Yet another 12v vs. 24v

Hey everybody. I have checked out other 12v vs 24v forum posts and can’t find the specific "why" behind some of the points being made. To be clear, I'm not calling out anyone for being wrong (I don't have the knowledge to do that!), I just feel like I need a bit more information to make the right decision for my specific project.

So the big question: 12v or 24v? What I can’t find specifically in other posts on the forum is why is a 3000 watt inverter is a bad idea to run off a 12 volt battery? This has been stated in a bunch of the forum posts I've searched. I have to ask the question; why does Victron make the 12/3000 Multiplus II if that’s not a good idea? I also keep hearing statements about wire size and efficiency.

When it comes to wire size, the Victron manual says for the 12/3000 I would need two 1/0 wires from the battery to the inverter. That wire run will only be 2 feet or so. That doesn’t seem like a big deal to me…what am I missing? As far as DC accessories in the rig, almost all are 12v, so not much savings on wire there if I did a 24v system. So the only real spot I would need heavier wire with a 12v system is between the battery and the inverter itself. With a 12v system, I also wouldn’t need the additional converter to take the 24v back down to 12v for most of the circuits. Please tell me what I’m missing! I could see if my lights, water pump, stereo, Maxx Air Fan, winch, usb charging receptacles, and whatever else were all 24v appliances, there would be significant savings on wiring, but they are all 12v.

Moving on to efficiency. Is it true that converting 12v to 3000w AC is inefficient in comparison to converting 24v to 3000w AC? if so, what's the efficiency difference? I wonder what that value is because if I end up going with 24v system, then I will have a loss through the 24v to 12v converter for all my DC circuits. Could that end up being close to a zero-sum game?

Thank you so much to anyone who can offer some insight on these things to help me and others who are wondering the same things. I truly appreciate it!

Wire is expensive, and 24V inverters are usually slightly more efficient. Those are the main reasons to recommend going up in voltage. That is fine advice when the inverter is the only DC load. Since you have other DC loads that are 12V, it makes little sense to choose anything but 12V. Every conversion from one voltage to another is going to add money, inefficiency, and failure points. Avoid that where possible.

Thanks bolhuijo! That makes sense.

I run a 3000w inverter on a 12v battery bank. It works great

Fuse everything

One thing that no-one has mentioned or just glossed over is why the wire size it's kind of a big deal and that's max discharge current. If your going with a 3kw 12v inverter your going to need 250 amps if your hitting 3kw, if you have a peak draw of say 4kw that's almost 350amp also a 250ah battery at full draw lasts you 1hr and the BMS is going to cut off at any current spikes over 3kw.

ThaiTaffy said:

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I would love to have an excuse to hook up to that

I generally recommend 2000w max on 12v. Not sure why an RV needs 3000 watts. I would go single 4/0 with 3000w over the 2x 1/0 wire. Probably very rare to go over 2000w anyway and the ampacity is fine at 4/0 (about 400 amps). No real need for surge performance that the Victron has to cover to meet specs. The Dometic microwave pulls about 140 amps from the 12v battery.

Custom cables here: https://www.customcableusa.com/products/custom-battery-cable-marine-grade-by-the-foot

If you do this, use windy nation welding wire rated 105c. It can take 440a so won't overheat if you hit 3000w.

Do you run an AC unit? If so is it AC current? If those two are yes 24v is a better choice even with the conversion loss back to 12v for everything else.

Do you have any high draw devices, I mean in watts? Do you ever plan to? If so, then 24v is the right voltage.

If it is all dc and never changing why do you need a 3000w inverter?

Running high currents means your connections have no margin for error. A bad crimp or a bad connection and you are asking for trouble. Perfect crimps with perfectly torqued bolts. With lower current there is more leeway for error.

Dadoftheturkeykids said:

I run a 3000w inverter on a 12v battery bank. It works great

Fuse everything

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Did you ever move the build out of your entertainment center in your bedroom? Did you end up putting fuses on both ends of all your wires in case there is a problem and current dumps from three batteries into one?


Does bring up a point - how many 12v batteries do you intend to parallel to get the watt hours you need to run?

Parallel batteries and short cables between 12 and 36 inches have enough resistance to cause serious heating when paralleling more than 2 batteries. If the cables are not way oversized and there is a short at the post of one of them can cause the others dump current into single cable and heat it well past the temp to melt the sheath off in a few seconds if you don't fuse both ends. The higher the current the faster that can happen.

robbob2112 said:

Did you ever move the build out of your entertainment center in your bedroom? Did you end up putting fuses on both ends of all your wires in case there is a problem and current dumps from three batteries into one?

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It's still in that entertainment center lol, I like the setup... other than my room is always hot,
I have not seen any temperatures over 115°f on any component or wire while stress testing. It usually runs about 100 at the hottest points i.e. inverter/shunt

I do have all the mrbf fuses now, just gotta take some time to tidy up my positive wire runs.

a 12/24v charge controller is only 50% net value at 12v

Double the voltage, double the panels, double the net output