diy solar

diy solar

12v, 24v, or 48v?

jbclem

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Joined
May 11, 2022
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As a solar novice I need to decide on the voltage of my system. I'm not sure what the advantages and disadvantages are of the different choices. Why is 12v better or worse than 24v or 48v? In my case, I want to start out with a portable system to deal with power outages (I'm in a brush fire area), but I want to be able to expand it later on. Initially I need to be able to run my refrigerator(full load amps: 6.5a, a few lights, my desktop computer with a router and modem, a small laser printer. Also a 1250 watt electric frying pan. The power outages usually last from 1-4 days.

Advice and suggestions would be welcomed...
 
 
My own rule of thumb is pretty close. I think your normal running battery current should be under 100 amps. If you need short peaks above that, you can decide, but if the constant load is over about 1,000 watts, I think you have hit the limit for a 12 volt system. And when your constant load goes over 2,000 watts, the 24 volt system starts to top out as well. With inverter efficiency and wire losses, that is close to 100 amps at the battery. By 3,000 watts, you are certainly going into 48 volt levels.

If the normal load is only 500 watts, you can use a 2,000 watt inverter and run a microwave once in a while on a 12 volt system. But you still then need cables and a battery that are able to handle 200 amps. If you go to 24 volts, then your running load of 500 watts is under 25 amps and the microwave only hits 100 so you can use smaller cables.

Look at all of the things you want to be able to run. Add up the wattage of what will run at any given time and that will decide on the inverter size. But then also think about how long things will run together and plan out the battery size for the current needed as well as the run time in watt hours. We call this "An Energy Audit".
 
Blurb time!

Well, I'll start the default answer to these questions and we can work from there. Here's you To-Do list:

1: Power audit! This will give you some important information on how big your inverter needs to be as well as how much battery capacity you'll need. There is a link in the FAQ section (I think, or someone here will post it shortly) so fill in the blanks and see what it comes up with. You'll probably need some sort of Kill-A-Watt to get accurate measurements. Are you going to be running a 12v system? 24v system? 48v system? What are the specs on your solar panels? VoC? Vmp? Being as this is a new build, throw together a wish list of what you want and estimate on the high side.

1a: Where do you live? Speccing out a system for Scotland is a LOT different numbers than Arizona due to the amount of light you actually get. Someone here can post the link to the PVwatts.com or JCR Solar Uber-Sun-Hours calculator sites to help figure out how much you'll have to work with. That will be a box in the Power Audit form.

2: Parts list: You don't need a make & model list, just a parts list to start from for reference. You'll need an inverter, a MPPT charge controller, fuses, shunt, buck converter, batteries, wire, etc. Once you have a basic list it can be fine tuned to make & models after that. If you're looking at the All-In-Ones check for correct voltage outputs (120v or 240v Split Phase for North America, 220v Single Phase for European type areas) and make sure it has enough capacity for a little bit of growth and fudge factor.

3: Budget!: Steak is great but doesn't mean anything if your wallet says hamburger. :) Figure out what you're able to spend now vs what you'll have to cheap out on now and upgrade later.

4: Tape measure! Figure out where you're going to stick all the stuff you'll need. A dozen 3000AH batteries sounds great until you're sleeping on the floor because there's no room left for a bed. Is there a compartment that can house all this stuff? Will the server rack batteries fit? Are you going to have to make space? Physics can be pretty unforgiving.

5: Pencil out what you think you need and throw it at us so we can tell you what you've missed (because we ALL miss stuff the first go-round :) ) and help figure out which parts and pieces you're going to want to get.

Well that's the thing about solar systems, there is no 1-Size-Fits-All answer. Your system will need to be designed to fit YOUR needs. When you design and built the system, it's not going to be the perfect system for me, or Will or 12vInstall or anyone else, but it Will be the right system for You and that's the goal.

As for where to get started, let me throw my standard blurb in here to help point you in the right direction. There's going to be a lot of math and research involved, but that's going to be a LOT cheaper than just buying parts off of someone's list and finding out that it doesn't do what you need.

Don't panic on the Power Audit, you'll actually be doing that a few times. When you do the first pass put in ALL the Things that you might want. AirCon? Sure. Jacuzzi? Why not. MargaritaMaster-9000? Go for it.

The second pass will be the "I Absolutely Need This To Survive" list that isn't going to have much on there.

The third pass will be the "This is what is realistic" audit that you'll use to design the rest of the system.

The Power Audit is going to tell you 3 primary things: 1: How big does your inverter need to be to power your loads? 2: How much battery bank do you need to last $N number of days with krappy weather? and 3: How much solar panel will I need to install to refill those batteries in a 4 hour day (the average usable sun hours rule-of-thumb).

Once you know what you Want and what you Need and what your budget can Afford there will be somewhere in that Venn diagram where those three things meet.

After that, THEN you can start looking at parts.

Yes, it's a long drawn out process, but it's worth it in the end. Not every house has the exact same floorplan, not every vehicle is the same make & model, and not every solar system is designed the same.
 
As a solar novice I need to decide on the voltage of my system. I'm not sure what the advantages and disadvantages are of the different choices. Why is 12v better or worse than 24v or 48v? In my case, I want to start out with a portable system to deal with power outages (I'm in a brush fire area), but I want to be able to expand it later on. Initially I need to be able to run my refrigerator(full load amps: 6.5a, a few lights, my desktop computer with a router and modem, a small laser printer. Also a 1250 watt electric frying pan. The power outages usually last from 1-4 days.

Advice and suggestions would be welcomed...
Higher voltage means lower amperage for the same wattage output.
Conductor size is determined by amperage.
So, Higher voltage means smaller Conductors.
Easier to work with, more efficient, and less cost.
Since you plan to expand your system in the future.
Start with 48v, so that you can still use everything later.
 
People usually state that the smaller battery cable size needed for 48 vs 12 volts for the same wattage output is an advantage, which is very true but there is also another advantage. When inverting to 120 AC volts it’s always more efficient to have the DC source voltage already four times higher to start with. The places that most beginners make a mistake is to buy an all in one inverter that’s too small that severely taxes your energy management. That’s not the place to go cheap and leaves no room for expansion like adding more batteries and panels later as needed. Another potential trap is putting batteries in series to make higher voltage. If you need 48 volts, buy or diy 48 volt batteries. Batteries are probably the most expensive part of the system, so consider a generator to charge during the day as a fallback plan. 4 days is a long time if you don’t have solar or enough.
 
As a solar novice I need to decide on the voltage of my system. I'm not sure what the advantages and disadvantages are of the different choices. Why is 12v better or worse than 24v or 48v? In my case, I want to start out with a portable system to deal with power outages (I'm in a brush fire area), but I want to be able to expand it later on. Initially I need to be able to run my refrigerator(full load amps: 6.5a, a few lights, my desktop computer with a router and modem, a small laser printer. Also a 1250 watt electric frying pan. The power outages usually last from 1-4 days.

Advice and suggestions would be welcomed...
Watts = Volts X Amps Keep it in mind when powering things and the choice of battery voltage becomes clearer. Many mobile inverters are 12vDC simply because automotive and marine systems use 12vDC batteries for operation. 24vDC has a smaller niche since Semi-Trucks and vessels with diesel engines would use that voltage to start harder cranking diesel power plants. 48vDC is pretty much becoming standard in the backup PV world as it allows for much larger setups that can power a home. Not sure if eventually 72vDC or higher may eventually come about since even 48vDC gets limiting once you get much above 12kW. (12000w/50v=240a)
 
Two years ago, I was a solar novice as well. I first thought I would go with 12V, because I was familiar with it from automobile systems. Then I discovered that the lower the voltage, the larger the wires needed to be. I did what others wisely suggest here, calculated my power needs, and built a 24V system that has worked very well. However, if I were to build that same system again, I'd go straight to 48V and not give it a second thought. As you're hearing now, wire and fuse sizes can be smaller, batteries can deliver more watts per AH rating, and you have more choices from manufacturers. Scaling the system also becomes easier. My two cents...
 
The other issue is you get a lot more bang for buck running 48V for the solar charging, versus 12 or 24V, for each amp of charger output.
 
With what you suggest, I think you want a 48 volt 10 kWh battery, with a 3 kw to 8 kw inverter, 3 kw to 10 kw of solar depending on your power requirement and location.

This is not really something you keep around for emergencies and ought to be run grid tied with a critical loads panel or on a seperate off grid system.

Once built, this system aside from being expensive will take a little bit of tweaking to get up and running to be sure its ready for when you need it. Using it all the time lets it pay for itself, or at least recoup the money spent. For my smaller portable solar generator I made, the payback period is 110 years.
 
i have a small system , but i have feel i have wasted much time and money on the 12v .

since the price has gone down and the availability of 48v of prebuilt battery's is so much greater

start with 48v and don't look back

even though i built as a back up only originally , i now find my self using to supplement my daily power usage

but honestly thats some expensive power i am now trying to get the most usage out it that i can to make myself feel better and move onto a 48v in fall maybe

when i take into cost and divide into kwh made at this point i am at 22.25 per kwh made since a bought a meter

if i went to current connect

Victron 48V Phoenix 1.2kVA 120V Inverter​

a Victron charge controller
a ebay 48v 50ah battery
the same solar panels
and a 200 dollar savings in wire ect alone
i could have done the same for less then half the cost have only losing the automatic transfer

now my current cost would be down around 9 dollars a kwh produces so far and much cheaper and easier to upgrade
 
48V is what I would do but need to be careful working with the higher voltage; not for the sloppy/easily distracted rushing in a bad mood kind of person.

Are you going to be the one transporting batteries? Hopefully you can get help because the more cost effective 48V batteries are kind of weighty.
 
A laser printer can be quite a power hog, which model?
It's an HP p1606dn, a small and simple older B&W laser printer. All it does is print, nothing else. Uses 400 watts (printing), 2.2 watts (ready), 1.6 watts (auto-off), 0.4 watts (manual off). I use it sparingly.
 
As others have said, go with the highest voltage you can reasonably cope with today. When you start pricing out wire at $5/ft you’ll start seeing another advantage of higher voltage.
 
Thanks for all the information, a trove of ideas. I should have mentioned I'm in the USA, Southern California. My house is 100+ years old, small, and I'm not sure the roof would support solar panels. It looks like a power audit is my first move...and could someone define "constant load" for me. If it's what it sounds like, then the only thing I have that's constantly on is my computer, router, and modem. The refrigerator is off and on, does that count as constant load?
 
I went with 24v for my system, and the number one change I'd make if doing it again would be 48v. The equipment would have cost slightly less, the wiring would have cost a lot less, and I'd have much better expandability for the future.
 
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