This guide follows 12 volts VS. 24 volts for Off-grid Solar Power Systems, a video from DIY Solar Power with Will Prowse that has passed 1.35 million views and roughly 1,800 comments in the seven years since it went up. The comment count is the useful part: this is one of the few voltage debates online where the replies come from people who built the system and then reported what it actually cost them. Below is the video's argument in order, with the numbers that settle each point, and answers to the ten comments readers ask about most.
One thing to clear up before the math: the video's headline claim is that 24V systems are always cheaper to build. That is true at the component level and only partly true at the convenience level. The honest version is that 24V wins on wire and controllers, loses on local availability and 12V appliances, and the crossover sits somewhere around 600W of array. Everything below is about locating that crossover for your own build.
Why Voltage Is a Cost Question, Not a Power Question
Voltage does not make a system more powerful. Watts are watts. What voltage changes is the current needed to move those watts, and current is what you pay for in copper, terminals, connectors and controller capacity. Double the voltage and you halve the current for the same load. That single relationship drives every cost difference in this article.
So the question is not "which voltage is better" but "at what array size does the copper and controller savings from 24V exceed the extra cost and hassle of converting back down to 12V for your appliances." For a two-panel cabin system the answer is never. For a 1,500W van build it is almost always yes.
Copper First: Where Half the Current Shows Up on the Invoice
Take 1,000W of load. At 12V that is about 83A. At 24V it is about 42A. Ampacity tables and voltage-drop math do not scale linearly with current, but the practical result does: the 12V version needs a cable two sizes up, and the lugs and terminals to match. Large-gauge copper lugs are not cheap, and they are the part people forget when they budget a build.
The Charge Controller Math Nobody Believes the First Time
This is the strongest argument in the video, and the one that surprises people. Charge controllers are rated in output amps, not input watts. A 20A MPPT controller charging a 12V bank tops out around 260W of panels. The same 20A controller charging a 24V bank handles roughly 500–520W, because 20A at 26–27V is twice the wattage of 20A at 13–13.5V.
One controller, same price — the video puts it around $130 — and it covers double the array. Scale that up: a 12V system with 1,000W of panels needs a controller roughly twice the amp rating of a 24V system with the same array, and controller price climbs steeply above 40–50A. On a medium build that is a $100–400 difference for one component, which is more than the cost of the buck converter you will need for 12V appliances.
The takeaway is not "always buy 24V." It's that the controller saving is large enough to pay for the 24V penalty several times over, which is why the "but I have to buy a converter" objection does not hold up once the array gets past a few panels.
Inverters Cost About the Same. Local Availability Doesn't.
Inverter pricing is a wash. A 12V 2,000W inverter and a 24V 2,000W inverter cost roughly the same online, and the video says so directly. What differs is where you can buy one at 7pm on a Saturday. Big-box stores and auto parts shops stock 12V inverters. Almost none of them stock 24V. If you source locally, 12V has a real advantage; if you order online, it disappears.
There is a second, quieter advantage to 12V: the whole ecosystem around it — battery chargers, jumper packs, accessory sockets, cheap solar maintainers — is built for 12V. None of that transfers to a 24V bank without another converter in the chain.
The 12V Appliance Tax: A Buck Converter and a Fuse Box
A 24V bank cannot feed 12V loads directly. In an RV or van that matters, because lights, vent fans, water pumps, fridge control boards and radio gear are all 12V. The fix is a DC-DC buck converter: the video quotes $30–40 for a decent one with overcurrent and thermal protection, plus your own small fuse block downstream.
If your system only ever feeds an inverter — a cabin with AC loads and nothing else — you can skip this entirely. If you have any 12V circuits, budget the converter and a fuse box, and treat it as the price of admission. Compare it against the controller savings above and the arithmetic is not close.
Alternator Charging Is the Real Break
Here is where 24V actually hurts, and the video spends real time on it. Charging a 24V house bank from a vehicle alternator is not a wire-and-fuse job. The alternator regulates around 14V; the bank wants 27–28V to charge. You need a DC-DC converter, a dedicated 24V alternator, or a second alternator on the engine.
The video is much harsher about the shortcut version: paralleling a deeply discharged lithium or deep-cycle bank with a starting battery through a smart isolator. The voltage difference is large, the discharged bank's internal resistance is low, and the alternator ends up delivering high current into a load it was never designed for. That is how alternators overheat and charging systems fail. Its recommendation is a dedicated second alternator — the kind shops install for RV and marine deep-cycle charging — or nothing at all.
Panels, Shading, and Series Pairs
Modern 60- and 72-cell panels run an open-circuit voltage around 40V, which a 24V MPPT controller handles comfortably in series pairs or small strings. The shading case is the exception. If your site has heavy tree shade and your plan is to parallel many 12V-nominal panels for redundancy, a 24V bank fights you: you need series strings, and each string is a single point that shade can drag down.
The video's compromise is series pairs, paralleled at the array level. Two panels in series get the string voltage up to roughly 40V, and paralleling several pairs keeps some redundancy when one pair is shaded. It is less shade-tolerant than a fully parallel 12V array, but it is workable, and it keeps the controller small.
Small, Medium, Large: Where 12V Stops Making Sense
The sizing rule in the video is simple enough to memorize: small systems stay 12V, medium systems go 24V, big or whole-house systems go 48V. "Small" means a handful of panels, one or two batteries, roughly 300Ah of bank at most, and something like four major components. "Medium" starts when the array passes about 600W or the bank passes that 300Ah ceiling, because that is where 12V current, cable and controller costs start compounding.
Above roughly 2kW of array, 48V is the better answer and the same logic repeats: half the current again, smaller controllers again, and a higher-voltage inverter that costs about the same as its 24V sibling. The 12V-to-24V question is really the first step of a ladder, not a binary choice.
12V vs 24V, Side by Side
| Factor | 12V bank | 24V bank |
|---|---|---|
| Current for the same load | Baseline | Half |
| Cable and lug size | Thicker, more copper | Roughly half the copper |
| 20A MPPT panel capacity | About 260W | About 500–520W |
| Controller cost for a given array | Higher (needs more amps) | Lower |
| Inverter price | About the same | About the same |
| Local availability | Everywhere | Online, or specialist dealers |
| 12V appliances | Direct | Needs a $30–40 buck converter plus fuse box |
| Alternator charging | Possible, but risky without a dedicated alternator | Needs DC-DC or a dedicated 24V alternator |
| Heavy-shade panel arrays | Fully parallel works | Series pairs required, less shade-tolerant |
| Practical bank ceiling | Around 300Ah | Well beyond 300Ah |
| Best fit | Up to roughly 600W, cabins, small RVs | Roughly 600W–2kW, vans, medium off-grid |
How to Decide Without Guessing
Answer five questions and the voltage usually decides itself. How many watts of panels are you actually installing — not planning to install later? Do you have 12V loads that must run without the inverter? Will you charge from the engine, and how often? Is heavy shade part of the site? And are you buying locally or online?
If the array is under 600W, the loads are mostly 12V, and you want to buy parts locally, stay at 12V and spend the difference on a better controller. If the array is 600W–2kW, the load is inverter-driven or you can afford one buck converter, and you order online anyway, go 24V and put the savings into more panels. If you are building a whole-house system, skip the debate and go 48V.
What Our Readers Asked (Top Comments on the Video)
These are the most-liked technical comments on the source video, with our answers. Where the creator replied, his wording is quoted verbatim; where he did not, the answer is ours.
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