Matching charge controller to solar panels

[Kelpie]

I'm looking into installing a pretty large solar setup on the boat. Domestic sized panels of around 280w are now getting incredibly cheap, so I am thinking of using two of them as part of a bimini design.
At 12v, this should work out at 47A (P=IV)

Charge controllers seem to come in 40A and 60A flavours, with the larger ones obviously costing a fair bit more.
Would it be risky to use a 40A controller for this much solar power? I'm just wondering if the working voltage will actually be higher, therefore the current lower, than I have calculated based on the simple 12v assumption. There's also the thought that getting 100% power out of both panels at the same time seems quite unlikely. But I don't want to fry a controller...

 

[RupertW]

I'm looking into installing a pretty large solar setup on the boat. Domestic sized panels of around 280w are now getting incredibly cheap, so I am thinking of using two of them as part of a bimini design.
At 12v, this should work out at 47A (P=IV)

Charge controllers seem to come in 40A and 60A flavours, with the larger ones obviously costing a fair bit more.
Would it be risky to use a 40A controller for this much solar power? I'm just wondering if the working voltage will actually be higher, therefore the current lower, than I have calculated based on the simple 12v assumption. There's also the thought that getting 100% power out of both panels at the same time seems quite unlikely. But I don't want to fry a controller...

Victron 100/50 would work well but wouldn’t leave much expansion if you added more panels. Having said that, a lot of poeple prefer an extra controller when adding a extra set of panels.

 

[lw395]

280W is 20A at a charging voltage of 14V, less than that at 14.4V or whatever absorption voltage you pick.
But running any electronics at close to 100% tends to reduce reliability.
You have to weigh up the extra cost of buying a bigger unit.
How often will the charging regime and other loads actually draw full current anyway?

I would hope that the Victron units protect themselves from short term damage from excess current in any case?

 

[William_H]

I am inclined to agree with OP that it is not likely that the full capabilities of the system will appear as charge current to the batteries (plus system load). I also understand that the domestic panels will give a huge capability for not much money. So excelent if you can mount them. Yes it is not good practice to run close to the claimed capability of the controller however one might imagine that the controller would be self current limiting anyway. Much then depends on the size of the battery bank. (or if you intend to run a large inverter when sun is shining). So probably Ok with the small controller but still a bit of a gamble. Or OP might set up a regime of disconnecting one panel if the sun is overhead and bright but still being able to extract a lot of power when over cast or low on the horizon. ol'will

 

[Laminar Flow]

A couple of years ago we installed 300w of solar panels on our wheel house roof. This was done by using 6 x 50w individual domestic panels. This type is also commonly used for navigational aids and cheap as chips. We could have also chosen two at 150w; the reason we did not do so was to reduce output loss by shading - a very likely scenario on a sailboat. Solar panels are normally fitted with with separation diodes, thus creating at least two separate fields per panel. Consequently, if one half is shaded the other is still productive. The more divisions the better.

In theory our 300w should produce a peak charge of 21A. In reality we have never seen that. Our MPPT charger can accept a max of 45A' leaving room for expansion. The cost for the larger controller was minimal.

In three years of cruising (three to four months at a time) and right into the fall in Northern Europe, we have not once had to run the engine to charge the batteries. Our house batteries have a total of 460A. On a sunny day they are fully recharged by 10am, on a cloudy day by 2pm. Our fridge runs 24/7, we have pressure water, charge devices, use lights when we want to, auto pilot is electronic.
Best A.

 

[Baggywrinkle]

Have a look here ....

Solar Charge Controllers - Victron Energy

... the link to the Excel sheet is better as it allows you to enter the parameters of the solar panels you actually have rather than standard Victron panels.

Use our MPPT Excel sheet or our Online MPPT Calculator for PV sizing calculations.

It will give you information across all panel temperature ranges, start-up voltages for MPPT controllers and allow parallel/series connection of panels too.

 

[GHA]

At 12v, this should work out at 47A (P=IV)

Oh no it won't...
Better rule of thumb is divide by 20 and add a bit, 100w panel will usually not go beyond about 6A on a perfect no shadow day. So more like 17a max per each of your panels. Could do a lot worse than a victron blue solar 100/20 on each panel.

 

[noelex]

Nearly all MPPT controllers will limit the current to protect themselves. So you will not blow the controller if you connect it to solar panels that can deliver more than the current rating (but check there are some models that do not have this protection mechanism). However, this does mean you are throwing away power and this is not sensible on a boat where solar panel area is limited. The other concern is that driving the controller at full power potentially reduces its reliability . Many run reasonably hot, especially the popular Victron models that do not have any active cooling. However, a 40A controller is just acceptable for 560w so if it is significantly cheaper it is worth some consideration.

MPPT controllers are limited by the output current. It does not help that the input current may be much lower.

The other thing to be aware of is that this self protection mechanism does does not apply to voltage. Exceed the voltage rating of the controller even slightly and it will usually instantly be destroyed.


One option to consider is to install multiple smaller solar controllers. Two in your case, one for each panel. This adds valuable redundancy and will extract slightly more power from the system. Two 30A controllers are sometimes cheaper than one 60A controller but even if it is slightly more expensive the advantages can make this the sensible option.

Large domestic solar panels require careful and knowledgeable installation. The voltages are often above the potentially lethal level and there are some traps. For example, some marine electrical components such as fuse holders etc are only rated for a maximum of 32v and are not OK for high voltage solar panels that may be 60v+.

 

[Kelpie]

Thanks for the info, very useful.
I'm looking at this MPPT controller: EV-Power | MPPT Solar Charger 12/24 V, 40A, Input 100V, LCD UI (XTRA4210N)
To go with these panels: EV-Power | Solar panel GWL/Sunny Poly 280Wp 60 cells, MPPT 32V (ESP280)

Astonishingly cheap compared to just a few years ago.