Solar panels in shade.

[Rhylsailer99]

I have 2 x 20w solar panels under the boom, usually one is in shade. I was looking to uograde to 1x70w panel that will fit the same space, but after some reading I think this would be a bad idea.
Am i correct that if some of a larger panel is in shade there will be little output.

 

[peteK]

A lot depends on how the individual cells are wired.

 

[Rhylsailer99]

Location of panels

 

[oldmanofthehills]

If one has two panels and they are directly connected together then the shady one will drag the output down a bit as its a sink not a source. If however both panels are fitted with their own controller then there is no back feed and which ever on gets sun, feeds the battery.

I suspect one large panel might have the some of the same issue as two directly connected panels, and I doubt you want to go to the cost of controllers for each of two separate panels. On the positive side the latest photovoltaic panels dont need direct sunlight so perhaps the shady part of a large panel is not so unhelpful. In othe rwords a 70W panel even if non optimum placement will still give more power than two 20s

 

[Baggywrinkle]

If one has two panels and they are connected together then the shady one will drag the output down a bit as its a sink not a source. If however both panels are fitted with their own controller then there is no back feed and which ever on gets sun, feeds the battery.

I suspect one large panel might have the some of the same issue as two directly connected panels, and I doubt you want to go to the cost of controllers for each of two separate panels. On the positive side the latest photovoltaic panels dont need direct sunlight so perhaps the shady part of a large panel is not so unhelpful. In othe rwords a 70W panel even if non optimum placement will still give more power than two 20s

If connected in series, then shading one panel will bring the output of both down as you said, if connected in parallel, shading one will not affect the output of the other.

@Rhylsailer99 - As already said, this really depends on how the individual cells in the panel are wired, some are far better in partial shade than others.

Have a look at https://www.youtube.com/@WillProwse

and ...

EXPLORIST life Mobile Marine & Off-Grid Electrical

My own experience in the Adriatic with 3 x 100W panels, subject to partial shade from the mast, was that the yield was to all intents and purposes identical whether they were wired in series or parallel - I did an experiment over two weeks with them wired in series for one week and parallel the next, looked at total yield for each week - practically nothing in it. Series raises the voltage so the start voltage is reached earlier in the day and the system shuts off later, parallel is better in partial shading, but runs with panel voltage so seems to start later and shut off earlier. It all depends on your individual setup and location though.

 

[B27]

A 'panel' is itself a series and parallel combination of cells.
So if you shade half a panel, it may depend on whether you're left with a reduced number of series strings, or a whole bunch of strings, each of which is shaded cells in series with illuminated cells.

Personally, I'd suggest fitting two panels. Maybe even tilt them a bit?

If the 20W panels are quite old, then a new panel of similar size might be 30W or so?

 

[Aja]

You can just see the arrangement I have of two 80w panels in parallel each side of the coachroof. They get shaded by the boom but in sunny conditions output does drop but not as significantly as I would have thought.

 

[Rhylsailer99]

A 'panel' is itself a series and parallel combination of cells.
So if you shade half a panel, it may depend on whether you're left with a reduced number of series strings, or a whole bunch of strings, each of which is shaded cells in series with illuminated cells.

Personally, I'd suggest fitting two panels. Maybe even tilt them a bit?

If the 20W panels are quite old, then a new panel of similar size might be 30W or so?

Yes, I just looked and I could replace with 2x35w panels in the same space.
I am looking to buy a portable fridge so will need extra power for the batteries.

 

[oldmanofthehills]

The advice on USA off grid sites, is to use blocking diodes (About $10) to prevent backfeed into a very shaded panel, so presumably a shaded panel is regarded as a lowish impedance load when not supplying. Certainly the rooftop solar panel on my wife installation has separate inputs from east and west bank to prevent back feed to a shaded area and that is standard usage on installations with separate collection zones (the integrator and inverter a heck of a lot more expensive than a pair of diodes but it has other functions). Roof top ones are all in parallel and at optimum give I think about 40v to battery. You certainly wouldnt want them in series as each bank would then give 320v

 

[B27]

Yes, I just looked and I could replace with 2x35w panels in the same space.
I am looking to buy a portable fridge so will need extra power for the batteries.

For what it's worth, I have one 50W panel rigged at the moment and we've not run out of power for our portable compressor fridge.
Running the heater in a couple of months' time might be a different story.

 

[noelex]

The advice on USA off grid sites, is to use blocking diodes (About $10) to prevent backfeed into a very shaded panel, so presumably a shaded panel is regarded as a lowish impedance load when not supplying.

If you have a solar controller that prevents discharge at night (nearly all models do this) installing blocking diodes, especially on lower voltage panels, is likely to be counterproductive. In other words it will result in less total output. This applies even in situations with multiple panels in shaded conditions.

If you have concerns about the discharge via a totally shaded panel this is easy to measure. It does vary depending on the type of solar panel, but typically the current is tiny.

If you have your panels in series, installing additional bypass diodes to bypass each panel is sensible.

 

[oldmanofthehills]

If you have a solar controller that prevents discharge at night (nearly all models do this) installing blocking diodes, especially on lower voltage panels, is likely to be counterproductive. In other words it will result in less total output. This applies even in situations with multiple panels in shaded conditions.

If you have concerns about the discharge via a totally shaded panel this is easy to measure. It does vary depending on the type of solar panel, but typically the current is tiny.

If you have your panels in series, installing additional bypass diodes to bypass each panel is sensible.

Blocking diode merely reduce received voltage at battery by 0.7V. As output on my 100w panel is 40v open circuit it wouldnt make much difference as at battery its about 13.5 in sun ie loses 25v under load anyway

As it happens my cheapy panel came with controller which not only gives permanent voltage reading but also has timer so i could just get it to feed anchor lights at night (intrusive and confusing though that might be)

 

[noelex]

Blocking diode merely reduce received voltage at battery by 0.7V. As output on my 100w panel is 40v open circuit it wouldnt make much difference as at battery its about 13.5 in sun ie loses 25v under load anyway

This 25 V should not be "lost". If you have a MPPT controller (and you should have this type of controller with a Voc of 40 V) the higher input voltage from the solar panel will be converted to more current at the lower battery voltage.

A blocking diode will reduce the input voltage and therefore reduce the output current. The effect is not great on a 40v panel, but you will still lose more than you will gain.

 

[dunedin]

We have two panels on the sprayhood. I would recommend sticking with two panels, wired in parallel into a single MPPT controller. Should work fine,

When on board tend to move the boom to the side to reduce shade and further improve. In winter marina berth (oriented E / W) we keep boom on the northern side.

 

[Rhylsailer99]

We have two panels on the sprayhood. I would recommend sticking with two panels, wired in parallel into a single MPPT controller. Should work fine,

When on board tend to move the boom to the side to reduce shade and further improve. In winter marina berth (oriented E / W) we keep boom on the northern side.

That's exactly how mine are set up with an mppt controller in parallel, I also messed about moving the boom but I think with just 20w each never made much difference.. So glad I never upgraded to the planned single panel underneath the boom, as only just realised by luck I had it right by having 2 panels there instead of one big one.

 

[oldmanofthehills]

This 25 V should not be "lost". If you have a MPPT controller (and you should have this type of controller with a Voc of 40 V) the higher input voltage from the solar panel will be converted to more current at the lower battery voltage.

A blocking diode will reduce the input voltage and therefore reduce the output current. The effect is not great on a 40v panel, but you will still lose more than you will gain.

The controllers contain blocking diodes - else the battery and any other panels would try and send current into the shady panel. Its simple electronics. There is voltage loss and heat dissipation across all electronic devices. One could use germanium diodes not silicon to reduce the voltdrop but as anyone who ever built amplifiers knows thermal overload kills germanium

I used 'lost" colloquially to describe difference between open and closed circuit voltage - quite important in some functions

 

[thinwater]

If the shade is from the boom, swing the boom way off to the side when anchored. Common practice.

 

[noelex]

The controllers contain blocking diodes - else the battery and any other panels would try and send current into the shady panel. Its simple electronics.

MPPT solar controllers typically do not use simple blocking diodes. They are too inefficient. The controller electronically detects when the solar panel array current has dropped below the self consumption of the controller, or when the solar panel voltage is close to the battery voltage. The popular Victron controllers disconnect the solar panels when the solar panel voltage drops to less than 1 V above the battery voltage.

When this occurs the solar controller will enter a "sleep mode" reducing its self consumption and preventing the very slight discharge from the battery into the solar panels. The controller periodically monitors the voltage of the solar panel array. It will "wake up" and connect the solar panels to the battery when certain conditions are met. For the popular Victron controllers this is when the solar panel voltage exceeds the battery voltage by 5 V.

This is more sophisticated and has a lower power loss than a blocking diode.

 

[William_H]

This 25 V should not be "lost". If you have a MPPT controller (and you should have this type of controller with a Voc of 40 V) the higher input voltage from the solar panel will be converted to more current at the lower battery voltage.

A blocking diode will reduce the input voltage and therefore reduce the output current. The effect is not great on a 40v panel, but you will still lose more than you will gain.

Yes with 40v panels and 12v system you do need an MPPT controller. Though 40v panel will work with direct connection or PWM controller but with huge losses of power. Typical 40v domestic panel will produce around 5 amps into a short circuit or a 12v battery. However an MPPT controller can get this to near double current by switch mode transforming.
Incidentally Old manof the hills mentioned domestic PV systems. Here they are wired in series as said to provide 320 or more volts. This goes into a MPPT controller which switches the 320v to 240v AC. (40% of homes in Oz have domestic PV systems from 1500w to 6kw on roof) ol'will

 

[oldmanofthehills]

The controllers contain blocking diodes - else the battery and any other panels would try and send current into the shady panel. Its simple electronics. There is voltage loss and heat dissipation across all electronic devices. One could use germanium diodes not silicon to reduce the voltdrop but as anyone who ever built amplifiers knows thermal overload kills germanium

I used 'lost" colloquially to describe difference between open and closed circuit voltage - quite important in some functions