How much solar panel power is enough?

[noelex 77]

Originally Posted by s/v Jedi

Mark aboard Reach has tried this and reports 25% increase in avg daily production when in series with a string on port and a string on starboard side.

This contradicts Mainsails experience.

Was the 25% gain due to separating the port and starboard arrays (this will produce more power in series or parallel) or were they always separated and the gain due to a switch from parallel to series?

Did the controller change?

Some details would help separate these conflicting claims.

[hellosailor]

Salty-
Bearing in mind that Nick is showing about 50% power on whatever you define as a cloudy or overcast day, and that you've got about 150% of the power you need in full sunlight, your proposed installation would still give you 75% replenishment on a cloudy day, so if you hit two cloudy days in a row, you're still not going to really hurt your batteries. Actually with an MPPT controller, that 150% is more like 160%, so you're going to be replenishing 80% on cloudy days, it will still take 2-3 cloudy days in row to bring you down to a 50% charge state.

Instead of bringing your solar array up to the necessary 200% to compensate for the cloudy days, you might consider adding a wind generator, since cloudy wx often means good wind and that supplements the weak point of the solar array.

Of course the downside is all the downside to windgens, including the separate dump controller for them, but unless you've got room to keep a BIG solar array unshaded and protected...that might be worth taking a good look at.

And then if budget is not an issue, you might take a look at www.solarstik.com where they have a mounting array for solar panels that can go outboard (i.e. on the transom) of the boat, is extremely rugged, and optimizes panel output by allowing for 3-axis rotation. Not cheap, but less than what a local stainless shop would charge you to fab up the same thing. Keeping the panels angled towards the sun from time to time, instead of fixed, can boost their output 10% for every 15 degree (one hour) shift of the sun.

[teneicm]

I'm thinking that you should be OK, especially if your 24 hour demand is only 125 amp hours. Here's our real world experience:

820 Watts of panels (4 x 205)
Outback 80 Controller
880 Amp Hour Battery Bank
Our overnight draw down is roughly 100 amps, so I'm guessing our daily usage is about 200 amp hours.
Most sunny days we're 100% charged by early afternoon. The panels will produce almost 60 amps from 11-2 when skies are clear.
Right now (11:00 am) it's overcast in Ft Lauderdale and even when the sun goes behind a cloud, the panels are still putting out 20 amps.

We recently had 2 fully overcast, very dark days with constant rain and thunderstorms. By the third day we were about 300 amp hours down (according to the LinkLite monitor). This was totally eliminated on the third day when we had a pretty sunny (but NOT completely cloudless) day. I figure we could go for about 3 very dark days before running into problems.

[s/v Jedi]

Originally Posted by noelex 77

The photo was taken under uniform low light.

Performance under shade is a separate issue

Here is another photo showing very low production (because of low light) with a Vmp of 15.0v still well above battery voltage.

With parallel setup of the panels, the output voltage you see is that of the panel that gives the highest output voltage. In your examples, that is the 15V or 18V as shown. You don't know the output voltage of the other panels, which might well be too low to be used. I can't believe you have no shading issues with the sun at very low angles, all boats have that when the sun is setting or rising.

With panels in series the voltage on the controller is an indication of how the panels perform because each panel output voltage is added to the total.

[s/v Jedi]

Originally Posted by teneicm

I'm thinking that you should be OK, especially if your 24 hour demand is only 125 amp hours. Here's our real world experience:

820 Watts of panels (4 x 205)
Outback 80 Controller
880 Amp Hour Battery Bank
Our overnight draw down is roughly 100 amps, so I'm guessing our daily usage is about 200 amp hours.
Most sunny days we're 100% charged by early afternoon. The panels will produce almost 60 amps from 11-2 when skies are clear.
Right now (11:00 am) it's overcast in Ft Lauderdale and even when the sun goes behind a cloud, the panels are still putting out 20 amps.

We recently had 2 fully overcast, very dark days with constant rain and thunderstorms. By the third day we were about 300 amp hours down (according to the LinkLite monitor). This was totally eliminated on the third day when we had a pretty sunny (but NOT completely cloudless) day. I figure we could go for about 3 very dark days before running into problems.

Check the statistics on the controller. Number in kWh mean much more than how long it takes to recharge your batteries. For example: you use power while charging the batteries, which means that not all the power generated goes into the batteries. The controller tells you exactly how much was generated, with a 128-day history in it's memory

[s/v Jedi]

Originally Posted by noelex 77

This contradicts Mainsails experience.

Was the 25% gain due to separating the port and starboard arrays (this will produce more power in series or parallel) or were they always separated and the gain due to a switch from parallel to series?

Did the controller change?

Some details would help separate these conflicting claims.

It's all been posted here several times... you can try to find it but I seem to always forget people's name on the forum here...

You will always see conflicting claims as there are too many variables. The only way is to compare output numbers with boats in the same area, like number of kWh divided by total array wattage gives a number to compare. When you're lower than others, you know you can improve it.
You need to find the Engineer kind of cruisers for this because others just don't care, plus you have to be one yourself because it takes time and effort to get it right.

[teneicm]

Originally Posted by s/v Jedi

Check the statistics on the controller. Number in kWh mean much more than how long it takes to recharge your batteries. For example: you use power while charging the batteries, which means that not all the power generated goes into the batteries. The controller tells you exactly how much was generated, with a 128-day history in it's memory

Yes- but the number of kWH or amp hours produced as reported by the controller is what was actually produced- not what could have been produced. Meaning, on most days, my controller throttles back the panels and goes into float mode by 2:00 pm. So that means there are 4 hours of light left where the panels are basically idling. The best measure of the panels potential are when the batteries are close to 50% discharged and the controller is pumping in as many amps as possible during the bulk charging phase. On days like that we'll see 325+ amp hours of production. On a typical day, we'll see maybe only 200 amp hours.

[roetter]

Originally Posted by noelex 77

The photo was taken under uniform low light.

Performance under shade is a separate issue

Here is another photo showing very low production (because of low light) with a Vmp of 15.0v still well above battery voltage.

The voltage shown is the open circuit voltage. As soon as the controller kicks in the Voc will collapse to way under 12 V and it goes back to snoozing. In the same low light with 3 panels in series the open circuit voltage would probably hover around 60V and with the controller kicking in it might still stay high enough to give you a little output.

[roetter]

My set-up:
840Ah LA bats - 5 Kyocera KD190 (950W) in series (Voc 150V) - Outback Flexmax 80

After a while of observing the charging behaviour, I programmed the Outback to 14.6V Absorption and 14.6V float. I rarely got the batteries full, except for high summer days. Our use was over 350 Ah/day. However, I wanted to get all I could into the batteries, and since the power usage during that day fluctuates the controller could get confused and stay in float for a long time not realizing I had a period of heavier use. With the voltage staying at 14.6 the batteries could absorb more amps and occasionally got a little bit of an equalization. I would not leave it at this setting if I am not on the boat.

I checked the battery water periodically and never had to top up.

[noelex 77]

Originally Posted by s/v Jedi

With parallel setup of the panels, the output voltage you see is that of the panel that gives the highest output voltage. In your examples, that is the 15V or 18V as shown.

This is not true of the 15v.
The controllers are reporting Vmp when the panels are generating power.

The voltage you see on the controller is the Vmp of the array.
The voltage is the best voltage, not the highest Vmp of a panel in the array.

(The 18v was an unusual case of the controler reporting Voc because no power was being produced)

[noelex 77]

Originally Posted by roetter

The voltage shown is the open circuit voltage. As soon as the controller kicks in the Voc will collapse to way under 12 V and it goes back to snoozing. In the same low light with 3 panels in series the open circuit voltage would probably hover around 60V and with the controller kicking in it might still stay high enough to give you a little output.

The second photo shows the Vmp remains high until the output current is getting very small.
If you look at the I/V curves of solar panels under low illumination it shows, in theory, the same characteristic I am showing with the practical example.

[noelex 77]

Originally Posted by roetter

My set-up:
840Ah LA bats - 5 Kyocera KD190 (950W) in series (Voc 150V) - Outback Flexmax 80

After a while of observing the charging behaviour, I programmed the Outback to 14.6V Absorption and 14.6V float. I rarely got the batteries full, except for high summer days. Our use was over 350 Ah/day. However, I wanted to get all I could into the batteries, and since the power usage during that day fluctuates the controller could get confused and stay in float for a long time

You would be better to adjust the rebulk voltage.
With an appropriate setting it cause the controller to initiate another bulk phase if the batteries become discharged.
The default setting on the Outback is very low (6v) which effectively disables this feature.

[hellosailor]

14.6 ?? When or why would a "12" volt bank need that much voltage? With an MPPT controller, your advantage lies in using just enough voltage--and letting the controller put the rest of the power into amperage, which is what the batteries are craving. Setting too high a voltage will just boil off electrolyte, and ensure the batteries cannot get enough amperage to recharge completely. Since it is "just" programming, that should be simple enough to experiment with.



Too many variables? Only in the subjective terms like "cloudy". How dark is dark?

But there are plenty of objective sources for data, like Sandia National Labs in the US. They have performance charts, based on repeated, standardized, quantified, objective results from how panels perform at various temperatures, in various latitudes, on every specific day of the year to accomodate the changing position (elevation and distance) of the sun as well.

The amount of power that you can and will get from a solar panel, wherever you are, versus the nominal rated power--assuming the manufacturer has followed the standards for that rating, which can also be confirmed--are out there. It shouldn't take more than 15-30 minutes to turn them up online.

Whether one controller is 5% more efficient than another, or how efficiently a battery bank charges based on chemistry and state of charge, sure, these are variables. But not terribly significant ones in the bigger picture.

The numbers *are* out there. Firm numbers, reliable numbers, numbers recorded and verified by multiple independent sources and the funny thing is, they all pretty much add up the same way.

Not that "paper" is any better than results from installations, but the folks who have published are often simply a more objective source for comparisons, i.e. their panels are unshaded and no seagulls allowed to land on them.

[Bash]

Originally Posted by SaltyTanned

Thanks Nick,

Then 3 panels will not be enough on cloudy days.

It is a problem of space to mount them of course. 3 panels cover the bimini exactly. So I will need to add an arch and install 5 panels for the electrical independance I dream of.

I disagree. If those three panels are not enough to achieve energy independence, then what you really need to do is invest in getting your consumption down. Easy to do with such things as LED lightbulbs. And this will require much less investment than building an arch and acquiring two more panels.

Honestly, 750 watts is a lot of power. And you've got a huge battery bank by most cruisers' standards. You'll be able to survive back-to-back cloudy days in the Caribbean without any problem. And, if once per year the batteries need a bit of a boost, you can always run the engine for an hour or two.

[s/v Jedi]

Originally Posted by noelex 77

This is not true of the 15v.
The controllers are reporting Vmp when the panels are generating power.

The voltage you see on the controller is the Vmp of the array.
The voltage is the best voltage, not the highest Vmp of a panel in the array.

(The 18v was an unusual case of the controler reporting Voc because no power was being produced)

Yes you're right, I'm trying to remember too many things

But the 18V is and you can do better with series connection; the question is how much and in which config. You just can't tell without testing or at least comparing with others in the same area that have parallel and series setups

I have six panels and they are mounted in such a way that "all parallel", "1 string of 6" and "3 parallel strings of 2" make sense and need to be compared. I wish I had the time for that, too busy with other projects at the moment, but it is on the list

It also matters where you are. In the Caribbean, most of the time you're anchored and have NNE winds. This presents starboard panels to the rising sun while shading port panels. The other way around at sundown. Now think about shading, multiple strings of panels and what will happen; it becomes very complex very quick