24v vs 48v

Masterjoe

I have a 24 v system with 18 250w panels and 12 batteries. What advantage would I gain upgrading to a 48v system that will cost almost $6000 more?

Dave Angelini

You would be increasing the GDP of our economy? For a serious answer you probably need to add a bit more like what batteries you have now, the equipment you have, and what is it that you want to do.

BB.

You have a nice sized system (at least solar panels + charge controller typical max output of ~59 amps) that is well balanced. A typical maximum AC inverter for a 24 volt system is around 2,400 to 4,000 Watts--Above that (if you need that much AC power), a 48 volt battery bank would make sense.

For a 10% rate of charge for your system, a 59 amp * 1/0.10 rate of charge ~590 AH 24 volt battery bank would be a nice design. Roughly, going over ~800 AH of battery bank, you should think about going to the next higher voltage battery bank.

Don't have information about your battery bank, AC inverter, and your loads/energy needs. So--As David says, we cannot really give you much more useful/accurate information at this point.

-Bill

Photowhit

• With 12 batteries, if I assume 6 volt, you would not have a configuration that would work, you would have to take 4 out or add 4. In general you don't add new batteries to old, unless the old batteries were less than a year old (6 months for me).
• If you have the wiring in place for 24 volt system, and you have an inverter large enough for your needs, and don't plan on adding panels to your array. I'd say "use what you got!"
• I have a 24 volt system and should have a 48 volt system. I have a similar array size. If I was building the system from scratch I would have gone with a 24 volt system, but I already had a system at my old place and had to move. A used 24volt forklift battery would have little value particularly when you are @50 miles away from anyone who would use it and have to arrange to lift it (1100lbs) onto a trailer. I also had an inverter, though it's really too small for this place at 1800 watts.
• I'll wait until the battery dies, and then replace the battery with a 48 volt battery and a larger inverter. 
• Once in place, there aren't huge advantages. Here are the basic advantages;
  
• Have higher capacity battery bank with fewer strings of batteries. (Multiple strings of batteries don't share the load equally so they wear out quicker.)
• Higher DC voltage allows for smaller gauge wires.
  
• Often fewer charge controllers. Charge controllers work on output amperage, so a 4Kw array would require 1 charge controller at 48 volts, but 2 at 24 volts.

There are some minor disadvantages;

24 volt inverters are somewhat more available with a larger selection and perhaps better prices for some entry level units.

Configuration for some arrays becomes difficult with some MPPT charge controllers. With 72 cell panels sometime strings of 3 run the VOC up against some charge controllers max, and strings of 2 are really too low to run efficiently on a 48 volt system. With 60 cell panels you would be limited to strings of 3 on some charge controllers.

Masterjoe

I will have to add further info on batteries and panels.  Thank you for getting back to me.  I think that is the whole issue right there, I have 1 MPPT 80 600 charge controller on a 4.5kw system.  My installer refuses to add a second controller stating it is not necessary.  True or not?  Should i be satisfied with the 24v system only capable of 2560 watts with an array producing 4500 watts.  Is the added almost $6,000 price tag really worth the effort to go 48v?  That according to the installer needs 4 more batteries to 16 and 4 more panels.  All I really am after is power in circumstances when utilities are down.  If i can send some back to the utility and offset the cost of the system also would be great.  

Horsefly

As I understand it....

Depending on your batteries, your current single charge controller may be undersized. If you get 80% of your nameplate power out of your panels you would get 0.8 x 18 x 250W = 3,600 watts. For a 24V bank charging at 29V that array could produce 3,600W / 29V = 124A, but your charge controller (which I assume is the MPPT80-600, not MPPT60-800) can only give 80A. You would be leaving the rest on the table.

If you went to a 48V battery bank the MPPT80-600 would be closer to the right size, since the maximum current when charging would be 3,600W / 58V = 62A. 

However, it may very well be that your batteries don't need more than 80A in the current 24V configuration. Again, you need to tell us about your batteries.

Masterjoe

12 6v trojan L16G-ac batteries rated at 20 hrs @390ah in 3 strings series.  I figure im leaving some power on the table, i am curious if anyone thinks it is economical to to bother spending that kind of money to get the full power?  Not something I need to rush into doing now, i'll wait for the batteries to die in i hope 8 more years.

Dave Angelini

It probably is current limiting and not a big deal. You get more power in bad solar weather. It does limit at 80A even though the breaker is 100 right?. More data gets more accuracy!!!. I often put 6KW on this @ 48 but I have to go as it is turkey time here. They run down the hill about 60 feet and get airborne and go up into an 80 foot Ponderosa for the night. And I am waiting to try and get a picture of the 747 back from dropping retardant. So much to do and tomorrow the smoke is suppose to go to Nevada

Masterjoe

Thank you for taking the time. Once I get the system back up I'll give better data as to how it is operating. Right now I'm just passing grid power through the inverter for the last 2 years.

Raj174

@Masterjoe
So your battery bank is really a little over 1100AH at 24 volts. 80 amps is only about a 7% charge rate. That's doable, kinda low, but if it meets your needs that's good. If not, then in my opinion, a second charge controller would be needed.

Rick 

Photowhit

It could be worth your time and money to add a 2nd charge controller. If you have the power company's okay and a hybrid inverter to back feed the grid. I think there is at least one hybrid inverter by schnieder that will do this from a 24 volt system. Dave would know more about this than I do. There are also Grid interactive inverters by Outback that should allow you to sell back to the grid Such as the GVFX3524. This would likely pay for it's self over a few years. If you are in a reasonable sunny area, maintaining the batteries and selling back 10-20 Kwhs of electric each day, it will take time, at $350-800 a year depending on your area. In this case not having to replace things.

https://www.solar-electric.com/outback-power-grid-interactive-sinewave-inverter-gvfx3524.html

Dave Angelini

Nothing that will sell to the grid from Schneider anymore at 24vdc. He has that BTW. The writing is on the wall for 24V. Use the XW+ 5548 or 6848 if you go 48 and keep all the system coms from the same company unless you like punishment.
They do make some excellent software for the CSW  24 and 48V to xero export or self consume if the utility is mean.

Masterjoe

I do have the XW 4024 hybrid inverter and net metering in place with the electric company. When it gets working again next week I'll have better numbers. Pretty sure I was at 28.6v with the batteries and our best days we were selling 2150watts back to the grid with roughly 750watt usage.

Photowhit

Masterjoe said:

I do have the XW 4024 hybrid inverter and net metering in place with the electric company. When it gets working again next week I'll have better numbers. Pretty sure I was at 28.6v with the batteries and our best days we were selling 2150watts back to the grid with roughly 750watt usage.

If you are setup this way, I would think getting a second charge controller would be very advantageous. More energy to sell!

Perhaps you have East and west facing arrays, about the only reason, and even then I would think it's a no brainer.

Dave Angelini said:

 He has that BTW. The writing is on the wall for 24V. 

I understood he had a 24 volt inverter, until now we didn't know it was capable of selling back to the grid! 

Regardless, if he didn't want to 'waste' the batteries an hopes they will last 8 more years, I suspect you would agree a grid tie hybrid inverter and additional charge controller would pay for them selves. Now that we know he has one, I would suspect you would back me on stating since he can sell the excess an additional charge controller would make sense.

Dave Angelini

I would really not agree that an additional CC would help him until he gets back with data. He can sell a max of 4KW now.
My suggestion would be to lose the XW4024 and go with an XW+ at 48V. My other suggestions I bill for.  
I think he was right at the start of this thread. Go to 48V

Photowhit

Dave Angelini said:

I would really not agree that an additional CC would help him until he gets back with data. He can sell a max of 4KW now.
My suggestion would be to lose the XW4024 and go with an XW+ at 48V. My other suggestions I bill for.  
I think he was right at the start of this thread. Go to 48V

No he can't really sell 4 Kws now as he'll quickly drop below what his array will produce and the voltage will drop below the set points. With 1 80 amp charge controller he would need to limit selling to < what the charge controller can produce in A/C through the inverter less loads. 27.2 voltsx80 amps=2176 watts. Around 2Kws after inverter use.

Since you need to sell your advice, I would hope someone with more knowledge about grid interactive inverters would jump in. I think the sell set point is too high. I would assume it would wait for fully charged batteries then, when it dropped to float sell to the grid? So a sell point slightly below float but above discharge for the battery bank would be in order? 26.8?

Masterjoe said:

Pretty sure I was at 28.6v with the batteries and our best days we were selling 2150watts back to the grid with roughly 750watt usage.

If this is 2150 Watt hours in a day, it seems like a travesty, if it's momentary, that's not bad for the current configuration and I would be curious how long it maintained that before the voltage dropped enough for it to stop selling. 

I went back to long as I thought this system was in use for a couple years. Perhaps not? 

I'm curious if Dave would just have him dump the system, and buy new batteries? or add new batteries and dump the inverter? A second MPPT 80/600 does cost nearly what an inverter would.

I think the current inverter is capable of using the all the power that the array is capable of producing and would suggest adding a 2nd charge controller would be the most cost effective route.

Dave Angelini

Wow having a bad day. Sorry to offend you! I don't need to sell my advice BTW. I am retired and add value to my clients or their money back.  I thought you of all would appreciate that but... I did add that we need data and the current inverter is going to be out of support life which you left out! The Turkeys are fling late tonight 

Masterjoe

Sorry I been away. I for sure will be back with more data when it gets working again in about 9 days. It is a travesty as I spent $27,000 on this and he refuses to put in a second CC. 48V he says is not worth it as I need 4 more panels and 4 more batteries. I don't understand the need for more panels as I am already wasting power. He is more salesman and knows very little about what he installs. Our batteries are charging to 25.8V max. Batteries begin charging at about sunrise for 2 hours ish then go to float and we sell to the grid until sunset. About as much as I remember from the month it worked. Installer says system is oversized so we charge earlier and later. Don't really buy that hogwash. Only married to him for 8 more years lol.

Estragon

If the batteries are flooded L16s, they should probably be charged to around 28-29ish (temperature dependent) volts every couple of weeks. The higher voltage (absorb) charge cycle is needed to mix the electrolyte, which tends to stratify over time with just float voltage.

Some extra panel wouldn't hurt if you went to 2 strings at 48v, but you could probably get by with what you have. For an off-grid home, I'd go for the extra (I have 18x250w panels for one string of L16s@48v) but with grid I think you could get by with less.

Photowhit

It might be that your system 'sees' higher voltage before it gets to float and that you rarely see it at anything but float. Float will be lower than absorb cycle.  Not sure what he's referencing with the 24 v vs 48 volt. The inverters will be nearly the same size output/capacity and the load wouldn't change much if any.

Dave Angelini

It is confusing as the solar panels do not need to be added as the CC is already high voltage and would support up to 6KW of solar with no change other than programing it to be 48V. The Inverter at 4KW could be changed to either a 5548 or 6848, just add money and the correct battery bank.

Joe, you need someone else! Good Luck!