What is the difference between a 12V, 24V, 48V solar System?

hmong2017 · May 2019

For an off grid Solar panels, breakers, controller, batteries and inverter.... Whats the REAL difference to choose from a 12V, 24V and 48V system? Why do others choose a specific system vs others?

I only see that they all produce electricity and makes 110 and 120volts....
Is bigger ones like 24v and 48V just simply charge batteries faster or what?

Estragon · May 2019

The main difference is the size of loads you need to run. Running a couple of 15a(AC) 120v loads means > 300a(DC) on a 12v system, requiring heavy wire etc., which gets pretty impractical. Running on a 48v system the same loads would be 1/4 the amps, so ~75adc.

A 12v system would also require 4x the charge controller capacity. Eg. 1200w would be 100a out from12v controller, likely needing 2 units. 25a out from a 48v controller easily handled by one. A 60a controller handles ~ 60x12v=720w of panel, same 60a controller is ~ 60x48v=2880w on a 48v system.

So, for larger systems, higher nominal voltage makes more sense. Offsetting this is, for smaller loads and systems, lower system voltage tends to be more efficient and cost effective.

There is no best choice, it depends on loads.

BB. · May 2019

As Estragon says, the higher the loads, generally, the higher the voltage for the battery bank.

For example, we say that roughly 100-150 Amps for the battery bank current, then for a 12 volt bank, I would suggest the largest AC inverter or DC loads would be roughly (ignoring losses and variable battery bank voltage) in the range of:

100 amps * 12 volts = 1,200 Watts maximum suggested
150 amps * 12 volts = 1,800 Watts not to exceed if avoidable

For 24 and 48 volt banks:

100 amps * 24 volts = 2,400 Watts max suggested
150 amps * 24 volts = 3,600 Watts not to exceed...
100 amps * 48 volts = 4,800 Watts
150 amps * 48 volts = 7,200 Watts not to exceed

In reality, if you heavier (and paralleled) copper DC cabling, you will find 12, 24, 48 volt inverters that are larger than the above recommendations, and they can work... But exceed the max suggested is better done with higher voltage battery buses (24 and 48 volts).

Sending 100's of Amps at 12 volts--Really only leaves you about 0.5-1.0 volts of drop on the DC battery bus for a "workable" system... For 24 volts, 1-2 volts, and 48 volt 2-4 volts drop--So you have more "headroom" with the higher bus voltages (and allows a larger DC load if needed and properly designed).

Similar issues for charging your battery bank... For example a good quality MPPT controller can output 80 amps... At 12/24/48 volts. So, a higher voltage battery bank means a larger solar array could be supported... For example, our rules of thumb would suggest the maximum array for an 80 amp charge controller:

80 amps * 14.5 volts charging / 0.77 panel+controller deratings = 1,506 Watt maximum "cost effective" array
80 amps * 29.0 volts charging / 0.77 panel+controller deratings = 3,013 Watt maximum "cost effective" array
80 amps * 58.0 volts charging / 0.77 panel+controller deratings = 6,026 Watt maximum "cost effective" array

The same $600 controller can support much larger arrays with higher battery bus voltages.

Anyway, details matter here. The above are good rules of thumbs to estimate a workable system without going into a bunch of detailed design issues. But they can be modified as needed.

-Bill

Photowhit · May 2019

Normally the first thing that we point out is the wire sizing to run high DC current to the battery bank. Had someone the other day have a 15,000watt 24 volt inverter... So a quick look at the NEC ampacity charge and some quick math at a full 24 volt and the wire size required is 1500mcmil often 4/0 is compared to the size of a finger, 1500 mcmil would be more comparable to your wrist!

If you are looking for inverter sizing, you will find that inverters that are UL1741 (for home use) rarely are far from the sizes Bill has mentioned above. Magnum makes a 2800 now 3000 watt inverter for mobile use and it only carries UL458 (mobile use) They do make the MS2000 12 volt which I think is rated for 2000 watts continuous. They also make a 4000 watt 24 volt, largest I'm aware of in 24 volt. I believe there are a couple 8000 watt 48 volt inverters, Outback's Radian, and Schneider? larger than that and you would have to 'stack' inverters. So just to be 'legal' you may find these ranges.

anthonytoro31227 · October 2019

Im from Puerto Rico and after huracan Maria we need an electrical alternative,solar system is the way to go,but Ive been looking in to using a forklift batt but I was recomended the 48v 24 85 13,my question is what other one is recomended as next better with higher capacity? Can any one help?

BB. · October 2019

Welcome to the forum Anthony,

I would suggest that you start a new discussion (thread) about your system needs.

Off grid solar power is pretty expensive--Something like 5-10x the cost of utility power. So you want to 1) make sure your power needs are reduced to the minimum amount you need through conservation/new energy efficient devices, etc. And 2) you need to measure your loads (average Watts, Watts*Hours per day of usage, etc.) so you can size the system to support your needs.

The voltage and Amp*Hour rating of your battery bank are based on the amount of energy you need per day. Fork lift batteries are among the longest life batteries (10-15+ years), but there are not cheap, and they are very large/heavy to move (1,000-2,000 lbs or so typically). Plus they need a larger solar array (+~30% larger) and need more maintenance (checking electrolyte levels and adding more distilled water) than other lead acid batteries (they tend to be "less efficient" and use more distilled water than other FLA (flooded cell lead acid) Batteries.

Any way, start a new discussion about your system needs and tell us more about your energy needs, and we will go from there.

-Bill

706jim · October 2019

12 volt system is good for modest loads maybe 1500 watts or so. Main advantage is that you can directly run 12 volt stuff like cell phone chargers, RV appliances etc directly off the battery. IMO you are far better off to go with a higher voltage system for all the reasons previously mentioned here and just run those minor loads off a 120 volt to 12 volt power supply rather than try to tap off the center of a 24 volt battery just as an example.

softdown · October 2019

My own vote is something like this:
12 volts - under 800 watts.
24 volts - between 800 and 1600
48 volts - living full time off grid with full complement of amenities - over 1600 watts of panels. The wiring is twice as efficient when voltage is doubled as has been pointed out. Better battery layouts are easier to achieve (says the guy experimenting with four strings - and loving it so far).

PNW_Steve · October 2019

706jim said:

12 volt system is good for modest loads maybe 1500 watts or so. Main advantage is that you can directly run 12 volt stuff like cell phone chargers, RV appliances etc directly off the battery. IMO you are far better off to go with a higher voltage system for all the reasons previously mentioned here and just run those minor loads off a 120 volt to 12 volt power supply rather than try to tap off the center of a 24 volt battery just as an example.

I went with 24v in my RV. I was able to find 24v stuff like without much trouble.. Lights, water pump, USB chargers are all 24 v. The only item that needs a 24v-12v converter ($15 from Amazon) is my Webasto. It is available in a 24v unit but I got a good enough deal that the 12v one that it was worth the compromise.

MrM1 · October 2019

Is it more efficient for an inverter to convert from 12v, 24v or 48v? It seems just thinking about it, that 48v would be the easiest/most efficient to convert to 120v, but I'm sure there is a lot more to it than that and it probably does not work out that way in the real world.

BB. · October 2019

It tends to be a bit more complicated... There is capacitance of the power FETs/Transistors, The wider the voltage swing on the input, the more loss to capacitance (charging/discharging the FETs).

Also, tends to be easier to find smaller wattage AC inverters at low voltage... 48 Volt inverters tend to be larger anyway. Although, in the last few years, the lower power 48 volt input inverters are at least available now.

Our host has a 48 VDC inverter down to 250 Watts:

https://www.solar-electric.com/residential/inverters/off-grid-inverters.html?nav_inv_input_voltage=439

https://www.solar-electric.com/victron-phoenix-48-250-120v-nema-5-15r-inverter.html

Output AC voltage: 120VAC +-3%
Output frequency: 60Hz
Continuous output power at 25oC: 250VAC
Peak power output: 400W
Max efficiency: 88%
Zero load power: 7.9 Watts
Output Socket: NEMA 5-15R
Dimensions(h x w x d): 3.4" x 6.5" x 10.2"
Weight: 5.3 lbs
Warranty:5 years

The 12 volt version of the same above inverter is only 4.2 Watts

https://www.solar-electric.com/lib/wind-sun/Phoenix-Inverter-VE.Direct-250VA-1200VA-DS.pdf

Not a huge difference... And for larger systems (which 48 volt systems tend to be), not going to make much of a difference.

-Bill

What is the difference between a 12V, 24V, 48V solar System?

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