Four questions about solar

[Dirk01]

Question 1: Can I imagine an MPPT Solar Charger as an "electricity sink"? If, for example, I connect two solar modules with different output voltages - say 18VDC and 20VDC - in parallel to one MPPT Solar Charger input, will a 12V battery get the full power of each individual solar module? That means every module spent the full power to the charger. Or do I have to consider something more or something totally different?

Question 2: What power do the solar panels give - say + / - 15° north or south of the equator? The performance specifications of the manufacturers refer to a "sun light output" of 1kW per m². However, the solar power in the area of the equator is (assuming a cloudless sky) about 2kW per m². Consequently, the modules (assuming constant efficiency) would have to deliver about twice the power. This in turn means double the current at the same voltage (to my knowledge the voltage does not get higher). This, in turn, would be important for dimensioning the maximum charging current that the solar charger can process. Is this assumption correct?

Question 3: Is there any experience of the peak values (cloudless sky, no shading)? To be precise, is it actually the case that the solar power of the modules is above the nominal values (in the above-mentioned +/- 15° around the equator) – above the information of the data sheets of the manufacturers? Alternatively, I can of course also convert Ah if the associated battery voltage and the nominal power of the solar panel is named (efficiency would be nice to make it more precise). Of course, it is important that the solar modules were never shaded or partially shaded. With conventional modules, depending on the number of bypass diodes, this leads to a significant to complete performance loss. Modern "hot spot free" modules have this problem to a much lesser extent.

Question 4: Which work (kWh) can realistically be harvested per day and per m² (specify current efficiency)?

It is clear to me that many other parameters (i.e. degree due to age of the modules, salt crystal coating, angle to the sun, etc. etc. etc.) make differences also. The more qualified information is provided, the sooner a "feeling" can be developed for where the journey is going. And maybe in the course of the posts there will be a hint for one or the another how to harvest the missing bit of energy without much effort.

Now I am very curious about the answers.

Cheers
Dirk

[hjohnson]

In regards to your first question, you want all panels wired in parallel to a given MPPT to be pretty close in characteristics. If you have two significantly different panels, you realistically want two smaller MPPTs. They work by loading and driving the panels to their highest power point (hence the name).

On my small boat, I have a pair of 60W 24v panels sitting flat on top of my dodger. The highest power I’ve seen out of it has been about 135W, which OSS pretty impressive.

[redneckrob]

Originally Posted by Dirk01

Question 1: Can I imagine an MPPT Solar Charger as an "electricity sink"? If, for example, I connect two solar modules with different output voltages - say 18VDC and 20VDC - in parallel to one MPPT Solar Charger input, will a 12V battery get the full power of each individual solar module? That means every module spent the full power to the charger. Or do I have to consider something more or something totally different?

Question 2: What power do the solar panels give - say + / - 15° north or south of the equator? The performance specifications of the manufacturers refer to a "sun light output" of 1kW per m². However, the solar power in the area of the equator is (assuming a cloudless sky) about 2kW per m². Consequently, the modules (assuming constant efficiency) would have to deliver about twice the power. This in turn means double the current at the same voltage (to my knowledge the voltage does not get higher). This, in turn, would be important for dimensioning the maximum charging current that the solar charger can process. Is this assumption correct?

Question 3: Is there any experience of the peak values (cloudless sky, no shading)? To be precise, is it actually the case that the solar power of the modules is above the nominal values (in the above-mentioned +/- 15° around the equator) – above the information of the data sheets of the manufacturers? Alternatively, I can of course also convert Ah if the associated battery voltage and the nominal power of the solar panel is named (efficiency would be nice to make it more precise). Of course, it is important that the solar modules were never shaded or partially shaded. With conventional modules, depending on the number of bypass diodes, this leads to a significant to complete performance loss. Modern "hot spot free" modules have this problem to a much lesser extent.

Question 4: Which work (kWh) can realistically be harvested per day and per m² (specify current efficiency)?

It is clear to me that many other parameters (i.e. degree due to age of the modules, salt crystal coating, angle to the sun, etc. etc. etc.) make differences also. The more qualified information is provided, the sooner a "feeling" can be developed for where the journey is going. And maybe in the course of the posts there will be a hint for one or the another how to harvest the missing bit of energy without much effort.

Now I am very curious about the answers.

Cheers
Dirk

A panel well never produce appreciably more than it's rated power no matter the irradiance, for example a 250 watt panel will never produce more than 250 watts. Also keep in mind that the nameplate power is usually at standard test conditions (STC) which is a temperature of 25 C. The hotter the panel is the less power it produces, and at the equator you'll be considerably hotter than 25 C and the panel will be considerably hotter than the air temp. Most panel manufacturers produce a temperature curve to show you the impact of that.

You can play around with the U.S. DOE's free PVWatts tool to determine how much power a system will make in a given location for a given month with a given azimuth and tilt. It's at https://pvwatts.nrel.gov/ and dead simple to use.

[HeywoodJ]

1. An MPPT controller works by selecting the 'best' voltage at which to run your modules. If one module produces maximum power at 18V, it will likely, because of the 'knee', produce almost no power at 20V. So, in your example the MPPT controller would select something near 18V. But that now means your 20V module is running at 18V, producing about 10% less than its capabilities under the current conditions.

2. Sunlight at the equator is not normally much greater than 1000 W/m2. At the top of the atmosphere sunlight is ~1350 W/m2, it becomes attenuated as it it passes through the atmosphere. The 1000 W/m2 test standard accounts for that 1350 passing through 1.5xatmosphere thickness (to account for sun not generally being directly overhead). In the equatorial regions it is typically 1.1-1.2 atmosphere and regular conditions get up to ~1100W/m2. So, in theory you could expect 10% over the nameplate capacity from your panel. However, rated conditions are at 25C module temperature. In sea-level equatorial regions it would be very rare to see anything close to that module temperature, so the the temperature de-rating largely balances the increased irradiance.

3. Absolutely you can get conditions where output power from a solar panel exceeds nameplate conditions. It is possible with cloud-banding, for instance, to get irradiance levels to 1300-1400 W/m2 and a corresponding increase in power output from a module. If your MPPT controller has headroom it may be able to pass this power through. If it doesn't have headroom then a good MPPT has the ability to be 'over provisioned', i.e. you have more solar on the input side than the controller can put out on the output side. In this case a good MPPT will raise the operating voltage on the input side, dropping the current (because of the knee) and thus the total power, and thereby limiting output to its capabilities. A bad MPPT controller may destroy itself. Good datasheets tell you how much you can over-provision.

4. The 'rule of thumb' is 4-5 hours of nameplate production from your module each day. So, for a 100W module, 400-500Wh/day of production. But this a) assumes no shading, and b) is highly dependent on actual irradiance. In the picture below you can see the highest solar intensity is actually about 15º N and south. You can also see that the 'typical' value matches the rule of thumb of 4-5x, but you get as high as 7x in the best insolation areas.

[Adelie]

1. Due to vagaries of how electricity works if the panels aren’t the same then they should each get their own MPPT controller.
This maximizes output.
On a related note if the is going to be any shading, even by shrouds, it’s best for each panel to have its own controller.
Shading or a lower voltage output will drag down the output of any panel sharing a controller. Multiple small controllers offer redundancy, if a controller fails, it’s panel can be added to a different controller until a replacement can’t be obtained.

2. Except for periods where the sun is close to the horizon you latitude will not affect irradiance much.

3. Nameplate Wattage is a pretty good estimate of peak wattage. In perfect conditions of air quality (not just cloudless but really clear) and panel orientation you probably would exceed nameplate wattage.

4. Rule of thumb for flat mounted panels in tropical and probably near tropical latitudes is that on average you can expect daily amp-hr produced to be 1/3 of nameplate wattage when using an MPPT controller, 1/4 when using PWM controller.

In higher latitudes during the summer you trade a lower angle of incidence on the panel for a much longer day and the rule of thumb is still a decent approximation.

If the panels track instead of laying flat output can go up significantly.

My sense is that underway when the boat’s course is stable and the watch is checking around regularly panels can be manually adjusted to track the sun.

At anchor, the boat will have a less stable orientation and there won’t be a watch to regularly adjust orientation so it would probably work best to lay them flat.

The low hanging fruit for power production on a sailboat is to mount as much solar as you can.

I would have a large panel on on arch over the stern, possible hinged so it could be tilted towards the sun.
The Bimini would have separate panels port and starboard (possible 2 each side) each with their own controller. Dodger would also have separate panels port and starboard each with their own controller.

[noelex 77]

Originally Posted by Dirk01

Question 1: Can I imagine an MPPT Solar Charger as an "electricity sink"? If, for example, I connect two solar modules with different output voltages - say 18VDC and 20VDC - in parallel to one MPPT Solar Charger input, will a 12V battery get the full power of each individual solar module? That means every module spent the full power to the charger. Or do I have to consider something more or something totally different?

The panels have to be matched in some characteristics if they are to be connected to one controller. For parallel connection the voltage of the panels has to be the same, or very close. If this is not the case you are unlikely to do any damage, but efficiency decreases significantly.

Originally Posted by Dirk01

Question 2: What power do the solar panels give - say + / - 15° north or south of the equator? The performance specifications of the manufacturers refer to a "sun light output" of 1kW per m². However, the solar power in the area of the equator is (assuming a cloudless sky) about 2kW per m². Consequently, the modules (assuming constant efficiency) would have to deliver about twice the power. This in turn means double the current at the same voltage (to my knowledge the voltage does not get higher). This, in turn, would be important for dimensioning the maximum charging current that the solar charger can process. Is this assumption correct?
Dirk

It is certainly possible to extract more than the STC power from a panel at least briefly and occasionally. As you note, this mainly because STC conditions assume 1kW per m² and luminances higher than this do occur. But double is very optimistic. The most you are likely to see is around 15%-20% above STC and even this will only be briefly and in conditions of very good solar insolation. These brief periods of exceptional output are, however, a sign that the panels and system are performing well.

Originally Posted by Dirk01

Question 3: Is there any experience of the peak values (cloudless sky, no shading)?

As posted above, the maximum peak value you are likely to see in a very good system is around 15% or maybe 20% above the STC rating. Many installations struggle to achieve the STC rating, even occasionally and briefly, but if you are not at least very close to the STC rating in very good solar conditions something is wrong with the system.

Originally Posted by Dirk01

Question 4: Which work (kWh) can realistically be harvested per day and per m² (specify current efficiency)?
Dirk

There are common rules of thumb related to the wattage of the panel. The most common quoted is that you will extract 1/3 of the the panel wattage in Ahrs (@12v). So a 100w panel would deliver 33 Ahrs per day.

Personally, I think these rules are near worthless. Depending on the season and geographic location an average from better than 1/2, to worse than 1/30 is seen. The range is so great that rules of thumb are of little practical value.

You need to look at the insolation in the area at the time of year to arrive at realistic answer. Rules of thumb such as 1/3 create an unrealistic expectation that average solar output will be consistent. In practice it varies considerably with lattitude, season and geographic conditions.

[noelex 77]

Originally Posted by redneckrob

A panel well never produce appreciably more than it's rated power no matter the irradiance, for example a 250 watt panel will never produce more than 250 watts.

This is not correct.

Peak outputs greater than the STC rating are not impossible, or even very unusual providing you are in a location and season where the solar insolation is reasonable and the system is performing correctly.

This is the peak output today from one of my 335w panels. As you can see at 369w it is well above the STC rating, and we are above 55°N. These periods of exceptional power will only be brief, usually when the sun peaks out from a cloud and the sunlight strikes the panels both directly and from the cloud edge. Today was actually a poor solar day, but the cloud together with brief periods of sun and less regulation creates conditions where the peak output tends to be highest.

These high peaks are perhaps of little significance to the average output, but importantly they do show that the solar system is performing as it should do. If the panel is defective, incorrectly advertised, or there is excessive loss in the wiring, poor connections etc this is most easily seen in low peak output.

[Rumrace]

I burnt my Bimini canvas. Now mounted on stainless bands 40mm from damaged canvas. Each plugs into a separate port in the charger. I have 4 solar ports generator port alternator port and shore power port.
I have 24,12,5,3.5V in various spots on the boat and creating a 3D printed magnetic wall mount iPad charger.

[redneckrob]

Originally Posted by noelex 77

This is not correct.

Peak outputs greater than the STC rating are not impossible, or even very unusual providing you are in a location and season where the solar insolation is reasonable and the system is performing correctly.

This is the peak output today from one of my 335w panels. As you can see at 369w it is well above the STC rating, and we are above 55°N. These periods of exceptional power will only be brief, usually when the sun peaks out from a cloud and the sunlight strikes the panels both directly and from the cloud edge. Today was actually a poor solar day, but the cloud together with brief periods of sun and less regulation creates conditions where the peak output tends to be highest.

These high peaks are perhaps of little significance to the average output, but importantly they do show that the solar system is performing as it should do. If the panel is defective, incorrectly advertised, or there is excessive loss in the wiring, poor connections etc this is most easily seen in low peak output.

I am guessing that if you're at 55N the temp was below STC (25 C)? I think we agree that the OP's idea that one could get a significant percentage above nameplate at the equator for an amount of time to be significant isn't at all realistic.

[Rumrace]

It’s great to read everyone’s experiences and opinions. Thanks everyone.
I’m winging it. As a power boater with a generator last two power boats and running three fridges dual screen. No way the boat could do anything but start an engine in 24 hours on the hook.
Bought a sailboat which was just 5 years old but sat dead in winter storage for 2. All batteries were dead swollen junk. So were faucets shower bilge pump.
So with no real clue re sailboat consumption I just winged it. I bought 6 lithium 100amp batteries which peak at 200amp for yada yada. 2 are in front and separate and 4 are under a Quarterbirth house banks. 3 flexible solar panels and another I have not installed yet are 105watts each. We have two fridges two plotter screens winches so fare amount of consumption. Other than air we have not used the generator much. No matter how poor the sun is we manage bring one battery up. The charger seems to favour the front batteries if we anchor use the windlass or bow thruster.
I say dive in give it you best guess. Buy local so you can complain

[ItDepends]

Originally Posted by redneckrob

A panel well never produce appreciably more than it's rated power no matter the irradiance, for example a 250 watt panel will never produce more than 250 watts.

This is just NOT true. It is true in a theoretical world, but the real world is more complicated.

Panels are rated for a minimum power output, and the variance from the manufacturing process is quite large. It is not at all unusual to receive panels that produce 5% or even 15% above the rated capacity. You can not count on it, but it happens quite often.

Our pair of panels rated at 600W total routinely produces 650W, sometimes even 670W.

To answer one of the OP's questions, our 600W array routinely produces 3kW-hr per solar day. Max we have seen is about 3.8kW-hrs in a day.

[noelex 77]

Originally Posted by redneckrob

I am guessing that if you're at 55N the temp was below STC (25 C)? I think we agree that the OP's idea that one could get a significant percentage above nameplate at the equator for an amount of time to be significant isn't at all realistic.

The STC uses panel temperature rather than air temperature and this would have been above 25°C yesterday during the times of peak sunlight.

Temperature does play a role, but that is not the factor that produces the higher production than the STC rating in this case. My panels have a temperature coefficient of -0.29 % /°C so lowering the panel temperature to 15°C would only produce a modest 2.9% gain. Panel temperatures lower than this in conditions of high irradiance (1kW per m² or greater) are hard to achieve.

The biggest factor that produces a production over the STC wattage rating of the panel is irradiance. 1kW per m² has been chosen as the irradiance at which the panels are measured. For example when buying a 100w panel an output of 100w has been measured from the panel (or a representative sample of the panel )when exposed to sunlight with an irradiance of 1kW per m². 1kW per m² is bright, but not exceptionally bright, and irradiances well above this figure are not unusual, albeit often for short periods of time. Conditions where it is brighter than the test irradiance is the primary factor that can result in the panel producing more than the rating despite the inevitable (hopefully low) losses inherent in the overall solar panel system.

Your comment about hotter air temperatures is still an important point to make. Make sure your panels are well ventilated. However, at the equator sun angles will be much closer to 90°. At my current latitude (above 55°N) the peak sun angle is well below 90°. This decreases production for my flat panels significantly, nevertheless as shown from my post the peak output can comfortably exceed the panel rating on occasions. This post was just the peak recorded yesterday and the panels have done better than this example.

I do not agree with your contention that solar production will never exceed the wattage rating of the panel and that includes boats at or close to equator, however, the original post, by Dirk suggests double the rated output is possible and this is not remotely practical. The truth is between these claims.

Note that these peak outputs are often only briefly sustained and they require very good solar conditions. You will not reach high peaks every day. Also keep in mind that if the batteries are close to full the controller will throttle back the solar output. This is normal. The true peak output of a solar panel will often only be seen when a significant load is used. A good result from this test requires the panel to be performing as advertised and rest of the system (voltage drop in wiring, resistance at connections, ventilation of the panels, and the efficiency of the solar controller in tracking the MPP and implementing the voltage conversion etc) to be performing well. This is why the peak production is a very good indication of the health of your total solar panel system, and something you should keep track of.

If you have the popular Victron controllers this number is recorded automatically each day. There is some lag in the recorded peak number on the app, but on the other hand most controllers are calibrated slightly optimistically so these errors tend to even out.

[Dirk01]

Hi folks,
many thx for the insights. Let me put all my learnings together:
Question 1: It’s not the best idea to have panels in parallel. If one of the parallel strings is shaded, even partially, the voltage drops and the MPPT-Controller will find it’s MPP somewhere inbetween. That results in a significant power loss. To put two different panels in parallel is a bad idea, because you build a systematically failure into your system (which might be equalized accidently from time to time, but that’s unlikely and the opposite of best practise). Two strings --> two MPPT-Controller. That’s the road to go.
Question 2: The assumption of 2 kW/m² +/-15° is wrong. It is rather 1.1k to max. 1.4kW/m². The temperature de rating largely compensates for the increased irradiance. Not to mention the flat screen mounting (I don't tend to adjust it, even from time to time), which worsens the yield.
Question 3: Yes, from time to time there may be an output power above the nameplate. If I want to harvest what is offered, a power reserve for the MPPT controller is recommended. In particular, not to work at the performance limits of the controller. Rule of thumb: 25% reserve would fit best.
Question 4: On the long run „The 'rule of thumb' is 4-5 hours of nameplate production from your module each day. So, for a 100W module, 400-500Wh/day of production.“ However, a cloudy week will drain my batteries and I’ll have to start the genny for two hours. Having 7x the above mentioned rate @80%SOC I would dump the energy to the hot water system, the cold plate ice box, the washing machine, the dive compressor, the anything else. To have to much energy on a boat is unlikely, even a catamaran.

I learned A LOT and now have a much better feeling what to do.
It is so impressive – 24 hours and pains turned into gains.

Many thx to all of you.

Cheers
Dirk

[noelex 77]

Originally Posted by Dirk01

Question 1: It’s not the best idea to have panels in parallel. If one of the parallel strings is shaded, even partially, the voltage drops and the MPPT-Controller will find it’s MPP somewhere inbetween. That results in a significant power loss. To put two different panels in parallel is a bad idea, because you build a systematically failure into your system (which might be equalized accidently from time to time, but that’s unlikely and the opposite of best practise). Two strings --> two MPPT-Controller. That’s the road to go.

One MPPT controller per panel will result in a higher overall yield than connecting two panels to a single controller in parallel. However, the difference is not great unless the solar panels are subject to very different conditions. A single MPPT controller will still search for the best voltage for the combined output. This is generally close to the MPP of the most shaded panel rather than an average between the two maximum power-points.

Multiple smaller controllers are usually only slightly more more expensive than a larger single controller and there is the advantage of redundancy. This is the preferred approach, but if this is not suitable, a larger single controller is only slightly worse in most situations.

Originally Posted by Dirk01

Question 2: The assumption of 2 kW/m² +/-15° is wrong. It is rather 1.1k to max. 1.4kW/m². The temperature de rating largely compensates for the increased irradiance. Not to mention the flat screen mounting (I don't tend to adjust it, even from time to time), which worsens the yield.

Double the rated output is wrong.

Originally Posted by Dirk01

Hi folks,
Question 3: Yes, from time to time there may be an output power above the nameplate. If I want to harvest what is offered, a power reserve for the MPPT controller is recommended. In particular, not to work at the performance limits of the controller. Rule of thumb: 25% reserve would fit best.

The times there is a yield above the rated wattage are only brief, so you will lose very little total power if your controller can handle the rated power of the panel and no more. However, slightly oversizing the controller prevents relying on the controller’s protection mechanism to prevent damage. The controller will also run a little cooler, which probably helps reliability.

Originally Posted by Dirk01

Question 4: On the long run „The 'rule of thumb' is 4-5 hours of nameplate production from your module each day. So, for a 100W module, 400-500Wh/day of production.“

This is certainly possible depending on your location, time of year and how successful you have been installing the panels in a shade free location. However, as a general rule it is on the optimistic side and in some locations wildly optimistic. Solar power is very variable and what counts most if you want to rely solely, or mostly on solar power is that when cruising poorer locations or seasons there is still adequate power. A more conservative approach to the system requirements is needed unless you want to confine your cruising to areas and seasons with very high solar insolation.

[Shanachie]

Yield from the panels is the wrong way to think about the problem.You need to think about how much amperage your batteries will accept.


For example, I have four group 24 battieres in my bank due to height limitations where they are mounted, I also have two 160-watt rigid panels and an mppt controller.


After a night at anchor, the very good controller starts charging the batteries after first light here in Florida. By the time the sun is up, the bank is getting 14-17 amps an hour on a normally sunny day.


But that charge rate doesn't last because the batteries accept amps at a lower rate as they charge. By early afternoon, the bank is accepting a couple of amps an hour as they approach capacity.


It wouldn't matter if I had 1,000 watts of solar at that point. The bank will still only accept that couple of amps. The rest is wasted.


This is also true with those 120-amp alternators. After a while, the battery bank is only accepting a few amps. Then the alternator is just stealing power from your diesel.



The good mppt controllers -- I have Victron -- will produce decent power on a cloudy day. The sweet spot is when you have enough panels to fill up on a cloudy day.


For me, that's 320 amps.