Solar 24v vs 12v system

[Talljohn]

I know there is lots of info out there, but I can't find a definitive guide on the best solution. People seem to have very complex systems with all sorts of extra inverters, fuses, breakers and shunts etc. Yet there are also quite simple systems. I am thinking of the latter. I have two 95ah AGM batteries and a long roof on a Possl Roadcar. I think something in the region of 300 to 360 watts would be good to sustain us for some off grid travel, and I can achieve this with one or two panels, matched to 6mm cable going direct to the batteries rather than my EBL119. If I added a battery master or equivalent or used a Votronic duo controller, that ensures leisure and vehicle batteries are covered. The only other question is therefore would I have 24v or 12v panels. Does it need to be more complicated than that or am I missing something and other than cable sizing does the voltage question make much of a difference? Thanks.

 

[royw]

The panels are at a higher voltage, its the charge controller that regulates it down to a 12 or 24 volt system,

 

[paddymcc]

As Watts =Amps X Volts having a higher voltage will lower the current. This might be relevant if you were using cable that could only cope with a low current. If you are putting new cable in it's probably not relevant - just make sure you size it correctly
With my EBL 101, by running the output from the Votronic through the EBL to the battery you can see the current on the display. Don't know if that's true for your setup but it is reassuring to see that the solar is working.

 

[autorouter]

Some solar charge controllers are intended for '12V' panels, and have a maximum input voltage of about 35V. Especially the smaller, cheaper PWM controllers. But many controllers are designed for either '12V' or '24V' panels, and can handle both quite easily.

The input voltage limit is a hard limit, not to be exceeded. A '24V' panel can easily exceed 35V, especially on a cold, bright day.

A popular choice is the Victron 75/15 which can handle up to 75V. The Votronic Duo models can charge both leisure and starter batteries, and are good if you don't have a BatteryMaster fitted.

 

[Sundowners]

Apart from the cable size issue---- I always favor a higher voltage system as I understand that in lower light you may get a voltage too low to charge --eg a "12" volt system could easily produce too low a voltage whereas a similar "24" volt system would still give enough volts to charge---- maybe I am wrong???
I believe most MPPT controllers will accept higher voltages-- from memory ours is 150v so we connected our array to work within this range---8 panels in pairs

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[Talljohn]

Yes, the low light benefits of 24v and the higher ampage achieved post controller suggests more charge bang for your bucks with 24v assuming you have a controller which accepts the higher voltage, which most decent ones do. And that is part of my query, I.e. why would people chose 12v over 24v given the advantages. Am I missing something?

 

[autorouter]

Yes, the low light benefits of 24v and the higher ampage achieved post controller suggests more charge bang for your bucks with 24v assuming you have a controller which accepts the higher voltage, which most decent ones do. And that is part of my query, I.e. why would people chose 12v over 24v given the advantages. Am I missing something?

It's just progress. Not long ago, few houses had solar arrays on the roof, and a 100W panel on a motorhome was as much as anyone thought they needed. Small solar controllers were developed, for small installations like road signs, security lights and cameras, remote-controlled barriers etc that were a pain to supply from the mains. These controllers were ideal for small motorhome installations.

Nowadays many houses and commercial premises have solar arrays, and many have battery power for storage/off-grid. Solar panels and associated electrics (controllers, inverters, chargers) have become cheaper and more widely available. Motorhomers have adopted these too.

 

[jongood]

I have a MH with 12v and 24v on board and Ive recently replaced the elderly existing PV set up. I did briefly consider going higher voltage but there is so much more choice of MH specific kit at 12 volt. Ive now got 600watts of panels going through 2 x Votronic 350 MPPT regulators and associated gubbins. All I need now is to go somewhere sunny to see it work properly.

 

[Raul]

Yes, the low light benefits of 24v and the higher ampage achieved post controller suggests more charge bang for your bucks with 24v assuming you have a controller which accepts the higher voltage, which most decent ones do. And that is part of my query, I.e. why would people chose 12v over 24v given the advantages. Am I missing something?

When you have a pwm controler, a higher voltage will not help you at all, hence the 12v panels are more suitable for that type of controler. Also where space is a issue, sometimes two 12v panels will fit, but not a 24v. The two 12’s can be wired as one 24.

 

[autorouter]

I think something in the region of 300 to 360 watts would be good to sustain us for some off grid travel, and I can achieve this with one or two panels, matched to 6mm cable going direct to the batteries rather than my EBL119.

The only disadvantage of going direct to the batteries is that the amps-measuring function of the EBL is bypassed, so the displayed battery state of charge will be wrong. Once you know that, it's no big deal, and the EBL can't handle anything over about 200W anyway.

If you want to know the battery state of charge, that's why a shunt is fitted, usually on the battery negative terminal. Alternatively a voltage reading gives an approximation to the state of charge, and many people are happy with that.

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[Talljohn]

Thanks for the advice. I think I may go down the route of a 24v Victron panel and same brand controller to access the blue tooth data. Space on the roof isn't an issue, so I can go big with one panel. I am just making up some brackets for the roof to secure aluminium extruded section for mounting so I can be totally flexible in future. Not sure I trust the glue option, though I know lots of people have been happy with it.

 

[Sundowners]

I have used a sikaflex type of "glue" and been very pleased with it---- every time I wash the panels off i have a good look and tug at fixing points----- the only problem comes in if you need to remove them-------I always clean the areas of contact carefully and what never seems to be mentioned is to work quickly once the "glue" is out of the tube---- much better to complete one panel before working on the next‐------- sikaflex forms a skin very quickly and I feel that unless you burst that skin the adhesions will not be totally successful.
I have seen a skylight "glued" with it and a TOTAL FAILURE!!!!

 

[Sundowners]

If you can fit more on dont be afraid to have more than you think you will need!!!
It is very easy to find ways of using power and in dull weather you have a much better chance of keeping the batteries happy---- obviously ensure that your mppt controller is capable of accepting the potential power---------- we fitted 2200w on our cabin so that we have power for air conditioning in summer---- this is the systems first winter and through several very dismal days we still had sufficient power------ the panels are the cheap part of the system!!!!

 

[Talljohn]

Looking at panels, the domestic panels from Panasonic etc. appear to be much better value with better guarantee longer warranty etc. Assuming your controller can carry the voltage, is there any reason they are not preferable?

 

[Raul]

I have two Panasonic’s on my van roof. I keep trying to tell folk that big grid tie panels are the way to go. One more advantage on panasonic: heterojunction staked cells HIT, no other panel has that.

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[mitzimad]

is it just me but ive not seen panels designated as 12 or 24v?

 

[Sundowners]

Must admit I have never seen 12 or 24v??
But most people seem to think house type panels are too powerful????
Others seem to be leisure panels------ from memory?? Ours are max 36volt?? Our
MPPT has max 150v from memory---- the reason I connected them in pairs.

 

[Raul]

A 36cell panel is a 12v nominal (18-22v actual), a 72 cell panel is 24v nominal ( 34-44v actual). A grid tie 60 cell panel is 30-38v. The voltages differ slightly from manufacturer, type, technology etc. Each solar cell produces approximate 1,5v each.
In a 36cell 12v panels there are 18 cells in series times two. They are paralleled via shotky bypass diodes, so one series can pass current when one is shaded.
In a 72 cell panel you got three series of 24cells in parallel, via diodes.
In a 60 cell panel ( most popular and versatile for different setups), you got three series of 20 cell in parallel via diodes. Each series of cells are joined by bypass diodes to cumulate all current in a common point by the box.

There are other panels Panasonic 96cells. And sun power. These are the only ones that make 5” cells instead conventional 6”, hence more cells can be fitted per equal space. These 96cells Panasonic have something special. Their cells are 3 layered bonded. There is a silica crystalline layer, amorphous layer and silica again. The bond in between this is called heterojunction, or HIT. What’s so special about them? They work better in various light conditions. The crystalline cells are tuned for a certain spectrum of light, and by staking with amorphous just increases that spectrum wider band. In a nut shell are better in low light, low temp derating, low degradation etc.
As a note, I had many times exceeded the peak power of the panels in right conditions. Out of two 295w Panasonic, I have seen as much as 660w sustained, and peaking at 680w.

Another good one, LG neon R, have a supper duper panel to, at 370w per 1,6m2 is the most efficient. Also these run the buss bars of the cells at the back, allowing more light on the cell.

 

[jezport]

I know there is lots of info out there, but I can't find a definitive guide on the best solution. People seem to have very complex systems with all sorts of extra inverters, fuses, breakers and shunts etc. Yet there are also quite simple systems. I am thinking of the latter. I have two 95ah AGM batteries and a long roof on a Possl Roadcar. I think something in the region of 300 to 360 watts would be good to sustain us for some off grid travel, and I can achieve this with one or two panels, matched to 6mm cable going direct to the batteries rather than my EBL119. If I added a battery master or equivalent or used a Votronic duo controller, that ensures leisure and vehicle batteries are covered. The only other question is therefore would I have 24v or 12v panels. Does it need to be more complicated than that or am I missing something and other than cable sizing does the voltage question make much of a difference? Thanks.

I Have a Campscout Revolution. With a similar setup as you are suggesting. I have 2x 95ah AGM a votronig duo MPPT regulator and 140w of solar. It works well for me and dont need EHU unless Im away on a cloudy winter trip needing prolonged heating.

 

[Talljohn]

Good to here you consider 140w is enough, a lot of people appear to want a lot more, but with only a fridge, shower pump, lights etc. and only relatively limted battery capacity, 300w seems plenty.

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[jezport]

Good to here you consider 140w is enough, a lot of people appear to want a lot more, but with only a fridge, shower pump, lights etc. and only relatively limted battery capacity, 300w seems plenty.

I would consider a good MPPT regulator is as important as the panel.
My van has led lighting, I use a laptop and 14" tablet for TV so can charge when bright out. I do have a 1000w Pure sinewave inverter for items that need 240v like bike chargers etc. Never have run out of power.

 

[karemart74]

Thanks for the advice. I think I may go down the route of a 24v Victron panel and same brand controller to access the blue tooth data. Space on the roof isn't an issue, so I can go big with one panel. I am just making up some brackets for the roof to secure aluminium extruded section for mounting so I can be totally flexible in future. Not sure I trust the glue option, though I know lots of people have been happy with it.

If apply correctly sikaflex 292i will provide a solid structural bonding. Please ensure you clean the surface and apply appropriate activator.

Structural Bonding

industry.sika.com

 

[MisterB]

If apply correctly sikaflex 292i will provide a solid structural bonding. Please ensure you clean the surface and apply appropriate activator.

Structural Bonding

industry.sika.com

Whichever bonding agent you use, get it in black and not white. It won't start looking green and mouldy in a couple of years like a lot of white ones do.

 

[Talljohn]

Good advice, thank you.