System Voltage

Bruce Clark

We have a 12 volt cottage off grid solar system with panels of about four KW charging capacity, four Outback MPPT charge controllers, three Outback 2812 inverters wired in series/parallel and a 14 KW pure sine wave backup generator.  The system was installed when 12 volt inverters were the only option and it has grown as well as the cottage electrical demands, which occasionally require five or six KW of power.  The Outback system components are about seven years old and the wiring and breakers we have are large enough to handle either voltage.

The system works fine but the batteries have reached the end of their useful life and cannot be revived.  In addition to changing the batteries, I am considering upgrading the system to 48 volts, in which case I would change out the three Outback 2812 inverters for two Outback 3648 inverters, which are the same size and can be installed relatively easily and fit in with the other components.  The charge controllers can be used with either voltage.

To change the system to 48 volts, I'd also have to have someone rewire the panels to a higher voltage, which would take some work but would not be insurmountable.  I have a few questions I'd appreciate comment on:

1.  Is a 48 volt Outback sufficiently superior to a 12 volt system to make it worth the time and cost to upgrade? 

2.  Could we buy batteries for the 12 volt system that could later be reconfigured for a 48 volt system, for example eight 6 volt batteries wired in series parallel then later changed to series wiring for 48 volts?  We have been advised that having four strings of batteries (which would be required for the 12 volt configuration) does not allow them to charge or discharge properly but there may be other possibilities I am not aware of.

3.  If I do change the panel wiring, which will require work on our roof, should I also change the roofing materials for longer lasting materials, for example a steel roof?  The shingles we have are also about seven years old.

Our system is obviously not ideal but really results from being added to from time to time over the 30 years or so we've had it.

Thank you for any advice you may be able to provide.

littleharbor2

A couple quick thoughts,
  Your used Outback equipment still has some value. You should be able to find buyers for your inverter and whatever charge controllers you don't end up keeping.
 As to the charge controllers, you should be able to run all your solar through one controller if changing from 12 to 48 volts.
 Wiring up your 48 volt battery bank, (lead acid) will require less cabling so you likely don't need to invest in more cables.
 I'm sure the pros will be chiming in soon with some deeper technical suggestions. 

Estragon

> @Bruce Clark said:
> We have a 12 volt cottage off grid solar system with panels of about four KW charging capacity, four Outback MPPT charge controllers, three Outback 2812 inverters wired in series/parallel and a 14 KW pure sine wave backup generator.  The system was installed when 12 volt inverters were the only option and it has grown as well as the cottage electrical demands, which occasionally require five or six KW of power.  The Outback system components are about seven years old and the wiring and breakers we have are large enough to handle either voltage.
>
> The system works fine but the batteries have reached the end of their useful life and cannot be revived.  In addition to changing the batteries, I am considering upgrading the system to 48 volts, in which case I would change out the three Outback 2812 inverters for two Outback 3648 inverters, which are the same size and can be installed relatively easily and fit in with the other components.  The charge controllers can be used with either voltage.
>
> To change the system to 48 volts, I'd also have to have someone rewire the panels to a higher voltage, which would take some work but would not be insurmountable.  I have a few questions I'd appreciate comment on:
>
> 1.  Is a 48 volt Outback sufficiently superior to a 12 volt system to make it worth the time and cost to upgrade? 

With your power requirements, a higher voltage system certainly makes sense to me.

>
> 2.  Could we buy batteries for the 12 volt system that could later be reconfigured for a 48 volt system, for example eight 6 volt batteries wired in series parallel then later changed to series wiring for 48 volts?  We have been advised that having four strings of batteries (which would be required for the 12 volt configuration) does not allow them to charge or discharge properly but there may be other possibilities I am not aware of.

IMHO, this is the crux of the issue. If replacing batteries, now is the time to really consider your load needs, and any foreseeable growth. Plan the optimal battery bank for those needs, then figure out what's needed to charge the bank and supply the loads. You could wire a new bank sub-optimally for 12v, but if an inverter upgrade is in the cards anyway (which is likely with even occasional 5-6kw demand), then as LH2 mentioned, it may make sense to get some value from existing 12v inverters, replace them with 48v, and run an optimal 48v bank instead of running a bunch of 12v strings.
>
> 3.  If I do change the panel wiring, which will require work on our roof, should I also change the roofing materials for longer lasting materials, for example a steel roof?  The shingles we have are also about seven years old.
>
Rewiring shouldn't be all that big of a deal. It's conceivable that there may be no rewiring needed at all. If I had panels working okay, decent mounts, and no leaks, I wouldn't get too worked up about reroofing. That said, I don't know if there's something in the string wiring or roofing that might complicate things.

SupraLance

I'm definitely not a pro here, but in hearing that parts of your system are 30 years old, I'm wondering if some of your pv has lost much of it's productivity?  And if it's been added to gradually over the years, how well matched the pv is an important consideration in moving it all over to one mppt.  So I'd recommend including more information about the pv in your post so better suggestions can be made regarding redesigning your system to a higher voltage while making best use of the the equipment you have. 
I wouldn't recommend running batteries at 12v before reconfiguring and then changing the configuration later, I would reconfigure now (quick and dirty if need be and you can safely) and install the batteries from day 1 in series configuration.

Estragon

Also not a pro. 30yr old pv might still be good for 70-80% of initial production, and a 7yr old shingle roof might still have 10-15yrs of life. Absent a reason to do otherwise, I'd aim to replace the roof and pv whenever reroofing became unavoidable. Obviously, if either the roof or pv have issues now, that would change my opinion.

Bruce Clark

Thanks very much for the suggestions, they are very helpful.  I will definitely just wire the batteries in series since this seems to be the best option.  I'll also probably keep at least two of the Outback MPPT controllers since they are properly wired and switched and hopefully a lower load on two controllers is preferable to a higher load on one controller.  

Probably just rewiring the panels for a higher voltage then updating these with newer panels when the roof needs replacement is also the best option.  I can see the wattage of the panels on my Outback mate and it is remarkable that virtually all of the power in from the 30 year old panels (Kyocera 50 watts) is still generated.  However, with the price of panels coming away down and the simpler snap in electrical connections on larger wattage panels, I will replace these at some stage.