Over Rated Power on Solar Controllers

[Captain Bill]

I recently listened to a webinar by MorningStar that recommended adding solar panels to an array so that the peak power output of the panels is greater than the rated capacity of the MPPT solar controller. They pointed out that the rated power of their controllers and most others is limited by the output side of the controller and if the input side exceeds this rating the controller will throttle back the input side and continue to put full rated power out on the output side. The input side is limited by voltage so one should not exceed the voltage rating of the controller on the input side.


The advantages are that you get to full rated power much earlier in the day and your daily average power gain is higher. You also get much more power on cloudy days simply because you have more panels.

All-in-all an interesting discussion.

[Rdubs]

Thanks for the info Bill. I think the same process applies a little differently as well - that the controllers reduce the power coming in also helps to prevent your batteries from being over-charged. Such as when ones boat is on a mooring ball and the electrical demands are near zero, if the controller was forcing a charge into the batteries they would be fried (probably after catching your boat on fire to boot).

[Dsanduril]

Bill, absolutely standard in the industrial solar plants I used to build. Our 'silicon ratio' was anything from 110% to 130% of the output capacity of the inverters (grid-tied plants). The inverters just use the MPPT control to drive the panel voltage high and send the solar power over the knee of the production curve when the solar power available exceeds the output capacity. The same is true of MPPT controllers on boats.

But, it's really a matter of economics. Yes, you'll get to the maximum output power of your MPPT controller sooner, but you'll also waste power in the middle of the day (assuming batteries could accept it) by being limited by the controllers. It comes down to a cost analysis of the extra silicon vs. the cost of MPPT controllers. I'm in the camp that most boat installations would benefit from an installation of one panel/one controller, and controller costs, while definitely a factor, are not so great that it would pay to have extra silicon that can't be used in the middle of the day.

As always, YMMV, it all comes down to the cost of MPPT controllers and the cost of 'lost' production.

[StuM]

That's been discussed at length in previous threads, including graphs of energy curves showing that you can gain more at lower insolation than you throw away at peak insolation. Meaning yes, that works.



However, if you put in a bigger controller, you still gain at the lower insolation without having to throw away anything


Added: As Dsanduril says, it's a trade of between lost production and cost of controller(s),

[Dsanduril]

In the power plant world the cost of land (amazingly) and silicon is sufficiently low compared with inverter costs that we're willing to 'clip' in the middle of day. There is also some benefit in grid-tied applications to being able to deliver your plant's rated power from 9-3 instead of 11-1, especially in markets where you are bidding for production slots.

On a boat the 'land' cost for solar installation is usually quite high (there just isn't much space available, and we keep adding power-hungry devices), wasting any production by under-installing conversion (MPPT) hardware may not make as much sense.

[Captain Bill]

In my case, between March and September I really can't use as much energy as I produce on sunny days and even on mostly cloudy days. Between September and March however because of low sun angle and short days I have a pretty hard time producing as much power as I need, especially on cloudy days. I certainly am not challenging the output capabilities of my solar controller. An extra panel to get me through those short winter days would be quite useful and they are relatively cheap and I wouldn't need another controller.

[billknny]

I am seriously missing something...

wouldn't you get more power out if you had an MPPT controller that passed through all that was made instead of limiting the panels? Other than reducing capital cost, what is the advantage of undersizing the MPPT controller?

From what I see the differences in cost as you go up in size make undersizing the MPPT unit a serious case of false economy.

[FlyingScot]

Solar controllers are cheap compared to space on a boat for solar panels. This makes no sense unless you want to do things sub-optimally. Buy underrating the controllers you don't gain more power. You clip excess power and never have the chance to use this. Making more power than needed is always good. It means you can do more things with electricity. Or you can handle cloudy days, etc.

I will also add that running your equipment at 100% tends to make them wear out sooner.

[Dsanduril]

Bill's use case is actually a perfect one for a silicon ratio > 1 on a boat. During the summer he has more power than he needs/can use. But during the winter he needs a little extra boost. None of his panels will be at full output then, so adding a panel to an existing controller is cheaper than adding both a panel and a controller.

IME it's pretty rare on a boat to have more power than you can use, but it does happen.

[sailorboy1]

I think the logic is twisted. Install the solar you can and size the controller based on that. You are going to always get the maximum power out of the system that it can provide and/or the batteries can accept at all time.

The only plus to the "logic" of exceeding the controller rating is if you are trying to talk yourself out of buying one.

[Dsanduril]

Originally Posted by sailorboy1

... and/or the batteries can accept at all time....

That's exactly it. In Bill's case his batteries are the limiting factor during the summer - they can't accept all of the power he can produce. In the winter his panels are the limiting factor - there isn't enough surface area to collect the lower insolation quantities. In no case is the controller a limiting factor.

So why spend the money on more controller? More silicon helps in the winter, and essentially sits idle in the summer whereas a new controller would sit idle 100% of the time.

[Redline452]

Interesting approach... when I was planning my installation, I was told the controller should be rated with a safety factor of (30%?) above the capacity of the panels, to insure it could safely handle worst-case (maximum) solar output.



I felt at the time that the margin was a bit high, but it wasn't as if the controllers were available in many small increments of capacity anyway. But I did note that the "buy big" recommendation was coming from the vendor of the the equipment in question, so there just might have been a bit of, um, upsell bias?


That said, I do appreciate my BlueSky SB3024IL MPPT controller, and the extra capacity was useful when I added a third 130W panel, so there.


On the other hand, I recently sailed on a friend's boat with three flexible panels on two Chinese-made (don't recall the brand) MPPT controllers. Two panels feed one, a single panel feeds the other, and only the first controller is connected to the display/monitor. There is no provision for daisy-chaining or parallelling the controller-display connection. He was wondering why his solar system appeared to be putting out less than expected, and the answer was finally that the display is only connected to 2/3 of his solar capacity. A bigger controller would have avoided that.

[Moontide]

I agree with the oversizing the solar. I had 720 watts of solar and an MPPT controller with max output of 60 amps. The panels routinely put out over 740 watts under very good conditions, but that doesn't happen often. (I don't do much adjusting to the tile so they are not positioned for max efficiency most of the time.)


I'm adding another 260 panel. That means during very good conditions I will be throwing away some power, but will have 36% more power for the rest of the time. Only have 6 golf cart batteries so really couldn't use higher charge rates (max charge rate of 10% of capacity) so in my case I'm much better off with extra wattage to increase the number of hours I'll be putting out 60 amps than a getting a larger controller that would either have to be throttled back or would damage the batteries with too high of a charge rate.



I downloaded the controller output and then played with it in a spreadsheet. Because you seldom see full rated output from the panels it turns out I will be throwing away juice less than 5% of the time, and will increase charging by 35% the rest of the time. That was this month so in the summer I'll be losing more, but also putting out 60 amps for more hours a day. Will easily net over 300 AH a day. If I have any unusally heavy usage during the day might even see 400 AH.


I guess the bottom line, as usual, is that the answer to a question depends on the exact circumstances. No single solution for all. This one works for my setup, and maybe for others as well.


Bill