Solar Panels for Marine Use

[Arthur Garfield]

As an aside,I spoke for quite awhile with huge solar panel wholesaler and he told Kyocera pretty much accepts returns without questions and he thought them best for cruisers.This company didn't even sell Kyocera! Though they are more spendy.

[socaldmax]

Originally Posted by tedsherrin

Yes, I believe it probably IS simple maths, but for something so simple I'm having a devil of a job getting simple confirmation of what this simple equation is. Is it the max amp size of the controller muliplied by the max volts? Or is it the max amp output of the panels muliplied by the number of panels?

Are you asking about ratings or actual output?

All of the MPPT controllers I have seen will list max. input voltage (Voc) and max. input wattage and will be rated at continuous amps of output. They will probably be able to exceed that rating by 10% or slightly more for short periods, but a 30 A controller shouldn't be expected to hold up forever at 33 A output.

Every controller has a different Voc, even if they're rated at the same current. I have one controller rated at 25 A, but it can't accept any higher than about 21 or 22v, which limits it to what people call "12v" panels, which generally put out about 18-20 Voc and around 16-18 Vmp.

So this particular controller could take 400w @ 18v (22.22 A) and will lose maybe .5 or 1 A in conversion, but put out about 25 A @ 14.8v. There are several things to consider. 400 w of solar panels will rarely, maybe never, put out full power. Maybe for the first 2 weeks when it's higher than rated, but after it settles down you might only see 380, 360, maybe 350 w out of the panels. So you might end up with 23 or 24 A of current out of the controller. If there is ANY shading at all, even a tiny amount, you'll see a much larger drop in solar output than most people would guess. Don't forget that unless you go with monster sized cables, there will be some loss between the panels and controller and controller to battery bank.

Another example is a 20 A controller I have. It will take 300 w of power for a 12v system, or 600w for a 24v system. It can take up to 42 V input, which allows the use of higher V, lower current panels, which exhibit less power loss in the cabling. Although it's rated at 20 A, I've seen it put out 22 - 22.5 A for short periods because there is 410 w of panels connected to it.

Keep in mind that if you want to run a 24 V, 36 V or 48 V battery bank, then you'll need to shop for a solar controller that can output those voltages and solar panels that are even higher, like 60 V. Another option is to connect in a series/parallel configuration like 4 x 40 V panels, 2 in series, 2 in parallel for a total of 80 V.


Now here is something to ponder. I generally tell people to size their solar array based on the amount of power they need.

For example, let's say someone says they need 50 A of power, I'd suggest a 75 A controller (so the controller isn't maxed out all of the time), with enough panels to supply about 55 A peak, and less off peak. That might be 800 w to 1,000 w of panels, depending on Vmp. With this setup, early in the morning you'd see very little power, mid morning you'd see 25 or 30 A, midday you'd see peak power, midafternoon maybe 25 or 30 A, evening almost nothing. These are just round numbers for illustration purposes.

However, in my case, I've attached 410 w of solar panels to a controller that is rated to handle only 300 w. What happens, compared to an identical controller attached to 300 w of panels is that the output rises faster, earlier in the day, peaks at 22 A midday vs 18 or 19 A, and continues to produce more power (closer to 20 A for longer) toward mid to late afternoon.

I would still gain more power if I attached the 410 w of panels to a 30 A controller, but I've come pretty close to what a 25 A controller would have produced. Since the 20 A controller was only $100 and the 25 A controller is about $210 delivered, I thought it was worth the risk of burning up 1 $100 controller just to see what it could do and if it would last. So far, so good.

For the record, the only way I could see these output numbers was to put an abnormally high load on the DC system. Normally these are sitting in storage, just keeping the golf cart batteries topped off, so they're normally just loafing along in float charge mode. But it's nice to see what they can do at full load.

[hellosailor]

socal-
I'm guessing that you are safe putting 410W of panels into a 300W controller, as long as your batteries/drain do not suck more than 300W through that controller. The controller "should" get toasted if anything downstream of it tries to suck more power, but conversely, if nothing pulls harder...it should never have to pass more than the 300W it is rated for. Just don't try to charge a big lithium battery, eh?

[mitiempo]

Originally Posted by HG02

[I].... I place a old seat cover over half the panel to represent mast boom shading to see what the voltage drop would be 14.4 was with out the old seat cushion cover thrown over the panel the 13.8 was with the cover on the panel it was an overcast day with rain

Voltage is not that relevant. You should measure the actual current going into the batteries with and without the shading, with the batteries in the same state of charge for each measurement. You can have a lot of current @ 13.8 volts and very little current @ 14.4 volts.

[HG02]

Originally Posted by mitiempo

Voltage is not that relevant. You should measure the actual current going into the batteries with and without the shading, with the batteries in the same state of charge for each measurement. You can have a lot of current @ 13.8 volts and very little current @ 14.4 volts.

I do not have time at the moment to get involved it this all Ive done is shown very quickly what Ive mounted and thats about it. its charging and running my batteries and keeping them topped up Ill get back this solar later Ive a lot of jobs to complete and all I wanted at this stage is keep my batteries healthy and topped up and its working

[bigpuff]

I believe the collapse in the price of solar panels in the last year or so was from Spain ordering 40,000 panels for solar farms, when the economy took a major hit so did the order, if fact thank you Spain, I wont go as far as thanking the bankers!

[socaldmax]

Originally Posted by hellosailor

socal-
I'm guessing that you are safe putting 410W of panels into a 300W controller, as long as your batteries/drain do not suck more than 300W through that controller. The controller "should" get toasted if anything downstream of it tries to suck more power, but conversely, if nothing pulls harder...it should never have to pass more than the 300W it is rated for. Just don't try to charge a big lithium battery, eh?

I wouldn't even go so far as to say I'm safe. It's just an experiment on an RV for now, and I would NEVER suggest anyone else do it, especially on a boat.

I was using an RV fridge in 110v AC mode, powered by a Xantrex inverter to create the large load. According to the inverter, it was drawing 29 - 30A and the solar controller was putting out about 22 A and the GC batteries were taking up the slack. I would never allow the controller to run at 110% while I wasn't looking directly at it, these things can get pretty hot at 100%, much less 110%.

Soon I'm going to remove those panels and add them to the other 14 panels for my house array (they all match) and buy a pair of used 155 - 165 w panels, that should be a good match for that controller, and plenty of power for that trailer.

[socaldmax]

Originally Posted by mitiempo

Voltage is not that relevant. You should measure the actual current going into the batteries with and without the shading, with the batteries in the same state of charge for each measurement. You can have a lot of current @ 13.8 volts and very little current @ 14.4 volts.

While current is the correct method to measure rate of charge, voltage is the "pressure" that makes the current flow. Charging at a lower voltage essentially leaves FLA batteries undercharged. Each of the different battery mfrs. have different recommended charging voltages, but I've seen them range from 14.8 V to 15.4 V. Without the voltage to drive the current, no current will flow. If the battery is resting at 13.5 V and the charger is at 13.5 V, you're at equilibrium and no current will flow.

A lot of people think of batteries like a bucket, you just pour electricity into it and when you need it, it just pours out. That's not a very good analogy. It's more like a balloon or air tank. You could have your charger set for 14.8 V and if it shuts off at that V, your batteries may only be 80 or 85% charged. They need to be held at that voltage for 3 or 4 hours (assuming your charger is properly sized for the bank) for enough current to flow into the batteries to not only overcome losses but to fully charge the batteries. Without the higher voltage, the batteries will never fully charge. You could have them connected to a charger at 13.8 V and they will sit there at 75 or 80% charge forever.

All "smart" chargers and almost all chargers are set from the factory with very conservative settings. They have no idea what brand of batteries you have, so they are set low enough to not fry the lowest charging V batteries on the market, and if your brand requires a much higher charging V, then you just never reach full charge.

[mitiempo]

Originally Posted by socaldmax

You could have them connected to a charger at 13.8 V and they will sit there at 75 or 80% charge forever.

I am fully aware that voltage equals pressure but there is some pressure as long as the charging voltage is higher than the terminal voltage of a fully charged battery. A fully charged battery measures 12.6 volts or higher.

They would charge eventually. Alternators were set at 13.8 in years past - about 20 years or so ago - and the batteries did fully charge eventually, just took a bit longer.

My point was that voltage alone is not necessarily a true indication of current flowing.

[Rustic Charm]

I know some if you are electrical guru's and have degrees in batteries, power and ecky thump, and some of you have worked for years in the science of power, solar etc, and I'm more than conscious that all I know about power is that it bites if it don't like the way your stroking it, but honestly I think some of you get caught up in the size of the appendage rather than the simple question of how to put it in. All this talk of pressure, temperature and convoluted ways etc.

For me, I asked my local battery guy and to work out the size controller I needed was simply by adding up the max voltage on the panels, in my case with two panels then 15amps so 20amos is plenty.

[socaldmax]

Originally Posted by tedsherrin

I know some if you are electrical guru's and have degrees in batteries, power and ecky thump, and some of you have worked for years in the science of power, solar etc, and I'm more than conscious that all I know about power is that it bites if it don't like the way your stroking it, but honestly I think some of you get caught up in the size of the appendage rather than the simple question of how to put it in. All this talk of pressure, temperature and convoluted ways etc.

For me, I asked my local battery guy and to work out the size controller I needed was simply by adding up the max voltage on the panels, in my case with two panels then 15amps so 20amos is plenty.

As I posted in a direct response to your question, it's not that simple.

Your solar panel voltage cannot exceed your controller's input voltage. There are plenty of solar controllers that can only take 21 or 22V input, and plenty of solar panels that put out 37, 40, even 60 V. Those won't work with a controller that will only take 22 V.

260 w of panels is a good match for a 20 A controller, it would max out at about 15 A.