Wouldn’t it be great if you were able to make the right decision the first time you were presented with a choice? You 
would have the confidence to say, “double my order, I’ll be using this a long time!” Unfortunately life is not that simple, and neither was my choice for my power system’s battery bank voltage. Some choices have long term consequences, and none more than your battery bank voltage. Once you select this, almost all other components are selected based on it.
Back in ’98 when I did the initial design of the ‘vacation cabin’ solar power system. I made several choices that now 12 years later I regret. Just like many companies running old legacy software, I need to live with it until I can afford a set of expensive upgrades. Wow, this sounds heavy, like a ‘cry in your beer’ story, its not that bad, really.
Why choose 12V?
I chose to use a 12V battery bank for several reasons that seemed to make sense at the time.
- The First reason was that it would provide a a great supply of 20A for my amateur radio transmitter and associated equipment. Yes, I already had a 20A 120Vac supply, but the nearly perfect battery bank seemed attractive for this use.
- The second reason was the vague sense that I might be using other 12V appliances in the cabin at some point and that 12Vdc would come in handy.
- The third reason, or rationalization, was that voltage drop over the 15′ to my 2 solar panels was small enough to not waste solar charging power.
While all these reasons seemed to make sense, they force you to purchase a large 12V inverter/charger to create the normal 120Vac for the cabin. To make things a little more difficult, I purchased two DR2400s from Trace Engineering (now Xantrex) that would slave together and produce 240Vac. This was done to provide emergency well power for our 370 foot deep well.. Unfortunately all my assumptions didn’t hold up to the test of time.
Why NOT 12V?
- The modified sine wave output of the inverters tended to generate a lot of noise on the shortwave bands that I wanted to use in amateur radio. I found I could get a nice quiet RF noise background only when the inverters (ie house power) was shut off! Not popular with the family.
- The other vague 12Vdc appliances do not exist except for the small 12Vdc pressure pump inside the house.
- The 2 solar panels grew to about 12, and the distance to the battery bank went from 15′ to 75′ now. To keep that ‘small’ voltage drop small, we spent a couple hundred dollars getting quadruple ought aluminum wire!
So now all the reasons that were used for the 12V choice have evaporated, but we live with the large legacy investment based on that choice! If you would please wave that magic wand that gives me a free “do over”, I’d be ever so thankful! I would choose a minimum of 24V or higher. That would make my monster wire half the size, or allow much more flexibility in where the solar panels go.
To be fair, there was one advantage to the 12V decision. You can add one solar panel at a time, where at higher voltages, it may be 2 , 3, or 4 at a time. There is a similar situation when purchasing groups of batteries for your battery bank.
Good News Flash:
There is a new type of solar charge controller called a Maximum Point Power Transfer(MPPT) controller. Since this device can handle higher DC charging voltages I can make some nice changes this spring when I hope to upgrade. We can rewire my solar panel array to produce 24V, or more by change the series-parallel wiring arrangement. There will be 4 instant benefits:
- Cut power lost in solar panels to charger wiring by 50% or more!
- Increase daily solar power from the efficient use of series connected panels and MPPT technology.
- Have increased charging capacity. I’ll have 60A charge capability, compared to the old 40A passive charger.
- Charger will work when I upgrade inverters / battery bank to higher voltage since it can output 12-48Vdc charging current.
Ok, sounds like a sales pitch for a new MMPT controller? Yup, I’m sold. This will be the first upgrade to my system this year , but the second purchase after the new battery bank!
Do you have one of the MMPT controllers? Have you made a good choice in system voltage? Please share, leave a comment!
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I know your pain! At least my investment is small, but on purpose. So, I could learn about home solar from the DIY stand point. I have a MPPT charge controller, and I love it. I’m about to rewire my panels to 24v, but keep my batteries at 12v. I use a 12v 3000w inverter, 12v flood lighting in my back porch, and in a pinch; my lawnmower engine is rigged with 12v alternators to recharge the batteries. By changing to 24v the voltage loss is actually cut by about a factor of 6.
For example a 300w array at 12v will loose 7.500w over 30ft of 6ga wiring. While a 300w array at 24v will loose 1.875w over 30ft of 6ga wiring.
Also, if you keep your batteries at 24v. They will last longer! The overall advantages of a 24v system is nice! And when you invert 12v to 120v your actual amp hrs is different. Meaning that if you have 1200aH battery bank at 12v; inverting it makes it 120aH at 120v. But at 1200aH at 24v is 660aH at 120v. There is so much to know, and most of it is by trial and error.
Later from sunny FL!
Thanks for stopping by!
Your calculations on capacity are valid at DC, but your AC conclusions don’t seem right. When we speak of capacity, the best term to use is Watt-Hours (or KWH). In your example both systems have 1200AH x 12V = 600AH x 24V = 14400WH or 1.44KWH
That system is 1.44KWH in either case, minus 5 or 6% for inverter efficiency.
We just finished 5 weeks of clouds here, and today is sunny for solar, just like FL!
My suggestion is to go 48V now if possible but this would involve an upgrade of the inverter at the same time. BTW just use DC-DC converters if you really need straight 12V DC. I happen to like Victron brand because I had good results on a military project where they hammered my design for a whole year before accepting it and ordered 124 more systems. I also like the fact they use a switch mode design that is lighter in weight, power factor corrected, and is very flexible by being able to select either 60Hz or 50Hz so for European or Asian bound customers the same box will work in either situation. And lastly, adding more inverters is as simple as plugging in a synchronizing cable and designating one inverter as the master and the others as slaves. You can parallel five 3kw inverters and even develop 3 phase power if your battery bank can handle the load current.
Outback comes a close second and being based in Arlington WA is closer to home should repairs be needed. This is what my wife’s uncle uses in his off-grid installation. Outback can be had in a NEC code compliant package complete with a backer board with DC and AC breakers for branch circuits. Unfortunately being EI iron core laminations the transformer does not lend itself to frequency changes. You have to pick one or the other and order accordingly.
For driving heavy loads like deep well pumps, heat pumps or even air conditioning I prefer to spec a 230V inverter and use a step down transformer for the relatively light 120V loads.
Outback learned a lesson from the old TRACE design where you end up with unbalanced neutral currents. Not so with a straight two wire 230V output. And you can make it NEC code compliant.
While down in FL last December to wire up a system I found a 4 kVA step down transformer to drop the 230v down to 115V that only cost $109 and ran fairly cool and completely silent. The 6 Kw inverter system can surge to over 12 kw but the 120V AC circuits only consume about 3 kw maximum including an all electric galley with counter top cooking appliances.
For deep well applications using a Franklin 230V pump, (or similar) add a soft start module to
minimize the start surge. Last fall I was quoted $179 Cdn for such a module. Sure eases the start surge and thus the size of inverter needed.
BTW it was so cold in Florida that month we ran 4kw of electric heating from the inverter to keep warm. Not a recommended practice for off-grid but useful in a pinch. We recharged the battery bank by using the 15 amp yard outlets to drive the chargers ( similar to your genverter idea) whereas before he found he could not run all the heaters with out popping the breakers. The secret being the heaters cycled and the battery bank buffered the surges and the 15 amp cords maintained a steady power delivery to the chargers that were fine tuned to avoid tripping the breakers. In case you wondered the bank was a 900 amp hour 24 V Sonnenschein bank built up from 2V cells. Much more efficient than a bunch of Surette,Trojans or Concorde 6 volt batteries wired series parallel.
Ok…so 24 or 48 then? I will be needing so little compared to most, just not sure where to start but need to start LOL. Found these good prices but not sure if they are good units, mind taking a look and give me some feedback plz? I am thinking I only need about 80 watts but not knowing what is coming down the road if bigger IS better and its a good deal then I would consider it but finances are an issue and have to get controller, inverter etc yet. Thanks so much! Katherine
http://www.solarblvd.com/Solar-Panels-&-Systems-Individual-Solar-Panels-55—95-Watt-(10)/c1_25_41/p1331/Solar-Cynergy-80-Watt-12-Volt-Solar-Panel/product_info.html
http://www.solarblvd.com/Solar-Panels-&-Systems-Individual-Solar-Panels-175-Watt-+-(11)/c1_25_203/p2272/Suntech-Power-175-Watt-24-Volt-Solar-Panel/-Black-Label/product_info.html
http://www.solarblvd.com/Solar-Panels-&-Systems-Individual-Solar-Panels-100—170-Watt-(12)/c1_25_42/p896/Sharp-NE-170U1-170W-24V-Solar-Panel/product_info.html
Kathrine, The first panel would work with inexpensive passive solar charge controller. Your Vardo wiring lengths would not be much of an issue. The other panels are the newer breed and have an optimum voltage of 34V. I belive there is a company, bluesky that has MPPT controllers for lower power ranges and would work well. Arlid may chime in with the URL for them. If you can afford it the second and third will work well.
Thank you Marshall
I don’t even have my head wrapped around 12v yet…now thinking about 24,48 etc has my head spinning again. I know Arild recommends it and I know he likes Bluesky products. We spoke quite a bit offlist and he did his best to guide me 
I have decided not to use the “Freezerator” which has decreased my power budget alot. Ok, back to studying…Katherine
one advantage to high solar panel (or array) voltages is that you get higher performance in adverse solar conditions like partial shade or snow. See part 3 of MPPT series.
I took a look at that, makes perfect sense to me, LOVE your analogies btw. For some reason when I see too many numbers in a sentence my head goes blank, my eyes glaze over and my head spins like that chick on the Exorcist. No pea soup thankfully! So for me, what Arild and I discussed was using a power pack with a solar array to charge it and my truck as back up. I looked at them last night (again) and they all have MSW inverters on them. So need to figure out if that is going to work for me. I know he recommends having more than one inverter as well so not sure what all can be “added” to that. I did see where someone added an extra 60AH battery to it for extra storage with good results. Need to look into it more, difficult atm as life is getting in the way a bit lol. Katherine
Oh..btw went through purchasing two power packs, both were DOA. I guess letting them sit on the shelves really effects those batteries. Decided to build my own power pack with two 110ah AGM’s, a 400 watt inverter and 4 outlet DC component. Put it on a furniture dolley (sp?) so I can move it around easier. Had to put a switch on the line to the DC outlet as it has a light on it that would burn constantly otherwise. There is a fuse in the inverter but I am still going to add a fuse on the positive between the battery and the inverter. Also ran #8 wire from my truck battery to the back for charging, works great
Need to add a fuse to that too probably. Easy connect with Andersons (Thanks Arild!!) Katherine