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Why solar PV panels don't work so well in the heat

Posted by Chris Rudge on 19 July 2010 at 9:32 am

During these last few weeks of hot sun, which are a welcome change from the rain of previous  few years, we have been getting calls from customers asking why the output from their solar PV panels is down, even though there is bright sun beaming down onto it.

The simple answer is solar heating!... Unlike solar thermal systems which rely on the sun's heat to do their work, solar PV works on light, and would like to stay as cool as possible. You'll notice your solar PV panels will produce as much output on a clear sunny winter's day as in the summer, even though the sun is lower in winter.

In winter your solar PV panels will be far colder, maybe almost zero degrees, which will improve their efficiency. In the summer sun, the solar PV module temperature will soar far over the Standard Test Condition (STC), rating of 25 degrees C, causing output to slump a little.

A recent example showed our office solar PV system drop off by 10% in the recent 30 degree plus days. So dont worry that a module is failing. Your system will perk up when the weather is cooler.

Photo: Wichita Renewable Energy Group

About the author: Chris Rudge is a qualified electrician who specialises in renewable energy.

If you have a question about anything in the above blog, please ask it in the comments section below.

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15 comments - read them below or add one

Cathy Debenham

Cathy DebenhamComment left on: 15 October 2013 at 11:53 am

Hi Wilma

My guess is that your inverter limits generation to 3.6kW - which is something to do with compliance with the grid.

As to the figures, a 4kW system in Bournemouth would be expected to generate 4,372kWh a year according to the figures in the new MCS guide. If you are averaging 12kWh per day, then that's 4,380kWh a year - pretty much spot on!

Of course, generation will vary enormously depending on the time of year. December is generally the worst month for generation, and May the best.

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WilmaComment left on: 14 October 2013 at 2:01 pm

I've had 4Kw system installed for 15 months now on a SE facing roof in Bournemouth and have never got more than 3.6Kw summer or winter.

My best ever day was 25 units but the daily average over the last 15 months has been  12 units. 


Should I be getting more?

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Caitlin Moran

Caitlin MoranComment left on: 25 September 2013 at 4:00 pm

@samoxby, thank you for your comment, please have a look  at our blog on Thermodynamic panels.  Thermodynamic panels are used for hot water whilst Solar PV panels are used for electricity,  you will see in the blog how the two compare.

The YouGen Team

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samoxbyComment left on: 25 September 2013 at 3:32 pm

I've been reading up on solar and thermodynamic panels.

Do thermodynamic panels not eliminate this issue of diminishing output? Would these be a better investment than solar PV panels?

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warranpComment left on: 21 August 2013 at 5:17 pm

It is with interest in the loss of performance, that I am in discussion with the installer to see if a test of the system would indicate loss of performance due to panel degredation or panel failure. He suggests that the system is surpassing the SAP Performance predicted, but I contend the SAP is generic and the recorded performance of the system is real world numbers. 

I fitted 16 250W Sanyo Panels 12 months ago. The panels have been working well until Jun this year when a drop off in daily peformance was significant.  The daily peak for clear sunny was 33+ units generated.  From Jun and certailnly through the very hot july period it peaked at just 27 units. A 20% or so drop. 

This seems concur with Jayhawks view that 20% loss or more is not unusual.

The system still Maxes at 3.7KW, the inverter limit, when in full sun and the panels are clean.

The blurb suggests 3% performance loss per 10 degrees rise and the industry norm is a fall off of 1% per year. The normal working temp is 45deg C the 20% performance loss suggests the working temperatures are in excess of 90deg C.  

So since the July heat I have not seen any recovery in performance.  The installer suggest I keep monitoring and wait.

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Mikey_mComment left on: 17 July 2013 at 5:08 pm

Hi Chris;

We live in a dusty area (it blows off the ploughed sandy field behind our property), but the panels are quite clean as I hosed them down the day before yesterday. I do that quite frequently. It caused the output to go up from 3kW to 3.2kW - i.e. still 1kW down from what they were a month ago (when conditions here were just as dusty). 

Seems to me that a 10% drop due to high temperature would be expected but a 25% drop???

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Rudge Energy

Rudge EnergyComment left on: 17 July 2013 at 4:11 pm

Hi Mikey

You may find that there is a layer of dust has accumulated on the surface of the modules. Especially after this extended period of zero rainfall.

If you are able to wash off with clean water, then you will find this will make a difference.

High temperatures will slump the output too of course, and this effect will vary from manufacturer to manufacturer.


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Mikey_mComment left on: 17 July 2013 at 3:35 pm

Further to yesterday's comment, today - a sunny and cloudless day here in Suffolk -  my 4kW system only made it up as far as 3.1kW. Surely the heat should not be causing such a marked reduction in output!??

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Mikey_mComment left on: 16 July 2013 at 5:07 pm

I live in coastal Suffolk and I have a 4kW PV system that is 7 months old. During the hot sunny weather we have at present, the peak output is only 3.2kW on a really fine day, compared to the 4.2kW I got in sunny but cool weather in June. Is this 25% reduction reasonable for temperature-related drop-off or should I be thinking that the system has developed a fault?

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Current Energy Solutions Ltd

Current Energy Solutions LtdComment left on: 18 June 2012 at 9:35 am

Hi Daisy,

If, as you say " have 2 separate arrays" then your installer has wired up your system totally incorrectly.

It should have been wired in one string of 11. This would give a string voltage that would enable the inverter to operate at all module temperatures.

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InventoComment left on: 15 June 2012 at 11:00 am

Hi Daisy,

It sounds like you have a major problem with the wiring of the panels or with the inverter settings, and may need to get your installers back.  I suggest you register with the Navitron forum  which is much more active than this one. If you post a full description of your system and details of the symptoms, you will soon get some good advice from an array of experts.

Good luck,


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Daisy Potter

Daisy PotterComment left on: 14 June 2012 at 4:23 pm

I have 11 sanyo hybrid panels H250 and a sunnyboy inverter 2500HF. The panels are west facing. When it is overcast and cold the panels work until 9o'clock at night. But when it is sunny they don't work. In our short summer, lasting 1 week, in May they didn't generate any electricity at all. The PV voltage was around 200 and not high enough to register. I have 2 separate arrays. Why is the system producing too little in the hot sunshine?  It seems to be the opposite to other people's experience. 

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Eco InnovationsComment left on: 26 November 2010 at 9:35 pm

It will be interesting to see the application of thin film PV. Ive noticed our installations are providing greater power outputs now the temp has dropped on a bright day. 

Eco Innovations

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GM Renewables

GM RenewablesComment left on: 29 September 2010 at 5:26 pm

PVT panels can increase output of the PV side of the panels by cooling.

Increases in efficiency of up to 40% are possible and there is an enormous supply of heat, ideal for swimming pools for instance.  have full details.

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Jayhawk International Ltd

Jayhawk International LtdComment left on: 20 July 2010 at 1:10 pm

I am very pleased to read this statement by Chris about the drop in efficiency during hot weather of PV panel installations.

We are soon to introduce a PVT panel, German design and German quality as you would expect.

In their 2 years of factory testing and in the TUV testing it was found that a 30% drop in efficiency could be shown when outside tempertures were 30C +

The PVT panel provides electricty from the top and heated water underneath but not to tempertures of 70c but 40c as the lower the temperture flowing across the back of the PV panels, the higher the efficiency increases.

The ideal installation for the PVT panel is when the swimming pool needs to be heated and any comercial building where a pre heated water is required in volumes. Like a Brewery

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