System Voltage: 12V, 24V or 48V??

samitrix · March 2011

hi guys,

is there a single factor or several factors that decide the system voltage? is it the solar array that eventually dictates a system voltage? somewhere i read that higher voltage systems perform better. true?

cheers,
sam

petertearai · March 2011

Re: System Voltage: 12V, 24V or 48V??

I put in a 12 volt system and wish I had gone to 24 volts or higher .The reason for 12 volts was cost and not having enough info at the time.
The higher the voltage the smaller the cables in the d c side need to be.
I originally put in a 125 watt panel with an oc voltage of 22 volts.
Had I had found this forum earlier, I may have made the effort to get 2 panels and go 24 volts.
You have found a great forum and will get some very useful info from some very patient and Knowledgeable people.
All the best with the solar power
Regards
Peter

mike95490 · March 2011

Re: System Voltage: 12V, 24V or 48V??

One rule of thumb is "for every 1,000 watts of AC load, add 12V.
so
0-1,000W = 12V dc
1,001 - 1,999 = 24V dc
not much gear for 36 V systems Jump to 48V
2,001w + 48V system

BB. · March 2011

Re: System Voltage: 12V, 24V or 48V??

Basically, how much power/current do you need and how far do you want to send it...

Using some rough numbers (using typical AC inverter cutoff voltage):

12 volt battery, 11.5 volt minimum operation voltage, 10.5 volt cutoff
24 volt battery, 23 volt minimum operation voltage, 21 volt cutoff
12 volt battery, 46 volt minimum operation voltage, 42 volt cutoff

So, lets assume that you want 100 amps at nominal voltage:

12v * 100 amp = 1,200 watts
24v * 100 amp = 2,400 watts
12v * 100 amp = 4,800 watts

Now, AC inverters are "constant power" devices (Power=Voltage*Current). if you run the inverter at minimum voltage it will draw more current. And the wiring/fuses have to be designed 1.25 larger for NEC safety rating (also, remember inverters are ~85% efficient):

1,200 watts * 1/0.85 * 1/10.5v cutoff * 1.25 = 168 Amps minimum
2,400 watts * 1/0.85 * 1/21v cutoff * 1.25 = 168 Amps minimum
4,800 watts * 1/0.85 * 1/42v cutoff * 1.25 = 168 Amps minimum

Now notice the difference between Vbatt-minimum and Vcut-off of the Inverter that can be used for wiring/voltage drop:

11.5 volts - 10.5 volts = 1 volt drop
23 volts - 21 volts = 2 volt drop
46 volts - 42 volts = 4 volt drop

So, you now can have higher voltage drops and still have a functional system... You could, for example, on the same gauge wire run the 4x the power 4x farther on a 48 volt verses a 12 volt system.

Also, this is why I really like using 120 VAC inverters when possible even though they may cost you ~15% extra power.

For a 12 volt system, if you want to send 240 watts some distance... At 12 volts that is 20 amps and a 1 volt drop... Lots of expensive copper to support that.

At 120 Volts, you can have a 3.6 volt drop (3%) or more (120 volts - 3.6 volts = 116.4 volts) and 2 amps--Much less copper required.

And, look at the operation voltage of a 12 volt battery bank... 10.5 to 16 volts (near dead with wiring drop to maximum charging voltage during equalization).

Many 12 volt devices are designed for cars... Maybe 12 volts when the engine is off and 14.2 volts when the engine is running. Quite a few of these generic 12 volt adapters either don't work well on a Solar power battery bank or even get toasted during charging.

Using a well designed AC inverter which is designed for 10.5-16 volts and outputs 120 VAC can make things much easier, more robust, and may save some money in the end (copper costs and replacement 12 volt adapters).

Another things to plan for is conservation... Some of the Energy Star rated 120 VAC appliances actually approach the efficiency of the dedicated DC/Off-Grid appliances, at 1/4 the price, and with longer life/more features (put that money into a couple extra panels and a good inverter).

Whatever you do--Pick your devices, understand your loads, and define your requirements before spending your first Dollar (or Yen). It can make a huge difference in your satisfaction/functionality in the end.

-Bill

samitrix · March 2011

Re: System Voltage: 12V, 24V or 48V??

petertearai wrote: »

I put in a 12 volt system and wish I had gone to 24 volts or higher .The reason for 12 volts was cost and not having enough info at the time.
The higher the voltage the smaller the cables in the d c side need to be.
I originally put in a 125 watt panel with an oc voltage of 22 volts.
Had I had found this forum earlier, I may have made the effort to get 2 panels and go 24 volts.
You have found a great forum and will get some very useful info from some very patient and Knowledgeable people.
All the best with the solar power
Regards
Peter

thanks for your input Peter! im learning a lot from here so yes indeed this is a great forum!!
see ya around,
cheers,
sam

samitrix · March 2011

Re: System Voltage: 12V, 24V or 48V??

BB. wrote: »

Basically, how much power/current do you need and how far do you want to send it...

Using some rough numbers (using typical AC inverter cutoff voltage):
12 volt battery, 11.5 volt minimum operation voltage, 10.5 volt cutoff

24 volt battery, 23 volt minimum operation voltage, 21 volt cutoff

12 volt battery, 46 volt minimum operation voltage, 42 volt cutoff

So, lets assume that you want 100 amps at nominal voltage:
12v * 100 amp = 1,200 watts

24v * 100 amp = 2,400 watts

12v * 100 amp = 4,800 watts

Now, AC inverters are "constant power" devices (Power=Voltage*Current). if you run the inverter at minimum voltage it will draw more current. And the wiring/fuses have to be designed 1.25 larger for NEC safety rating (also, remember inverters are ~85% efficient):
1,200 watts * 1/0.85 * 1/10.5v cutoff * 1.25 = 168 Amps minimum

2,400 watts * 1/0.85 * 1/21v cutoff * 1.25 = 168 Amps minimum

4,800 watts * 1/0.85 * 1/42v cutoff * 1.25 = 168 Amps minimum

Now notice the difference between Vbatt-minimum and Vcut-off of the Inverter that can be used for wiring/voltage drop:
11.5 volts - 10.5 volts = 1 volt drop

23 volts - 21 volts = 2 volt drop

46 volts - 42 volts = 4 volt drop

So, you now can have higher voltage drops and still have a functional system... You could, for example, on the same gauge wire run the 4x the power 4x farther on a 48 volt verses a 12 volt system.

Also, this is why I really like using 120 VAC inverters when possible even though they may cost you ~15% extra power.

For a 12 volt system, if you want to send 240 watts some distance... At 12 volts that is 20 amps and a 1 volt drop... Lots of expensive copper to support that.

At 120 Volts, you can have a 3.6 volt drop (3%) or more (120 volts - 3.6 volts = 116.4 volts) and 2 amps--Much less copper required.

And, look at the operation voltage of a 12 volt battery bank... 10.5 to 16 volts (near dead with wiring drop to maximum charging voltage during equalization).

Many 12 volt devices are designed for cars... Maybe 12 volts when the engine is off and 14.2 volts when the engine is running. Quite a few of these generic 12 volt adapters either don't work well on a Solar power battery bank or even get toasted during charging.

Using a well designed AC inverter which is designed for 10.5-16 volts and outputs 120 VAC can make things much easier, more robust, and may save some money in the end (copper costs and replacement 12 volt adapters).

Another things to plan for is conservation... Some of the Energy Star rated 120 VAC appliances actually approach the efficiency of the dedicated DC/Off-Grid appliances, at 1/4 the price, and with longer life/more features (put that money into a couple extra panels and a good inverter).

Whatever you do--Pick your devices, understand your loads, and define your requirements before spending your first Dollar (or Yen). It can make a huge difference in your satisfaction/functionality in the end.

-Bill

Bill you are everywhere! you are faster than the current passing in solar circuit

anyway, i really appreciate the detailed explanation and the knowledge you bestow upon us mere mortals;)
i'm still digesting the information you provided but i get the overall message.

arigatou gozaimasu (thank you very much),
sam

BB. · March 2011

Re: System Voltage: 12V, 24V or 48V??

You are very welcome Sam...

And I am most assuredly, not perfect... If something does not make sense, ask. I probably messed up somewhere.

-Bill

tonygcan · March 2011

Re: System Voltage: 12V, 24V or 48V??

Hi Bill,

Thank you for all your help.

For clarification, in this thread, are you comparing the merits of a straight 12 volt dc output system compared to a 120 volt AC with inverter?

BB. · March 2011

Re: System Voltage: 12V, 24V or 48V??

Trying to demonstrate that if you have to send the energy any distance, it may make sense to use a 120 VAC inverter even though you pay for the inverter and loose ~15% of the power converting from 12 to 120 volts.

Because the current in the 120 volt circuit is ~1/10th that in a 12 volt circuit for the same wattage load.

Also, for many appliances these days, the 120 volt versions are much cheaper and more reliable than the specialized 12 VDC versions (for example off-grid DC refrigerators). It is cheaper to add another solar panels and AC inverter than to buy the DC version.

And with energy star, many of the AC appliances are almost as efficient as their DC off-grid versions (at least in the US).

It is a balancing act/personal choice. Do the research and make your decision based on what makes sense for your home/location.

-Bill

RandomJoe · March 2011

Re: System Voltage: 12V, 24V or 48V??

I also started with 12V, in large part because most of my intended loads at the time were 12V. Ham radio bench, networking gear, some "emergency lighting".

The wire sizes are INSANE for even relatively low current levels, due to the complete lack of tolerance for voltage drops. The cost of the cabling, and the difficulty working with it, made it quite frustrating.

Another issue was the difficulty keeping the battery bank balanced. I had 4 parallel pairs of T-105 batteries. Overall they did well, but as time went on I could see issues creeping up.

I also kept finding devices I'd like to run on solar power, but didn't use 12V directly. I could either spend an excessive (to me!) amount on DC-DC converters for them, or run an inverter all the time.

I finally decided I wanted to be able to use the power in other parts of the house as well, and didn't particularly want to deal with the humongous wire needed for a 12V inverter of any size. So I bought a separate 12V AGM battery and solar panel for the ham radio bench, then converted the main system to 48V and bought an Outback inverter (I already had their FM80 charge controller), wired that up to a small breaker panel and put in dedicated "solar power" circuits in a few places in the house.

I've been much happier with this setup! The batteries have all settled down, the cabling is so much easier to deal with (and looks a lot cleaner to boot), and it's easy to get "grid backup" if needed (say, lots of cloudy weather) since the OB inverter can switch between grid and inverter on command.

DavidOH · April 2011

Re: System Voltage: 12V, 24V or 48V??

Thanks for that guide Mike, BB.

That shows my thinking was right.
With a low wattage system I am working on I thought 12v was the way to go.
About 500 watts and short connections it works.
KISS no conversions. All devices the same voltage.

The larger systems and those that need longer cables higher voltage make more sense.

bobdog · April 2011

Re: System Voltage: 12V, 24V or 48V??

Can I back up for a second? I was just thinking about 12 vs 24 vs 48 volt systems today and wished I had a 24-48 volt system. BUT, I can't as part of the cabin was already wired DC for 12 volts. Also, I couldn't figure out WHY I needed to have anything more than 12 volts anyway. So, other than wire sizes and saving money on them, is there any other reasons why you would want to go with a 24-48 volt system? I realize I would've saved money on wire size, but is it more efficient to use 48 volts versus 12 volts?

mike95490 · April 2011

Re: System Voltage: 12V, 24V or 48V??

1) series batteries instead of parallel.

2) lower wire losses

BB. · April 2011

Re: System Voltage: 12V, 24V or 48V??

Basically, you get 4x the power (same current, same wire size) from 12 to 48 volts...

And you get 4x the workable voltage drop... Which means you can send the power 4x farther or use 1/4 the size of wire...

So the overall difference is ~16x more "something" between 12 to 48 volt bank...

1/16th the copper, 16x father to send the power on the same size wire, or mix and match, 1/8th the copper 2x farther, etc..

Also, in terms of many electronics--it is the current that defines the size of inductors, transistors, etc...

60 amps * 14.5 volts = 870 watts into battery bank maximum
60 amps * 58 volts = 3,480 watts into battery bank maximum

For example, the same 60 amp charge controller can operate in a 12 volt system with 1x array, or in a 48 volt system with 4x the size of array... So, you save money as the system voltage goes up... Either a smaller charge controller, or fewer charge controllers to control the same amount of power.

The other issue is the number of parallel strings of batteries... Personally, I recommend 1x string of batteries, and 2-3 maximum parallel strings.

In general, more parallel strings of batteries becomes more expensive (fuses/breakers per string), more time on maintenance (more battery caps to R&R), and failures become less obvious (an open or shorted cell or bad cable "hides" very nicely in large paralleled battery bank).

To a degree, the advent of a cheap and reliable DC Current Clamp meter ($60 or so)--Has made it much easier to monitor parallel battery strings for proper current sharing on charging/discharging vs the old days of using volt meter to poke around for voltage drops on wiring (to find that string that is taking too much or too little of the current).

And, to be sure, there are many people out there with their 4+ parallel battery banks that are very happy--In spite of my recommendations.:roll:;);)

About the only down side is that there are typically a lot more 12 volt native devices than 24 or 48 volt...

However, if you bite the bullet and run mostly 120 / 240 VAC power from an inverter--then the 12/24/48 volt issue becomes more of just picking the matching components.

-Bill

bobdog · April 2011

Re: System Voltage: 12V, 24V or 48V??

Well, that answered my questions and more. I wish I could switch to a 24 or larger voltage system, but alas it was not meant to be. It is for a small cabin so in the end it probably doesn't matter much. Thanks Mike and Bill.

DavidOH · April 2011

Re: System Voltage: 12V, 24V or 48V??

it depends on what devices you are powering.
It does make sense to use 48 volts for high powered items like drive motors
Power equipment... Tools. Transmitting high power radios.

XRinger · April 2011

Re: System Voltage: 12V, 24V or 48V??

DavidOH wrote: »

it depends on what devices you are powering.
It does make sense to use 48 volts for high powered items like drive motors
Power equipment... Tools. Transmitting high power radios.

My main PV system was designed as a 48v back-up system (w/ 48v inverter).

My Ham radio transceivers all use 12V (they were designed for mobile use in cars)..
So, I have a small 12V battery+PV just for radio use.
But, it's only for back-up, in case of lost grid power.

If I need to, I can also run my Ham gear off AC (MSW) from my 48v backup..

Paul54 · April 2011

Re: System Voltage: 12V, 24V or 48V??

I started with a small 12 volt system and have kept it to use for lots of my home lighting. I use a lot of the 12 volt LED lamps through out the house. At 3 to 5 watts a peice you can run a bunch of these every day off of a small system. I use # 8 low voltage lanscape lighting wire to distribute the 12 volts through the house. You can find it pretty cheap on the internet even with shipping a 100 ft. spool. My larger system is 24 volt only because I already had a 2.5Kw 24 volt inverter. When I replace that I will definetly reconfigure my battery bank to 48 volts.

I have a small inverter hooked up to the 12 volt system but never use it any more. I think for low wattage use, a 12 volt direct system is quite usable. The other evening I had my main PV system shut down, and my grid service off so I could work in the main service panel. It was really nice to have lights for the house and to work by from the 12 volt system.

desertratt666 · February 2014

Re: System Voltage: 12V, 24V or 48V??

i too was warned about 12 volt systems but the better availability of 12 volt accessories and the fact i never pull over about 200 watts ac at any one time (running propane refrigerator from my RV still) 12 volt works for me + if an inverter goes out i don't have to special order it as im 15 minutes from the nearest truck stop that stocks plenty of 12v accessories.bottom line you'll have to decide whether or not the convenience outweighs the drawbacks of the higher voltage systems.

desertratt666 · February 2014

Re: System Voltage: 12V, 24V or 48V??

p.s. if you do go with a 23 volt system you might want to consider a dc to dc converter for a ready source of 12v power for led lights and small appliances that usually use a small inverter - even better if you can find one that puts out 6v/9v/12v.(at least 5 amps for most would be sufficient i think)

masterpawn · July 2017

Hello bill.
I just noticed this thread and wondered if it was still active. I would like to ask you a question

Estragon · July 2017

Probably best to start a new thread with your question.

BB. · July 2017

You may, or just start your own thread. That way, we can address your questions and not get confused with other setups and such.

-Bill

System Voltage: 12V, 24V or 48V??

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