Solar setup for 500 sq ft cabin

[Ghfalls]

I’m just hoping to get some advice on a system I’m in the process of building. I built a 500 sq ft cabin with all 12v lighting and rv appliances with wood stove as well for heat. 12v unique brand stand up 24” width fridge. It’s set up to be fed with power from a 30a rv cord for the 110v plugs. I’ve built a 6’-6”x8’ power shed with single sloping roof and have mounted 6 250 watt solar panels on the roof. I picked up a set of used discovery tubular cell batteries (12) 2v to have a 24v system. I would like some advice on how I should wire the panels. At this point, high end inverter and charge control are not in the budget. I was looking at Amazon 3000 watt inverter and possibly 60 a charge controller. Or should a guy do 2 30a charge controllers and wire the panels in two sets of 3? I’m not at home right now but I think the voltage is 30.7 and 8.24 A. Any advice would be very much appreciated.
Thanks

 

[Ghfalls]

And I’ll also need a dc to dc converter to step the voltage down to 12v for the low voltage stuff inside the cabin.

 

[12VoltInstalls]

And I’ll also need a dc to dc converter to step the voltage down to 12v for the low voltage stuff inside the cabin.

What are your consumption voltage/amps? If under 100A at 24V why not just use 12V straight up?

 

[Rednecktek]

Just to throw the idea out there, have you thought about going with one of the Growatt SPF-3000 or MPP LV2424 24v AIO's? It'll do your solar charge controlling AND AC output, you can wire the 30a RV plug into the AC Input, but you'll still be able to use your 120v stuff without the RV plug doing anything. Since you have the power shed you can house the AIO out there and not have to worry about any noise.

Cost comparative between one of those units OR the MPPT controllers to handle that much wattage AND a 3Kw inverter separately should be about even with the bonus of taking care of the AC input from a generator or other source.

As to the step down buck converter, I liked my Uxcell 60a version as it started getting $$tupid expensive per amp beyond that. Do realize that step down converters don't really handle surge loads like a true inverter does, so be sure to oversize your buck converter as much as you can.

What are your consumption voltage/amps? If under 100A at 24V why not just use 12V straight up?

I suspect wanting 3Kw of inverter AND having 1500w of panel gets expensive and dangerous to keep everything 12v. If the AC load wasn't nearly that high (say 1500w or so) I would instead suggest a pair of 60a MPPT controllers, a pair of 3s strings, rig the batteries up in 12v config (2p6s) and just keeping everything at 12v.

But there's a reason your handle isn't 24VoltInstalls. ?

 

[12VoltInstalls]

But there's a reason your handle isn't 24VoltInstalls. ?

Yes, I’ve spent years making 12V stuff work

However, I am not strident on 12V. The higher voltage systems make sense once you are 2000W, maybe 2500W (or with the victron 3000W, too) for several reasons, but if one’s setup is 12V heavy and regular ongoing demand isn’t close to 2000W or so it doesn’t make sense to me to not run a 12V integrated system.
When you can run a vacuum and shopvac on a 1200W Giandel pure sine, for example, I don’t get how 24V makes sense.
If one is running a whole house with an ever-present 15- or 20A draw at 120VAC with big surges from a well pump then I can’t see why one wouldn’t use 24V.

 

[Ghfalls]

Right now the only constant consumption is the fridge running all the time. We will likely get Starlink out there too as it’s remote with no cell coverage there. That way we could run a couple security cameras that we could monitor from home. In 10 years once the kids are gone, we will sell our house and move to there full time. I really like the look of those two inverter/ charger/ controller setups that were suggested and the price is great too. I’m not sold for sure to go to 24v vs 12v I just thought it made a bit more sense to go that route as I got the 12 2v batteries for a great price. But I could wire them as two sets of 6 and keep it at 12 v as well and that would eliminate to need for a dc to dc step down converter. We don’t really have any big power draws when we’re out there other than the wife constantly runs a small vacuum to keep the floors nice and clean. Would it be best to wire the panels in 2 groups of 3 in series? Would that still get me 48a of charging in full sun?

 

[Rednecktek]

Would it be best to wire the panels in 2 groups of 3 in series? Would that still get me 48a of charging in full sun?

With your panels and if you kept it as a 12v system, your best bet would be to put 3s on each SCC and parallel them. That way each set would be pumping out about 250w * 3panels = 750w / 12v = 62.5a on paper, in reality probably right about 60a and about 150 VoC which is the max an off-the-shelf SCC can do before you start having to spend $tupid money on an SCC. With 2 in parallel that would get you about 120a-ish of charging in full sun.

 

[OffgridMatty]

Right now the only constant consumption is the fridge running all the time. We will likely get Starlink out there too as it’s remote with no cell coverage there. That way we could run a couple security cameras that we could monitor from home. In 10 years once the kids are gone, we will sell our house and move to there full time. I really like the look of those two inverter/ charger/ controller setups that were suggested and the price is great too. I’m not sold for sure to go to 24v vs 12v I just thought it made a bit more sense to go that route as I got the 12 2v batteries for a great price. But I could wire them as two sets of 6 and keep it at 12 v as well and that would eliminate to need for a dc to dc step down converter. We don’t really have any big power draws when we’re out there other than the wife constantly runs a small vacuum to keep the floors nice and clean. Would it be best to wire the panels in 2 groups of 3 in series? Would that still get me 48a of charging in full sun?

Watch out for those vacuums. My wife loves to run ours and it pulls 900-1000W. She loves a clean floor and I keep telling her it's a cabin! Thankfully she can vacuum the whole place in a short time, so not really a big draw as long as your system can handle to start up wattage.

 

[Ghfalls]

With your panels and if you kept it as a 12v system, your best bet would be to put 3s on each SCC and parallel them. That way each set would be pumping out about 250w * 3panels = 750w / 12v = 62.5a on paper, in reality probably right about 60a and about 150 VoC which is the max an off-the-shelf SCC can do before you start having to spend $tupid money on an SCC. With 2 in parallel that would get you about 120a-ish of charging in full sun.

Thank you for the response. This sounds like it makes the most sense. So if I’m understanding this correctly, if I set up these batteries to be at 24v, I’d have exactly half the charging amperage? If so, what is the advantage of going with the higher voltage systems?

 

[chrisski]

Thank you for the response. This sounds like it makes the most sense. So if I’m understanding this correctly, if I set up these batteries to be at 24v, I’d have exactly half the charging amperage? If so, what is the advantage of going with the higher voltage systems?

Half the charging amperage at twice the volts equals the same power. So 10 charging amps at 12 volts equals 120 Watts, 5 charging amps at 24 volts still equals 120 watts.

My limits for constant loads are

12 volts: 1000 watts
24 volts: 2000 watts
48 volts: 4000 watts

Its best explained in my signature block in batteries and inverters.

I do run an inverter for more than 1000 watts on 12 volt loads, but it is to pull 2000 watts for a minute or two five or six times a day, not pull 2000 watts for 50 minutes straight. i did use 4/0 wire for that system. I think a well designed 12 volt system can take a 3000 watt quality inverter, but not to deliver 3000 watts, just to cover surge for a 1000 watt load and also go to 2000 watts for one or two minutes, Five or six times a day.

 

[Ghfalls]

Half the charging amperage at twice the volts equals the same power. So 10 charging amps at 12 volts equals 120 Watts, 5 charging amps at 24 volts still equals 120 watts.

My limits for constant loads are

12 volts: 1000 watts
24 volts: 2000 watts
48 volts: 4000 watts

Its best explained in my signature block in batteries and inverters.

I do run an inverter for more than 1000 watts on 12 volt loads, but it is to pull 2000 watts for a minute or two five or six times a day, not pull 2000 watts for 50 minutes straight. i did use 4/0 wire for that system. I think a well designed 12 volt system can take a 3000 watt quality inverter, but not to deliver 3000 watts, just to cover surge for a 1000 watt load and also go to 2000 watts for one or two minutes, Five or six times a day.

Thank you. Decisions, decisions.

 

[Rednecktek]

Thank you for the response. This sounds like it makes the most sense. So if I’m understanding this correctly, if I set up these batteries to be at 24v, I’d have exactly half the charging amperage? If so, what is the advantage of going with the higher voltage systems?

The biggest advantages of higher voltage systems are the lower amps for the same power which reduces cable and fuse requirements, and the load side. Much like the above example where 10a @ 12v = 5a @ 24v, the same is true for the output side. For people who need larger inverters like the 6Kw and up crowd powering a larger house or higher draw devices more often, getting 6000w out of a 12v system would require 600+ amps worth of fuses and honkin huge wires to carry that load from the batteries to the inverter. Doing 6000w on a 48v system takes that down to 150a and MUCH smaller wire and fuses to provide the same output power to the house.

Likewise your poor charge controller, which is based on amperage, doesn't have to work as hard at higher voltages and can send more net watts through the wires without smoking itself.

There's a saying out there that "30a is a breaker pop, 300a is a flame thrower!"

 

[MichaelK]

So if I’m understanding this correctly, if I set up these batteries to be at 24v, I’d have exactly half the charging amperage? If so, what is the advantage of going with the higher voltage systems?

That's just a simple math problem. You said you bought 1500W of panels. Just do the math. 1500W/13V charging = 115.4A Even with 85% de-rating, that's still 98A. Have you looked at the prices of charge controllers rated at 100A+? Have you looked at the price of copper wire thick enough to carry 100A+ safely?

By switching to 24V the math becomes (1500W/26V charging) X 85% = 49A. I can tell you without even looking that a 50A charge controller is going to be cheaper then a 100A rated controller. And, the copper wire rated to carry 50A is going to be thinner/cheaper than the 100A wire.

Yes, you are likely to be able to make 12V work, but it is going to cost you more than trying to make 24V work.

 

[Ghfalls]

That's just a simple math problem. You said you bought 1500W of panels. Just do the math. 1500W/13V charging = 115.4A Even with 85% de-rating, that's still 98A. Have you looked at the prices of charge controllers rated at 100A+? Have you looked at the price of copper wire thick enough to carry 100A+ safely?

By switching to 24V the math becomes (1500W/26V charging) X 85% = 49A. I can tell you without even looking that a 50A charge controller is going to be cheaper then a 100A rated controller. And, the copper wire rated to carry 50A is going to be thinner/cheaper than the 100A wire.

Yes, you are likely to be able to make 12V work, but it is going to cost you more than trying to make 24V work.

Thanks a lot for all the reply’s. This place is great and I really appreciate the sharing of knowledge. I’m going to try and put a list together of what I think I need.

Here is a vid of my cabin.