank to the panels and from the panels to the tank.
The unit has a solar control unit that shows the temperature on the geyser tank.
The system is so efficient that I'm getting 85 C on a sunny day. The problem is that the water gets too hot and the other day the system was showing me 93 C. At 100 C will boil! so I'm scare the whole system might blow.
Any advice?
I have considered to install another 200 litters tank. Other option is to cover on of the 150 litters solar panel, during summer, but that is complicate (pick roof, hard to get, not practical).
Any advice please?

 

Thanks!

Anthony,
Gordons Bay, West Cape, South Africa

 

Anthony,

I'm not sure I entirely understand your question. Are you circulating water from tanks to help cool your modules? If yes, perhaps it would be best to increase the volume of water your circulating or the frequency in which the water is cooled.