12V or 24V? Which one and why?

[UpShinnCreek]

I am trying to plan out my electrical system for my skoolie. I will use it about a month out of the year, so not full time. I hope to 2 mini splits installed and all the solar I can fit on the roof. I do not plan to be hooked up to power for most of my use. As an added bonus, I can get the wire for free through work.

[john61ct]

The only need for 24V is if you have inherited a 24V system already in place, or plan to use specific load devices that require it.

The only other advantage is lighter gauge wiring for a given current rate.

Downsides are many, especially complexities in charging and conversion, maintaining multiple busses since many devices are not available or limited choices, much more expensive in 24V.

[Booyah45828]

To take your question at face value. 12v systems use devices that are typically more common and cheaper. 24v systems will have half the amperage and wire requirement per watt compared to a 12v system.

To go deeper though, we have to look at your usage. I understand the whole "only used 1 month out of 12 scenario" as I'm in the same boat where my bus gets used a handful of weekends per year. With that said, you have to realize that running 2 mini splits plus assorted other electronics is a pretty significant amount of electric. To run that all off battery/solar will require a pretty large solar and battery setup. Like a several thousand dollar type setup.

There are a few guys on here that have such a thing: kazetsukai, JDonTheGo, etc. so read their builds and learn what you can off them. What you'll likely realize is that a large solar/battery investment really only pencils out if you're a full timer. In my eyes, a weekender like you and I won't benefit from thousands invested into a solar system, and are better off with a hybrid type system.

The ROI just isn't there for solar power on a weekender rig.

I'm all for you doing this off solar, but financially, you'll be better suited scaling back your solar/battery goals to supplying only small items and running a generator for the mini-splits as needed.

[rossvtaylor]

At the risk of sounding repetitive (thanks John and Booyah!), there's not generally an advantage to going with anything other than a 12V system in a bus. In the off-grid house and village systems I used to do, we always went with 56V DC systems to keep the wire sizing down...and in those systems, we could always find 56V inverters to provide 120V or 220/240V AC power. There was no need to run DC appliances and thingies, so the DC voltage wasn't a factor.

In a bus, though, there are lots of advantages to running as much off the native DC side of the system and (as mentioned above) it's easier to find 12V appliances and thingies. We have a few buses which came with 24V systems and it's a complication that isn't really helpful.

[UpShinnCreek]

I fully expect to spend about $10K on solar and batteries. While I have quite a few generators, mostly gasoline, one dual fuel, I don't want to have to use them. One of the reasons for me building a skoolie is to be able to go anywhere and still be comfortable. I don't want to have to get desiel for the bus, gas for the generators, propane to cook, and still need fresh water. Last year in Texas, we had a week of below freezing temps, there was no power, no water, long waits to get whatever gasoline and propane was left. I want to be able to go to the bus and be able to survive. I don't want to just survive, I want to be comfortable. If I lose my house, I want to have the option of living in the bus. If society collapses, I want to be able to stay in the bus. I am toying with the notion to call my bus "The Last Resort." I want it to be nice and comfortable when I am in the middle of 1000 acres with no utilities or another person for miles, but I also want it to be a place I can go when I have no where else to go.

I am happy to hear that 12V is pretty much the way to go in the bus. It seems that the only real downside is that I need thicker wires. But since I can get wire from work, that isn't a big issue for me. I reckon it will take me a year or so to complete my build with a notice portion of that because I will be waiting to earn the funds.

[rossvtaylor]

Originally Posted by UpShinnCreek

I am happy to hear that 12V is pretty much the way to go in the bus. It seems that the only real downside is that I need thicker wires.

And, keep in mind that the thicker cables are only those from the batteries to the inverter...and those should be placed close to each other anyway, so the run will be short.

[john61ct]

From the battery to your 12Vdc load devices.

The inverter(s) are such as well, but should be minimised in favour of DC direct devices.

AV gear, screens, fridge/freezers, anything with a battery needs charging, lights, fans, pumps, all should be DC not AC.

Use propane or burn other fuel directly not electric for producing heat.

As much as you can stand anyway.

The fridge(s) are usually the biggest loads, in 12V Ah/day if you want to be mostly-solar.

[rffffffff]

I personally disagree with the 12v stuff. It’s easy to fine most things in 24v and when you’re talking about two mini splits you could run into the need for wire bigger than 4/0. I wouldn’t even consider 12v for a system with that amperage. Solar charge controllers will more than makeup any difference in cost and running less amps through the wires and connections will likely make them more reliable. Add a little 12v converter if you have a device or two that you can’t find in 12v but the difference between 600a and 300a is really significant. If it were me and i was making a bus with that kinda load I would run 48v honestly. The downsides are not nearly what they’re made out to be once the system is installed.

[Booyah45828]

Originally Posted by UpShinnCreek

I fully expect to spend about $10K on solar and batteries. While I have quite a few generators, mostly gasoline, one dual fuel, I don't want to have to use them. One of the reasons for me building a skoolie is to be able to go anywhere and still be comfortable. I don't want to have to get desiel for the bus, gas for the generators, propane to cook, and still need fresh water. Last year in Texas, we had a week of below freezing temps, there was no power, no water, long waits to get whatever gasoline and propane was left. I want to be able to go to the bus and be able to survive. I don't want to just survive, I want to be comfortable. If I lose my house, I want to have the option of living in the bus. If society collapses, I want to be able to stay in the bus. I am toying with the notion to call my bus "The Last Resort." I want it to be nice and comfortable when I am in the middle of 1000 acres with no utilities or another person for miles, but I also want it to be a place I can go when I have no where else to go.

I am happy to hear that 12V is pretty much the way to go in the bus. It seems that the only real downside is that I need thicker wires. But since I can get wire from work, that isn't a big issue for me. I reckon it will take me a year or so to complete my build with a notice portion of that because I will be waiting to earn the funds.

Well, that makes sense. And you'll receive no argument from me about the hassle of multiple fuels, it's why I have a diesel generator. But, you need to realize that you're requirements are more in line with full-time use, not part-time. So you should mirror the systems that full-timers have in place.

With that said, for a full timer, the difference between 12v and 24v is minimal when you're dealing with that much electric. Full-timers use 48v+ systems because it makes the most sense for them. They also nearly all use lithium over lead acid, because it's more economical to do so when on a large scale. Read the builds of the guys I mentioned before, as they've put a lot more intellect in their systems then I care to spend on the topic. Those 2 also put a lot of detail into their posts, and are regularly updating their changes.

Use as much dc as you can to minimize conversion losses is always a good idea. I don't know of too many 48+v dc devices though, and I'm not sure of an efficiency comparison between a dc-dc converter to a dc-ac inverter. I do feel that you want to run multiple inverters, instead of one large one, to keep idle power consumption down. And flip on/off inverters when needed. There's no sense in running a 5kw inverter to power an alarm clock. Maybe put the mini splits on a large inverter, and then power it down when ac/heat isn't needed.

Make sure your fridge is a compressor model, as no absorption fridge runs efficiently off electric.

And FWIW, minisplits/heat pumps don't function that well near freezing temps, and toaster/ceramic heaters are horribly inefficient electrically. So if surviving the 100 year emergency Texas had last winter is in the cards, plan on a backup heat source, like a diesel air heater, woodstove, etc. Or plan on moving, as when texas was freezing, I believe ohio was in the 50's.

[john61ct]

Originally Posted by UpShinnCreek

I am trying to plan out my electrical system for my skoolie. I will use it about a month out of the year, so not full time. I hope to 2 mini splits installed and all the solar I can fit on the roof. I do not plan to be hooked up to power for most of my use. As an added bonus, I can get the wire for free through work.

A 12V system with mostly DC very efficient load devices

and as much solar as you can fit on your roof

will still require shore charging or a small quiet inverter genset when insolation conditions are poor for a few days in a row

a House bank, say 300-600Ah of LFP, or

400-800Ah of deep cycling lead

per 2000W of panelage will allow for reliable fridge/freezer, computers, pumps fans & lights, maybe a few minutes of microwaving per day.

Do **not** try to build out a system where aircon or heating runs off solar/storage.

48V might be helpful for that, or for a system based on AC mains power appliances where nearly everything goes through huge inverters

but crazy inefficient IMO very impractical for boondockers and insanely high investment.

Run the genset for aircon and other thirsty loads, sized so they don't draw down the battery at all.

Honda Companion models let you connect two to get double the output.

The solar / storage part then just becomes a way to reduce gennie runtime.

[Doktari]

Fantastic project. It’s interesting to see all the variety of perspectives. As you won’t have RV air conditioners on the roof you will have a lot of “real estate” for solar panels. Ive seen 2500-3000 watts on a large bus. On hot sunny days you could run the mini splits for hours from the solar then switch to generator. Ive not done a auto generator start system (ags) yet but know those who have. And they did have some complications. When it works it must be very convenient. I like the idea of having a “escape pod” bus with some comforts. Ive suggested to several people to place their solar energy systems on a vehicle or trailer but no biters yet. I’m glad I did. I have 9 solar energy systems lol (2 buses, 2 motorhomes, 1 cargo trailer, and 4 small systems to keep starting batteries charged). I hope to have a small village for people to fall back on if things get worse. Oh, I’ve seen complications from big dual “synced” inverter systems also. Having a small inverter and a large inverter is the route I’d like to try for conserving power. Yes, inverters are one more thing to fail and the bigger inverters use 25 watts power to operate. A mostly DC system makes a lot of sense in some regards. But I’ve had inverters running for 20 years and not had one fail yet. When it does it might be when I need it most however so having a spare is probably smart. It wouldnt take long to wire in more DC loads however if a inverter isn’t available. What I do is run some extra load bearing wires when I’m building the bus so they are there if I need them later. Then the hardest part is done.

[dzl_]

Originally Posted by UpShinnCreek

I am trying to plan out my electrical system for my skoolie. I will use it about a month out of the year, so not full time. I hope to 2 mini splits installed and all the solar I can fit on the roof. I do not plan to be hooked up to power for most of my use. As an added bonus, I can get the wire for free through work.

The question should really consider 12v, 24v, and 48v. Some generalizations:


Take an hour to understand two fundamental relationships: (1) Voltage = Current * Resistance, and (2) Watts = Current * Voltage


The practical implications of the above is that doubling the voltage means you halve the current (amps) and quarter the power loss. For small loads (loads = energy consumers) or short wire runs this may be inconsequential, for large loads current can be a limiting factor/bottleneck so a higher voltage allows higher powered loads, or larger charge sources, and lets you get away with smaller gauge wire. Choice of voltage also affects product availability, high power devices are often only available in higher voltage (if they are good quatlity devices, cheap brands often push 12v beyond its limits on paper and struggle in reality).



Beyond this there are specific pros/cons to 12/24/48 volt systems, I believe I and others have gone into more detail about these in other posts, I wont repeat that here (its late and i'm tired) but a search should turn up some results I think.

[kidharris]

Originally Posted by Booyah45828

To take your question at face value. 12v systems use devices that are typically more common and cheaper. 24v systems will have half the amperage and wire requirement per watt compared to a 12v system.

To go deeper though, we have to look at your usage. I understand the whole "only used 1 month out of 12 scenario" as I'm in the same boat where my bus gets used a handful of weekends per year. With that said, you have to realize that running 2 mini splits plus assorted other electronics is a pretty significant amount of electric. To run that all off battery/solar will require a pretty large solar and battery setup. Like a several thousand dollar type setup.

There are a few guys on here that have such a thing: kazetsukai, JDonTheGo, etc. so read their builds and learn what you can off them. What you'll likely realize is that a large solar/battery investment really only pencils out if you're a full timer. In my eyes, a weekender like you and I won't benefit from thousands invested into a solar system, and are better off with a hybrid type system.

The ROI just isn't there for solar power on a weekender rig.

I'm all for you doing this off solar, but financially, you'll be better suited scaling back your solar/battery goals to supplying only small items and running a generator for the mini-splits as needed.

If you/he did do a large solar/battery setup, couldn't you use/adapt it to be back up power in a grid power down scenario or partial power to your house, provided that the rv/bus is stored at your house. I've never done solar, just curious if it could be easily justified by the dual use.

[kidharris]

Originally Posted by UpShinnCreek

I fully expect to spend about $10K on solar and batteries. While I have quite a few generators, mostly gasoline, one dual fuel, I don't want to have to use them. One of the reasons for me building a skoolie is to be able to go anywhere and still be comfortable. I don't want to have to get desiel for the bus, gas for the generators, propane to cook, and still need fresh water. Last year in Texas, we had a week of below freezing temps, there was no power, no water, long waits to get whatever gasoline and propane was left. I want to be able to go to the bus and be able to survive. I don't want to just survive, I want to be comfortable. If I lose my house, I want to have the option of living in the bus. If society collapses, I want to be able to stay in the bus. I am toying with the notion to call my bus "The Last Resort." I want it to be nice and comfortable when I am in the middle of 1000 acres with no utilities or another person for miles, but I also want it to be a place I can go when I have no where else to go.

I am happy to hear that 12V is pretty much the way to go in the bus. It seems that the only real downside is that I need thicker wires. But since I can get wire from work, that isn't a big issue for me. I reckon it will take me a year or so to complete my build with a notice portion of that because I will be waiting to earn the funds.

I find it odd that you think of "The Last Resort" as being "comfortable". My version of my last resort doesn't involve comfort at all.



BTW, it's kinda hard not to have another person within miles when you confine your self to 1.5625 sq miles (1000 acres). You are going to need 4 wheel drive and lots of fuel, maybe a helicopter, or set out on foot to get somewhere that there is no one for miles unless most everyone else is dead. It would help if you were already somewhere that there was no one around for miles.

[TaliaDPerkins]

"I find it odd that you think of "The Last Resort" as being "comfortable". My version of my last resort doesn't involve comfort at all."


Credit to Max Dunbar

[john61ct]

Inverters is a term used in many different contexts.

Solar installations on a house are usually grid tied, totally different from an off-grid system.

25W of wasted power while in use is nothing

I've seen 30% and more, so a multiple kW unit...

And that does not count the parasitic vampire draw of failing to switch them off while the fed load(s) aren't running.

[Booyah45828]

Originally Posted by kidharris

If you/he did do a large solar/battery setup, couldn't you use/adapt it to be back up power in a grid power down scenario or partial power to your house, provided that the rv/bus is stored at your house. I've never done solar, just curious if it could be easily justified by the dual use.

Oh it certainly could, and matter of fact, I wouldn't own a large system like that without it having the capability of backfeeding my house.

All you would have to do is add a nema 14-50R socket onto your bus, and then use the bus to backfeed into your house much like you would with a portable generator. Would cost a couple hundred dollars max.

But, you'd need a 12kw+ inverter and a massive amount of stored energy for that, so it'd be wise to augment your battery and solar with a mounted genset of some sort too.

If it were me, and money/time were limitless, I'd have a battery and solar system similar to kazetsukai's, and then install an onan diesel genset that can provide additional power when needed, and also recharge the batteries during cloudy days. Then, install twin 100+ gallon saddle tanks on your bus like the semi's run, and you'd have virtually unlimited power, that can backfeed into your house if needed.

Or, since it's mounted on a bus, if SHTF, you unplug from the house, bug out, and take your power with you.

Truthfully though, and very few get this, but if "S" would truly hit the fan, I'm bugging out with the bare minimum, because anything more then that, like being tied to a school bus, makes you that much more of a target. And that's also why I feel preppers with 10k + rounds of ammo is comical.

I hope mankind is decent enough that it never comes to it.

[kazetsukai]

Originally Posted by Booyah45828

If it were me, and money/time were limitless...

lol, neither were "limitless" for me. My (48V 840Ah) LFP bank was intended as a 10 year investment, with backfeed capability like you mention in mind.


The answer to the OP I have is as high as practical. For me this is when you start talking 100A charge/discharge.... 12V is insane imo if you're regularly pulling/pushing 2kW.


And for practicality of bug out, I say buy land now in the middle of nowhere and get ready to bug in there instead... way better strategy imo.

[rossvtaylor]

I've gotten to know and really respect so many of the group who have taken the time to offer input here (and elsewhere) in this thread. I know my "go with 12V in a vehicle" approach is a bit controversial to some. And, yes, in any off-grid stationary setup...especially those with higher wattage inverters...I would always go with a higher battery bank voltage. But in a vehicle, with a short run between the batteries and the inverter...and a relatively small battery bank compared to a home/village system...and relatively short-duration draws (because most systems won't store enough energy to allow for a long-duration draw)...I'm hard pressed to see the advantage of anything other than a 12V system. Yes, the current flow between the batteries and the inverter will be 2X the current draw of a 24V system. And that short run of cable will be large in cross section, indeed. But I just don't see the advantages, when weighing all the pros/cons, that a higher-voltage system has in a vehicle.

Having said that...I appreciate all the different perspectives. And my way is just that...my way. You do you, as they say.

[Booyah45828]

Originally Posted by kazetsukai

lol, neither were "limitless" for me. My (48V 840Ah) LFP bank was intended as a 10 year investment, with backfeed capability like you mention in mind.

Lol, never said it was. But for me to do what you did, my priorities would have to drastically change, or I'd have to stumble onto a large pile of money.

The grid here is extremely reliable, and I'm not reliant upon it for my life, and I have 2 gas gensets and the bus's mounted generator that I could use if needed in a pinch.

For me, a large battery/solar setup isn't something that I'd utilize. So it'd be a complete novelty, a very cool one, but a novelty none the less.