Need help with high level build architecture for 6x12 off-grid work trailer

[gregtzar]

Hey there fellow off-grid gang! I'm a software engineer and this is my first solar build. I'm converting a 6x12 offroad cargo trailer intended for full-time working and living. I'm a minimalist (living out of a suitcase for years). I am currently in the high-level architecture stage, and looking to spec out some basic design constraints for the system. Things like whether to go with 12v or 24v, and the minimum energy inputs and outputs I'll need for the battery bank, solar panels, charge controller and inverter. I'm hoping some people with more experience could help assess my situation and provide some general direction. I've outlined the essentail parameters below. Thanks!!

* Needs to power:
* Laptop, phone, device charging
* Starlink (current gen) (continuous)
* Vitamix (optional, will downgrade if this alone puts me in another energy needs class)
* Small DC water pump (for a kitchen sink)
* LED lights
* Fridge (continuous)

* Solar charging with backup gas generator charging
* Alternator charging (optional, possibly later from second dedicated alternator)
* Cooling system (optional, possibly later and willing to run from generator)
* Since this is a dedicated work station the primary goal is continuous power to starlink and laptop charging for 8-10+ hours/day
* The trailer will be static setup but its a mobile setup in that I can move/relocate the trailer easily and often
* For starlink I need to have a good line of site with open sky which also helps the solar
* I'm willing to use the roof of the trailer to mount panels, either onto the rack (which covers about 30%) or potentially directly onto the body. I'm also willing to have some or all of the panels which I can remove and sit on the ground.
* Cost is a consideration of course but I'm more interested in understanding what the optimal setup might look like regardless of price, as a point of reference, and I can make concessions from there if needed.
* I know the 12v/24v decision is a big one and the main impact I can see (aside from cost) is that I may need 30-50% more panels for the 24v system which seems like a big downside given the limited space.
* I also had planned to just stack batteries in parallel to increase capacity as needed but now I'm questioning that decision due to some things will has said about the longevity of parallel systems. Now I'm questioning that and wondering if a single ginormous (yep thats the technical term) battery would be better.

 

[Mattb4]

One basic design constraint is area needed for solar panels is small (6 X 12 trailer) When you consider that it takes 1 square meter (there is 10.764 sqft to sq meter) to get around 200w there simply is not much room for a lot of panels directly mounted.

 

[gregtzar]

One basic design constraint is area needed for solar panels is small (6 X 12 trailer) When you consider that it takes 1 square meter (there is 10.764 sqft to sq meter) to get around 200w there simply is not much room for a lot of panels directly mounted.

Thanks Matt. I'm not yet clear on how many watts I'll need to be targeting, but I did suspect the available real estate was going to be a problem. The roof is empty except for the roof rack (which I can use) and a ceiling fan/vent (which I don't think I should cover over). I suspect I'll need to build both a roof-mounted array (which will cover basics but not full needs) and an additional ground array I can pull out and supplement with. And that this might be necessary for both a 12v and 24v system but the 24v system would need a much larger ground array.
One thing I didn't mention is that I will probably need to sustain about ~6 hours of work time after sunset which would give me a kind of energy debt the next day.

 

[Mattb4]

You will need to add up all your loads to get the running watts and also determine the time that each will be on so that watt hours can be calculated. (Ex. a 13w LED uses 13w and if you turn it on for 10.5 hours you used 136.5wh). Once you have your total daily load than you can start to figure out how to supply it.

BTW whether 12v or 24v the array size is determined by your watt-hour needs. It does not change just because the voltage changes.
Watts = Volts X Amps.

 

[gregtzar]

You will need to add up all your loads to get the running watts and also determine the time that each will be on so that watt hours can be calculated. (Ex. a 13w LED uses 13w and if you turn it on for 10.5 hours you used 136.5wh). Once you have your total daily load than you can start to figure out how to supply it.

BTW whether 12v or 24v the array size is determined by your watt-hour needs. It does not change just because the voltage changes.
Watts = Volts X Amps.

I see... yeah adding up all the loads is definitely something I still need to do and will post it here soon.
As for the solar array size for 12v/24v batteries I drew that conclusion based on one of Will's videos I was watching (

) where he said that 24v requires a higher solar panel array voltage. But I guess that doesn't necessarily mean I'll need a larger array -- but I was thinking in terms of wiring up 12v solar panels in series in which case I think the number of panels would need to double to support the same battery bank in 24v vs 12v. So ideally if I wanted 24v I would just get panels with a higher voltage output and in that case the surface area would not need to increase?

 

[Mattb4]

RV panels are most often rated at 21-23Voc. This is often called a 12v panel. Most SCC today are MPPT that can handle a large Voc and output at various battery voltages(12, 24, 48). But you are correct that in order to charge a higher voltage battery you need several volts higher solar input.

For instance you take 2 RV 100w panels at 21.6Voc and 5.55Vmp wired in series. There output is 43.2Voc at 5.55Vmp. This is sufficient to charge a 24 or 12vDC battery from the SCC. The 12 v battery would charge at 200w/12vDC = 16.67a max and the 24v battery would charge at 200w/24vDC = 8.33a max W=VA

If you were selecting a SCC you would need a 20a model for that scenario of 200w panels and 12v battery.

 

[gregtzar]

RV panels are most often rated at 21-23Voc. This is often called a 12v panel. Most SCC today are MPPT that can handle a large Voc and output at various battery voltages(12, 24, 48). But you are correct that in order to charge a higher voltage battery you need several volts higher solar input.

For instance you take 2 RV 100w panels at 21.6Voc and 5.55Vmp wired in series. There output is 43.2Voc at 5.55Vmp. This is sufficient to charge a 24 or 12vDC battery from the SCC. The 12 v battery would charge at 200w/12vDC = 16.67a max and the 24v battery would charge at 200w/24vDC = 8.33a max W=VA

If you were selecting a SCC you would need a 20a model for that scenario of 200w panels.

Okay so if I'm understanding this correctly, in the two scenario you gave the final "watt hours" you would derive would be nearly the same regardless of whether you were charging a 12v or 24v battery bank, the SCC is just going to transfer that power in different ratios of volts/amps based on whether your target is 24v or 12v. (Sorry for any over-simplifications, still early in the learning process). But I think I get the gist of it.

 

[gregtzar]

So if I have a separate ground array and roof-mounted array of panels, do I need to funnel them through the same SCC before hitting the battery bank or can I run two separate SCCs to the battery bank simultaneously (one for each array)?

 

[Mattb4]

You can use more than one SCC with its own PV array to your battery bank. You just can not hook up one array with 2 SCC.

There is a lot to consider when setting up an adequate solar power supply to meet the load.