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OffGrid system - design options

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SolarSpinout

New Member
Joined
Dec 10, 2023
Messages
3
Location
Muswellbrook, NSW, Australia
G'day All from Down Under!
Have taken the plunge to go Solar on our small 5 acre patch. Have mains connected - 240v single and 415v 3 phase.
Wishing to power our home and a second smaller dwelling. We use atm 17 to 20 kW/day
Need to run the usual host of appliances, as well as our household water pump.
Air con ... maybe. It was 101 deg today and will be near that the next week or so.
I am prepared to leave mains connected for the ($350/yr connection cost) convenience eg charging batteries if needed / workshop machinery.
Have secured 24 x SunGEL Ultra carbon batteries (2SGU specs at batteryenergy.com.au/products/sungel-ultra/technical-overview
Also 60 new 415w PV panels.
QUESTION 1 : Setup as 24v or 48v? I am getting so many contradictory answers / reasons
Q 2 : How big an inverter / s
More info needed? I will see what I can do :)
Hoping to get things a little clearer in my mind!
Thankyou in advance.
John
 
48v for that amount of power.
Inverter is sized for max load, not average load.
Don't know if you can easily convert 3 phase to single phase. May need two inverters. So figure max load for each.
 
Stuck with 12 at the desert nest for the moment. Monthly solar budget is geared toward 48v.
When I was sent to work on 3 phase without training at the power plant took my other option and retired.
 
SolarSpinout said:
G'day All from Down Under!
Have taken the plunge to go Solar on our small 5 acre patch. Have mains connected - 240v single and 415v 3 phase.
Wishing to power our home and a second smaller dwelling. We use atm 17 to 20 kW/day
Need to run the usual host of appliances, as well as our household water pump.
Air con ... maybe. It was 101 deg today and will be near that the next week or so.
I am prepared to leave mains connected for the ($350/yr connection cost) convenience eg charging batteries if needed / workshop machinery.
Have secured 24 x SunGEL Ultra carbon batteries (2SGU specs at batteryenergy.com.au/products/sungel-ultra/technical-overview
Also 60 new 415w PV panels.
QUESTION 1 : Setup as 24v or 48v? I am getting so many contradictory answers / reasons
Q 2 : How big an inverter / s
More info needed? I will see what I can do :)
Hoping to get things a little clearer in my mind!
Thankyou in advance.
John
Click to expand...
Welcome to the Forum John, glad to e-meet you from the opposite side of the globe.
You have a very large solar PV planned for a smallish daily load. 60 x 0.415 x 5 sunny AU hours per day = 124kWh per day ie five times your expected loads. It may be best to run your local area through a PV calculator and see what size array you need on average.
Inverters are picked based on: max instantaneous load you may need (starting amperage for motors, well pumps A/C) and what voltage/phase you are using. I am no expert with 3-phase, but in my system the few 3-phase motors I have are run off my single phase inverters using VFD's and this works for me. Not sure what 3-phase loads you run, and how often. More info would be helpful.
For 20kWh per day system you will want 48volt DC, not 24v.
 
This is a good example of newbie mistakes, buying stuff before having a design plan. The answers to all your question are there in just a few simple math problems. Let's start with the panels. Assuming your battery starts charging at 25V, then (60 panels X 415W)/25V = 996A of charging current. There is no single charge controller I am aware of that can handle anything like that amount of current. So, 48V. Even then, that's 498A. Even with 85% de-rating, that's 423amps.

So, you will NOT get to use all of these panels in one installation. What is the second dwelling? Workshop, or maybe a mother-in-law cottage?

Since you are in Australia, I'd suggest going with Outback. Take a look at their FlexMax 80A, and FlexMax 100A controllers. I suspect your 415W panels are high Voc, maybe 50+Voc, so I would go with the FlexMax 100.

Let's say you divide up the panels between the two buildings, maybe 2/3 and 1/3, are even less. With a 100A controller, that means you can handle at most (100A X 50Vcharging)/85% = 5882W of panels. Maybe 15 of those 415W panels. Maybe 3 parallel strings of five panels in series. Here is where the panel specs become very important, along with your winter lows. Assuming each panel puts out 50Voc and 5 in series gets to 250V, at 0C (freezing) the panel string might bump to to 280Voc?

At 33 degrees South, I'd say you are likely to make 3.0sunhours in June, and maybe 6.0sunhours in December. So, with that single 5S3P array, you are likely to make 18-20kWh of power in June, with maybe 36-40kWh in December.

One way to utilize more of your panels is to put additional series strings facing due East, and also due West, in addition to those facing North. Let's say two strings facing facing East, three facing North, and another 2 facing West. That should keep an air-conditioner running without any battery depletion till 6-7pm in the summer.

Now, the batteries. They are each 2V. What the specifications did not say is their amphour rating. Are they the 2SGU1000 batteries, the 2SGU1400 ones, or the 2SGU2000? Assuming they are the 1000Ah batteries, and you want to charge them at 1/10th of C, then one FlexMax 100 could handle that. With the larger batteries, you might get better performance dividing the current between two or three FlexMax 80s. You would have to reduce your series strings to maybe 3S5P to handle the 80's 150V limit. Again, we really need those panel specs. Since you need all 24 of them to make a 48V bank, you'll need to buy additional batteries for the second dwelling, if you want a stand-alone system there.

An excellent inverter for you is Outback's Radian 8000. It's a very high-quality low-frequency transformer-based inverter, which means it can handle the starting surges of your air-conditioner, and your water pump.
 
MichaelK said:
At 33 degrees South, I'd say you are likely to make 3.0sunhours in June, and maybe 6.0sunhours in December. So, with that single 5S3P array, you are likely to make 18-20kWh of power in June, with maybe 36-40kWh in December.
Click to expand...
I put in John's location Muswellbrock NSW, into PV watts and it pulled up a local monitoring station within 1km of this city.
(I expect Australia has other online solar calculators, just used the one I am familar with for an example)
Pugging in his proposed 60 x .415 = 24.9kW array, resulted in monthly totals from 2558 to 3678 kWh (about 85kWh per day avg on worst month, 122kWh per day during best month)
For a "20 kWh per day" load, this would mean most of the collected solar is wasted, unless sell back is possible and attractive to the OP, or they have plans for a couple of EV's charged on this system one day.
 
@SolarSpinout - don't take the comments personally, we are all here to help you:
Looking at the loads/PV in the more usual way, starting with loads and calculating the PV actually needed:
Say 20kWh/day x 1.25, x 31 days per month = target 800kWh/month.

Plug into PV watts for OP location 1kW array to see output = 103kWh/mth worst month, so needs about 8kW array.

I would recommend looking at installing maybe 10-12kW of the PV (since it appears he bought these panels already) check what will be easily connected to the inverter he ends up choosing - we need some more info to assist with inverter selection - especially the 3P or 1P requirements.
He could look at selling the exess PV to recover some money, or adding other loads (like selling to the grid, EV's or workshop or future plans, or storing half of the panels) but it certainly appears that the 24.9kW in panels is far too many for the reported loads, hope they were a bargin.
 
Thankyou for the fast replies!
I wont take anything personally!!
I had a designer give an outline for a 'bullet proof' design set up. ie to run offgrid,with no 240v mains or genny backup.
As such, no fancy wiFi / monitoring etc - just Robust components to give problem free operation with a biggish built in 'safety' factor.
His own system runs some 35kW of panels and battery storage some 20% bigger than what I have. 24v system.
Both PV panels and batteries were purchased opportunistically
The panels were purchased 'new' as a 'bulk' lot from a testing facility. Ended up costing $0.21c/W landed.
I have a lot more than the 60 I had set aside to actually use.
The 60 set aside for my use all have Vmpp 31.5v and Impp 13.2 - or extremely close.
The PV panels do still have a Govt STC rebate available once installed (if I ensure ALL components comply) which is worth approx $160/ panel :)
ie should end up costing me NIL $ for the panels ... I am hoping. Worst case, they are still low cost panels.
I knew I would have too many and figured on re-selling some. They are in lots of 12 panels per brand
The batteries are the 2SGU1400s. Came from a Telco remote facility - 4y.o. New they are approx $1600ea. All up, delivered (all 4000lbs of them) came to $410 ea.
Basically, I secured these components NOW as they are not often available .... or are hundreds of miles away.
Was looking at mounting panels in E, N and W facing directions.
Re the mains currently connected .... ? keep connected as a standby in the event charging is required?
Gives an option for Inverter / charger combo v inverter + charger
The two buildings are presently connected to the same mains meter box - not individually.
Power useage WILL increase when we occupy the main dwelling. The smaller one (1200sq ft) becomes a 'Granny flat'
I have no intention to Export surplus, but aim to have sufficient power thru 4-5 days of rainy weather - even if it means cutting back to 'Eco' mode!

Hope this fills in some of the gaps.
FYI - most grid connect system here are in the 10kW area, only a small portion using batteries.
 
OK, if your batteries are the 1400s, then that changes the math a bit. The standard rule of thumb for lead-acid batteries is that they "like" charging most at 1/10th to 1/8th of C. Since C in your case is 1400, then the resulting charge rates are 140A and 175A respectively. That is a LOT of amps. You might want to contact the manufacturer directly to find out what their optimal is.

No single controller on the market that I am aware of can handle those kinds of amps. So, what you will need to do is purchase multiple controllers that can be linked together in a slave/master configuration to sync their charging profiles.

Be careful here. Do NOT skimp on your charging rate, because that is where most people kill their batteries, via inadequate charging. Going cheap on the electronics is going to be a serious mistake in the long run. So, that means at least two of the Flexmate 100s, or maybe three of the 80s. Remember that your string voltages will change going from one model to the other. It will probably be easier to wire up two of the 100s because you'll end up with fewer solar strings.

I don't know what code is in Australia, but here in the US, more than two solar strings need fuse/breaker protection. If you panels have a Vmp of ~31V, then I'd expect the Voc to be ~ 39-40V. So that means maybe a six panel string will work best for you. What are your winter lows like? Your area goes below freezing in winter?
 
SolarSpinout said:
@MichaelK - we are only at 400', so winter is mild here. Coldest overnite we would get be 20deg F, but usually between 30 and 40 would be our min. overnite
Click to expand...
Using Midnight Solar's quicky conversion factor of 1.12X at 32F/0C, this is what you would get with six or seven panel strings, with a Voc of 40V.

40V X 6 panel string X 1.12X = 269Voc
40V X 7 panel string X 1.12X = 313Voc

So, six panels in series might be OK down to somewhere below freezing, assuming the Voc is 40V.
Seven panels in series will exceed 300V and possibly destroy your controller at some point somewhat above freezing.
So maybe a series string of five to guarranty a completely worry-free installation.

To get a more accurate voltage prediction, we really need the Vmp and Voc of your panels. Take those numbers and plug them into Midnight's solar string calculator

MidNite Solar - Classic Sizing Tool.

MidNite Solar is the industry leader and manufacture of quality Renewable Energy System electrical components and E-Panels.
www.midnitesolar.com

For now, start planning on no more than five to six panels in series, and two FlexMax 100 controllers. No more than two or three strings per controller. Three if you keep the series string down to five panels.

Whether or not you'll need one Radian inverter or two will depend I think mostly on what kind of air-conditioning you have.
 
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