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Solar Panels faraway from maximum peak voltage on sunny days


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Hello, guys

I recently bought 2 x 420 watts solar panels. And yesterday it was really sunny and hot, but both could only get 350watts together. I use a Victron 100/50 (with a fuse protector in case the solars would reach more than 50A). Any ideas of why that could be? I literally expected much more. At least 550 watts. Am I dreaming high? Or is it impossible to get something like that in the UK? I'm planning to test the current from both solars as soon it gets dry again. And the cables are 4mm, all installed by myself. I think that or one of the solars aren't working well or my expectations are too high for the British summer. Any thoughts? 

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Might sound daft, but dirty panels will have a low output, on my panels a single sheet of A4 on the panel has a massive effect, make sure they are clean and no patches of shade.

 

It is also possible that the batteries are reasonably charged and won’t take more than a few amps at a safe charging voltage.  This time of year the sun rises fairly early and by noon when the sun is high in the sky where peak power would be expected my panels output is very low, but that is because the batteries are almost full.

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30 minutes ago, bluemooncaptain said:

Hello, guys

I recently bought 2 x 420 watts solar panels. And yesterday it was really sunny and hot, but both could only get 350watts together. I use a Victron 100/50 (with a fuse protector in case the solars would reach more than 50A). Any ideas of why that could be? I literally expected much more. At least 550 watts. Am I dreaming high? Or is it impossible to get something like that in the UK? I'm planning to test the current from both solars as soon it gets dry again. And the cables are 4mm, all installed by myself. I think that or one of the solars aren't working well or my expectations are too high for the British summer. Any thoughts? 

 

Manufacturer's ratings are not going to match the UK's likely conditions, angle of sun, etc. Panels heating up will also show a reduced output, caused by resistance. Some panels will have diodes, and these would help in overcoming the odd shaded areas, here and there; where any shaded area would otherwise drop the efficiency of the whole panel without these diodes.  

 

 

Edited by Higgs
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You must be completely in the open to get full use of the panels, nearby trees etc with any shade will dramaticaly reduce the input. If they are also flat you will not get anywhere near the full capacity, they must be pointing at the sun all the time to maximise input.

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Agreed 

 

Make of panel can be interesting as well. 

 

I've got a couple of Canadian Solar 415w poly panels on one of the boats mounted flat. Hot and dirty they don't chuck out the full power but I'm not that worried. Will clean them in the winter when I need the power. 

 

 

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Panels are rated under standardised conditions of solar radiation, temperature etc. The standardisation means that comparing panels from different manufacturers is possible, but the conditions used are unrealistic for the UK, or indeed almost any where on this planet. Radiation is closer to what you'd find on a sunny day near the equator, but the panels are also held at a temperature much lower than they would realistically get out in strong sunshine.

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I reckon circulating cold water around either under or on top of solar panels would be worth doing. Route the water to a storage tank. 

 

If the panel was slightly angled it would be quite easy to put a soaker hose on a timer from the main domestic water tank along the top edge and a drain pipe at the bottom..

 

Could be quite an interesting way to do energy efficiency. 

 

I use a soaker hose for the bucket garden. They are quite clever in that they do distribute water evenly and at low flow rates. Ideal for cooling a solar panel. Must try it. 

 

 

 

 

 

Edited by magnetman
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You state that you have a fuse protector, I have no idea what you mean by that. P!ease be aware that some types of breaker/ fuse sold for solar are known to be poor such as the types shown below which can givine a high resistance due to poor contacts. The high resistance vauses voltage drop so the solar controller sees a higher voltage than the batteries actually are at so it backs of the charging current. You need to check the voltage directly at the MPPT output terminals and compare this to the battery voltage. Another cause of this is if your MPPT to battery cables are too long or too thin, you state your cables are 4mm, I hope the MPPT to battery cables are much more then 4mm2 for 50Amp.

 

See a thread a few weeks ago which covered this very topic showing one with high resistance. https://www.canalworld.net/forums/index.php?/topic/115089-solar-controller-over-voltage-cutout-problem/#comment-2791824

 

Some other advice, the Victron MPPT itself will not be able to deliver more than 50A, it is limited to that, you do not need to fuse it to protect against that. Victron will recommend a 60 or 70Amp fuse (I have a 150/60 MPPT and that is 80Amp fuse), the fuse is to protect the cable in case of a short circuit.

 

I have recorded 630W output from my 680W panels at this time of year around mid day with all panels in clear sunshine when I have had a large load on the inverter so that the batteries were not limiting the amount of current that could be absorbed. But that was a one off.

 

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Edited by PeterF
Forgot to add the pics
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2 hours ago, Jen-in-Wellies said:

Panels are rated under standardised conditions of solar radiation, temperature etc. The standardisation means that comparing panels from different manufacturers is possible, but the conditions used are unrealistic for the UK, or indeed almost any where on this planet. Radiation is closer to what you'd find on a sunny day near the equator, but the panels are also held at a temperature much lower than they would realistically get out in strong sunshine.

 

2 hours ago, magnetman said:

I reckon circulating cold water around either under or on top of solar panels would be worth doing. Route the water to a storage tank. 

 

If the panel was slightly angled it would be quite easy to put a soaker hose on a timer from the main domestic water tank along the top edge and a drain pipe at the bottom..

 

Could be quite an interesting way to do energy efficiency. 

 

I use a soaker hose for the bucket garden. They are quite clever in that they do distribute water evenly and at low flow rates. Ideal for cooling a solar panel. Must try it. 

 

 

Temperature coefficient of panel power is typically -0.35%/C (mono PERC panels), so if a cooling system could keep them 10C cooler in direct sunlight (which is quite a lot) you'd get 3.5% more power, about 13W for a typical panel at maximum output -- which it wouldn't often be, so in reality the saving would be less than this. But then you need the power to run the water pump, which would probably eat up most or all of the extra power... 😞

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I've got 160w of panels, they are semi flexible but mounted flat on home made aluminium frames and I don't bother to tilt them.  I am in Yorkshire. A few days ago the Victron controller reported a peak output of 161w.. I find this surprising.

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1 hour ago, IanD said:

 

 

Temperature coefficient of panel power is typically -0.35%/C (mono PERC panels), so if a cooling system could keep them 10C cooler in direct sunlight (which is quite a lot) you'd get 3.5% more power, about 13W for a typical panel at maximum output -- which it wouldn't often be, so in reality the saving would be less than this. But then you need the power to run the water pump, which would probably eat up most or all of the extra power... 😞

You'd need a water pump for a solar hot water system anyway unless using the excess electric and an immersion heater. I don't think a thermosyphon system will send the heated water downhill. Unfortunately. 

 

Also a solar collector would take extra space on top of the boat. 

 

It does make sense to an extent to combine the two (PV and water heating) using a thermostat of some sort attached to the solar panel to switch the pump on at the required temperature. 

 

Possible side effect is keeping the panels clear of debris. This can be filtered out before adding water to the beer keg hot water tank..

 

ETA unless you are a very high consumer with a large and efficient battery bank it's likely you will be getting surplus power so the immersion heater is probably the better option overall but it doesn't cool or clean the solar panels .

Edited by magnetman
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5 hours ago, Chewbacka said:

It is also possible that the batteries are reasonably charged and won’t take more than a few amps at a safe charging voltage.

This ^^^^

If your batteries are not demanding a charging current something above 25 amps (if it's a 12v system), your panels won't be able to deliver more than the 350 watts your seeing  - even if you had 10 x 420w panels instead of the pair.  I'd suggest that, unless you have an atypically large battery bank, by lunchtime much of the heavy charging effort has already been done and what you're seeing is fine.

 

With a large alternator rather than solar, you may typically see in excess of 100a charging current on morning start up, but it'll drop back to the low tens in short order. You won't see that high start up current on solar as the power builds slowly as the sun comes up, but by the time maximum solar power is available you'll have put hours of charge in already hence charging demand has fallen.

 

If you want to prove this to yourself, you could choose a forecasted sunny day when moored with no shade, isolate your solar the afternoon before so you have no solar charging next morning.  After an evening on batteries so they're partially discharged, switch the solar back on at peak sunshine near noon. The initial bulk charge demand by the discharged batteries will then cause a high charging current which will let you see your panels deliver closer to your power expectations.

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30 minutes ago, Sea Dog said:

If you want to prove this to yourself, you could choose a forecasted sunny day when moored with no shade, isolate your solar the afternoon before so you have no solar charging next morning.  After an evening on batteries so they're partially discharged, switch the solar back on at peak sunshine near noon. The initial bulk charge demand by the discharged batteries will then cause a high charging current which will let you see your panels deliver closer to your power expectations.

Or when you have a sunny day, turn on a large load if you have one available, if you have an inverter, something to draw 1kW, then the solar should deliver a high current.

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... or fit lithium batteries which will absorb everything your solar panels can produce until they are full.

 

Switching from lead acid made my solar become about ten times more useful, simply because there is somewhere to store the sunshine.

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3 hours ago, magnetman said:

You'd need a water pump for a solar hot water system anyway unless using the excess electric and an immersion heater. I don't think a thermosyphon system will send the heated water downhill. Unfortunately. 

 

Also a solar collector would take extra space on top of the boat. 

 

It does make sense to an extent to combine the two (PV and water heating) using a thermostat of some sort attached to the solar panel to switch the pump on at the required temperature. 

 

Possible side effect is keeping the panels clear of debris. This can be filtered out before adding water to the beer keg hot water tank..

 

ETA unless you are a very high consumer with a large and efficient battery bank it's likely you will be getting surplus power so the immersion heater is probably the better option overall but it doesn't cool or clean the solar panels .

 

I'm not saying that a solar hot water system is pointless, quite the opposite, you can get quite a lot of heat collected as well as power. You can now get solar panels which are also solar heaters, but they're *very* expensive and you need a closed-system with piping on top of the roof which you need to stop from freezing in winter -- if you spend the same amount of money on more plain old solar panels (and have enough space for them) and use the electricity to run an immersion heater, you'll get more heat for your money.

 

I'm saying that the idea that cooling panels by pumping cooling water past them just to get more electrical power out *is* pretty much pointless, unless you then store the hot water -- which as it heats up, makes it less good at keeping the panels cool.

 

The oft-repeated "fact" that solar panels lose a lot of output when they heat up is simply not true, as the numbers clearly show 😉

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4 minutes ago, Tracy D'arth said:

Rigid panels spaced off the roof or stick ons?

The hot power loss I quoted was for rigid mono panels, which is what the vast majority of installations use because they're by far the cheapest per watt and the most efficient.

 

CIGS panels -- the most common flexible ones -- are very similar (-0.36%/C).

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4 minutes ago, IanD said:

The hot power loss I quoted was for rigid mono panels, which is what the vast majority of installations use because they're by far the cheapest per watt and the most efficient.

 

CIGS panels -- the most common flexible ones -- are very similar (-0.36%/C).

But stick on panels will get hotter than rigid with an air space underneath = less output.

 

Panel in parallel or series, makes a difference if there is any shading?

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8 hours ago, Tracy D'arth said:

But stick on panels will get hotter than rigid with an air space underneath = less output.

 

Panel in parallel or series, makes a difference if there is any shading?

Actually stick-on panels can often run cooler because they're attached to a steel roof which conducts heat away from them. Rigid panels with air underneath are only cooled by it if there's a crosswind to move the hot air out, otherwise it just sits there getting hotter -- it acts as an insulator like in double glazing, so the panels end up hotter than the roof (which is cooler).

 

If there is shading then parallel panels can lose less output than series ones, though this does depend on the panel (internal connections, diodes etc.). With a lot of panels the best solution is to split them into groups with separate MPPT controllers, but few boats have enough panels for this.

Edited by IanD
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Just curious reading this thread. If You had a bank of rigid panels laying flat on a surface on a very hot day, if you faked rain on them using a hose would their output go up?

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45 minutes ago, robtheplod said:

Just curious reading this thread. If You had a bank of rigid panels laying flat on a surface on a very hot day, if you faked rain on them using a hose would their output go up?

Slightly, depending on how much cooler they got. The water evaporating has a much bigger effect than the water being cold due to latent heat of evaporation, it's why a damp towel over something (like a barrel of beer...) helps keep it cool 😉

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