Why don't solar panels ever produce their peak?

[Hannah Barnes]

Can someone please help me understand why solar panels don't ever produce their peak? I understand it may have to do with the angle and the sun etc. and I think at around midday mine are all optimal but I still get a maximum of 1.2 KW coming in through 6 x 270 watt panels which is roughly 200 watts per panel. 

Is it related to how they are connected? 

And will the efficiency improve with lithium ion batteries? 

[Coulomb]

Short answer: because they get hot, and the rated power is at 25 °C. The higher temperature increases the panel's available current, but decreases the voltage, and the latter effect is the higher. You lose some 0.4% per °C, which typically comes to over 10%. The rest is non-ideal angles and atmospheric effects (early and late in the day). Typically, it means that much of the time, you only get 80-85% of rated power even two hours either side of noon with no cloud or shading.

They do occasionally produce their rated output or even 10% more, with cloud edge effect. That's when the sun burts out from behind the edge of a dark cloud, the panels have had time to cool, then there is a blast of solar production before the panels have a chance to heat up. Edit: but this effect lasts only a minute.

Edited April 14, 2019 by Coulomb

[Mistral]

Thanks for this clear explanation.

[Hannah Barnes]

38 minutes ago, Coulomb said:

Short answer: because they get hot, and the rated power is at 25 °C. The higher temperature increases the panel's available current, but decreases the voltage, and the latter effect is the higher. You lose some 0.4% per °C, which typically comes to over 10%. The rest is non-ideal angles and atmospheric effects (early and late in the day). Typically, it means that much of the time, you only get 80-85% of rated power even two hours either side of noon with no cloud or shading.

They do occasionally produce their rated output or even 10% more, with cloud edge effect. That's when the sun burts out from behind the edge of a dark cloud, the panels have had time to cool, then there is a blast of solar production before the panels have a chance to heat up. Edit: but this effect lasts only a minute.

Thank you this makes so much sense. But then changing to lithium ion batteries would have no positive effect on the solar panel production if I understand correctly? 

[Hannah Barnes]

47 minutes ago, Coulomb said:

Short answer: because they get hot, and the rated power is at 25 °C. The higher temperature increases the panel's available current, but decreases the voltage, and the latter effect is the higher. You lose some 0.4% per °C, which typically comes to over 10%. The rest is non-ideal angles and atmospheric effects (early and late in the day). Typically, it means that much of the time, you only get 80-85% of rated power even two hours either side of noon with no cloud or shading.

They do occasionally produce their rated output or even 10% more, with cloud edge effect. That's when the sun burts out from behind the edge of a dark cloud, the panels have had time to cool, then there is a blast of solar production before the panels have a chance to heat up. Edit: but this effect lasts only a minute.

And another question - if my 6 panels are bolted directly to an IBR roof with obviously very hot conditions most of the time in summer where I live, would this also be having an adverse effect on the heat of the solar panels? Will it help to put brackets between the panels and the roof? I'd like to try and do it all optimally when we install the other  6

[___]

6 minutes ago, Hannah Barnes said:

changing to lithium ion batteries would have no positive effect on the solar panel production

Changing to Lithium has a huge effect when charging above 80% SOC. With lead acid batteries your absorption rate slows down as the battery fills up. So typically as the battery goes above 90% SOC, the charge current drops off to as little as 2% or even 1% of the charge current you started with, and getting the battery 100% full is a really really slow process. During that entire time, the MPPTs will throttle the PV modules and if you don't have any loads that can consume the power, it will be lost.

An LFP battery charges to 99% almost at full power and then stops like a plane against a mountain. It definitely has a positive effect.

[Hannah Barnes]

1 minute ago, plonkster said:

Changing to Lithium has a huge effect when charging above 80% SOC. With lead acid batteries your absorption rate slows down as the battery fills up. So typically as the battery goes above 90% SOC, the charge current drops off to as little as 2% or even 1% of the charge current you started with, and getting the battery 100% full is a really really slow process. During that entire time, the MPPTs will throttle the PV modules and if you don't have any loads that can consume the power, it will be lost.

An LFP battery charges to 99% almost at full power and then stops like a plane against a mountain. It definitely has a positive effect.

I get it. Thank you very much. 

[___]

@Hannah Barnes, tell me more about your ministry? Just a one-liner, I'm curious

[Coulomb]

2 minutes ago, Hannah Barnes said:

my 6 panels are bolted directly to an IBR roof with obviously very hot conditions

Never bolt panels directly to a roof. I've seen even Sunpower panels (one of the more expensive brands) ruined by this. You must have air under the panels, which will happen automatically when you use proper racking systems. It's not just reduced production, it's early (few years) death.

[SilverNodashi]

7 minutes ago, Hannah Barnes said:

Thank you this makes so much sense. But then changing to lithium ion batteries would have no positive effect on the solar panel production if I understand correctly? 

No, but it would help with your storage needs Lithium Ion batteries have higher cycles and don't suffer from the Peukert's Effect. In laymen's terms, a lead acid battery looses capacity over time. i.e. today it can hold 100% capacity but after, say, 100 cycles it will only hold say, 95% capacity. Capacity diminish over time. And the Peukert's effect also state that a Lead Acid / Gel / AGM / etc battery can't deliver the same performance linearly at higher Amperage drawn. i.e. at 5A it might deliver say 1920Kw, but at 10A it won't deliver 960W, instead it might deliver say 900W or so.

 

 

Back to your original question, solar panels are specked at a certain rating, at 25degrees celcius, with 1Kw/sqm solar radiation. Solar radiation changes throughout the day, as the sun / clouds move. The wind also plays a role. And as @Coulomb said, as soon as a solar panel goes above 25degrees celcius, it looses some volts, or efficiency.

Stationary panels also produce less power than panels that track the the sun perpendicularly.

[SilverNodashi]

4 minutes ago, Hannah Barnes said:

And another question - if my 6 panels are bolted directly to an IBR roof with obviously very hot conditions most of the time in summer where I live, would this also be having an adverse effect on the heat of the solar panels? Will it help to put brackets between the panels and the roof? I'd like to try and do it all optimally when we install the other  6

Yes, definitely! Try and lift the panels by at least 150mm to 200mm off the roof. If the roof is slanted, at say 30 degrees, it helps a bit with natural ventilation. But flat roofs have less natural ventilation so the space between the panels and roof will help

[Hannah Barnes]

1 minute ago, Coulomb said:

Never bolt panels directly to a roof. I've seen even Sunpower panels (one of the more expensive brands) ruined by this. You must have air under the panels, which will happen automatically when you use proper racking systems. It's not just reduced production, it's early (few years) death.

Oh dear. Thank you. I will see to this. Best place to get a proper racking system? 

[SilverNodashi]

2 minutes ago, Coulomb said:

Never bolt panels directly to a roof. I've seen even Sunpower panels (one of the more expensive brands) ruined by this. You must have air under the panels, which will happen automatically when you use proper racking systems. It's not just reduced production, it's early (few years) death.

I had a client once, about a year after he brought a 4Kw PV array from me, but installed it himself, tell me one day that solar is crap. Well, he used other words but you get the jist. He wanted a refund. So, trying to get to the bottom of the problem, it turns out he put the panels flat on a metal roof, and used some brackets he made by bending some flatbar in a vise and drilled some holes in it. Bottom line is the panels overheated and under-produced badly.

@Hannah Barnes even though the proper mounting structures cost a lot of money, it helps getting the most out of your system.

 

[SilverNodashi]

3 minutes ago, Hannah Barnes said:

Oh dear. Thank you. I will see to this. Best place to get a proper racking system? 

I can courier you some. Send me a PM with your address, roof type and amount of panels you have.

[___]

3 minutes ago, Hannah Barnes said:

Best place to get a proper racking system?

Most solar places will be able to provide you with commercial racking solutions. I personally quite like Renusol. Talk to @Jaco de Jongh, he supplied me with the bits and pieces I needed. See some of my pictures here. For a corrugated iron roof that is already tilted, I'd probably go with the MS+ series.

[Hannah Barnes]

5 minutes ago, Coulomb said:

Never bolt panels directly to a roof. I've seen even Sunpower panels (one of the more expensive brands) ruined by this. You must have air under the panels, which will happen automatically when you use proper racking systems. It's not just reduced production, it's early (few years) death.

This is what it looks like at the moment. There is quite an angle so should be some ventilation but not enough if I understand? 

[Hannah Barnes]

11 minutes ago, plonkster said:

@Hannah Barnes, tell me more about your ministry? Just a one-liner, I'm curious

Ha ha i need more than one line :)....part of a Christian ministry run nationally. We run the course in Limpopo one weekend a month on the farm at the moment.  

[___]

Here is an example of the older MS system, MS+ is similar.

[Hannah Barnes]

1 minute ago, plonkster said:

Here is an example of the older MS system, MS+ is similar.

Very nice. Thank you so much. I will start the quote process. The farm had no power at all when I first bought and I had such a limited budget to get the solar and AC into the house so this was not a priority at all. But I will get it fixed as soon as possible. Again it is grace that my whole system has survived my ignorance for so long. 

[Hannah Barnes]

23 minutes ago, SilverNodashi said:

No, but it would help with your storage needs Lithium Ion batteries have higher cycles and don't suffer from the Peukert's Effect. In laymen's terms, a lead acid battery looses capacity over time. i.e. today it can hold 100% capacity but after, say, 100 cycles it will only hold say, 95% capacity. Capacity diminish over time. And the Peukert's effect also state that a Lead Acid / Gel / AGM / etc battery can't deliver the same performance linearly at higher Amperage drawn. i.e. at 5A it might deliver say 1920Kw, but at 10A it won't deliver 960W, instead it might deliver say 900W or so.

 

 

Back to your original question, solar panels are specked at a certain rating, at 25degrees celcius, with 1Kw/sqm solar radiation. Solar radiation changes throughout the day, as the sun / clouds move. The wind also plays a role. And as @Coulomb said, as soon as a solar panel goes above 25degrees celcius, it looses some volts, or efficiency.

Stationary panels also produce less power than panels that track the the sun perpendicularly.

Thank you for your input - I will pm you for a quote for the racks

[GreenFields]

3 hours ago, Hannah Barnes said:

 

Can someone please help me understand why solar panels don't ever produce their peak? I understand it may have to do with the angle and the sun etc. and I think at around midday mine are all optimal but I still get a maximum of 1.2 KW coming in through 6 x 270 watt panels which is roughly 200 watts per panel.

 

Your daily peak power should also vary throughout the seasons as the angle of sunlight coming onto the roof panel changes. Higher peak as the light comes in closer to the perpendicular in summer, and a lower peak in Winter as the light comes in at a sharper angle and spreads over a larger area. If your panels are not facing directly north, the daily peak will also be lower. And I guess some 2-3% losses due to inefficiencies of the inverter could be considered.

[Coulomb]

2 hours ago, Hannah Barnes said:

This is what it looks like at the moment.

That looks all too similar to the roof that ruined the Sunpower panels. It looks like there is some ventilation, but I suspect the conduction from the rest of the roof has more to do with it. You seem to be covering most of the roof with the panels, so it may not be too bad.[ Edit: Ah, I remember now, a colleague told me that the channels fill with leaf litter and block the flow of air. I suppose if you're vigilant and clean regularly you might get away with it, but there are better things in life to be doing. ]

I ended up with those cooked panels, and actually attempted to repair them, hoping that it was just a few lifted contacts. Sunpower panels are one of the very few with back contacts, making repair feasible. But it turned out that more than 50% of the cells needed repair, and it was a never ending process. They were thoroughly ruined; I ended up trashing them.

Edited April 14, 2019 by Coulomb
As noted.

[___]

3 hours ago, Hannah Barnes said:

This is what it looks like at the moment.

I see a bit of rust as well. That may well be because the alluminium is reacting with the zinc and the steel which have a different galvanic index.

[GreenFields]

Other thought, maybe, not to let your cables lie chafing on that metal roof in wind and weather, but to route them more safely and securely. 

[SilverNodashi]

9 hours ago, Hannah Barnes said:

This is what it looks like at the moment. There is quite an angle so should be some ventilation but not enough if I understand? 

That angle will help a lot with ventilation but it would be best to lift them off the roof. I will send you a quote for this.

Edited April 14, 2019 by SilverNodashi