Off-grid solar south coast NSW

I need to install an off-grid system in NSW (south of Sydney) that will power a very large house. Grid connection is not available so I need to relay 100% on the solar system with a back up generator. I’ve received a quote from a company that has been very helpful in working out what I need but I want to make sure the quote seems reasonable before I go ahead.

We’ve worked out that I will use approx 50kWh per day. Its important that I get a system I can add to down the track if needed. Ive been given a few options:

Option 1. $110,000
AC Coupled - 48v, 21.08kwp, 27kVa Victron Controller, 69kwhBYD lithium storage

  • 5x BYD B-Box 13.8 LV PRO (IP20) Lithium -ion (LFP) battery unit (69kWh: 92% usable)
  • 68x Trina Solar TSM-310 HoneyM Plus monocrystalline (BF) solar panels (21.08 kWp)
  • 2x Fronius Primo-8.2-1 Solar inverter

Option 2. $140,000
Optimised AC Coupled - 120v, 21.08kwp, 20kVa controller, 66kwh Powerplus LFP lithium storage

  • 20x PowerPlus: High Performance 120V rack mounted L-ion (LFP) batteries inc. 800/600/1950 IP21 20 module cabinet (66kWh: 100% usable)
  • 68x Trina Solar TSM-310 HoneyM Plus monocrystalline (BF) solar panels (21.08 kWp)
  • 2x Fronius Primo-8.2-1 Solar inverter

Do these prices seem reasonable? Which system would be best? Is there any way we can keep costs down without sacrificing quality and that will still provide us with enough power?

Unfortunately these quotes are more than double what we originally allowed for in our new home budget and we struggling to find the extra. With Option 2 (above) we can do it in 2 stages which may help. We will initially be living in a shed while the house is being built so wont need as much power to start with. Stage 1 includes everything except for 10 of the batteries and is $96,000. Stage 2 adds the other 10 batteries and is $48,000. After we move into the house, the solar system will stay on the shed but power both the house and shed.

What are your thoughts?

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First of all I must say 50kWh per day is huge? I’m assuming this a home and business operation? The size and cost of an off-grid system is directly related to the amount of daily energy consumption.
With this amount of consumption you will also require a back-up generator, have you discussed this as well?

Those prices are very reasonable for the size of the system. The batteries alone are worth around $50,000 plus you have the huge 20kVA + inverters and large solar arrays. There is a lot of specialist labor which goes into installing and configuring off-grid power systems so it’s not cheap.


Thanks for the reply, Jason.

Yes, it is a big system as we have a big house and run a business from home in a large shed. We’re currently using up to 70kWh per day but the new house is being designed to be much more energy efficient so we think we can get that down to 50kWh.

When we first set out a budget we were advised a large off-grid system would cost around $50k but at that stage nobody (including us), had worked out just how big it would need to be. That’ll teach me for not checking it out earlier.

We’ve only been quoted by one company and they have been very helpful so I was just looking for an opinion on the fairness of the price before we go ahead. Sounds like its reasonable so we’ll get moving on it.

I would advise to have more panels, Generation power is essential, solar panels are not expensive. I would say 30kW of panels or more.
You can have less batteries, battery wold be easy to add later if needed.
Panels as well will save you on generator fuel.

And one more, I believe you still can be with in $50k may be 60max

Would there be any problem for the grid inverter if excess power generated is not in use? For example, if the AC couple inverter that charges the inverter shut-down when the battery is fully charged, and the consumption is less than supply. What will happen to the un-use power? Can it damage any of the inverters?

Hi @Ashland

No this would not damage the inverter, this is a pretty standard situation in off-grid systems. I‘m now really sure what happens to the excess power that the panels are producing though to be honest, maybe it just gets earthed? Hopefully someone can tell us.


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Off-grid systems (AC coupled) are able to ramp the solar inverter’s power up and down to prevent over-charging the batteries when they are full, or the load is low. There are a few ways this is achieved…

  1. Some systems use direct communication between the main battery inverter/charger and solar inverter such as Selectronic and ABB/Fronius. This is an extremely reliable and robust way of controlling solar output.

  2. Other systems use a more complex way of ramping down power output is using ‘Frequency ramping’. This is where the solar inverter is programmed to react to changes in the frequency. The master battery inverter/charger is able to slightly tweak the frequency from 50Hz up to 51 or 52Hz and the solar inverter responds by ramping down the solar output (SMA and Victron both use this method). I’m not a big fan of this as it can cause issues with sensitive electronic equipment.

NOTE: DC coupled systems using solar charge controllers are the most basic. The charge controllers simply stop producing power when the battery is full and ramp up power output when there is energy being drawn from the battery (as the battery voltage drops more with higher load they increase power output).

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That number one is better and easier.

I just found this information regarding excess power that is not consumed or export;
It is from SMA website, and it shows the string inverter will decrease the generator’s (Pv) power as soon as power build-up and frequency increase to certain percentage of the setting.