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Off Grid Solar System

P1030615-OptimizedGem Energy has a team of dedicated engineers focused on off-grid, battery backup and hybrid systems.

Off Grid solar system or also known as stand-alone solar systems provide grid like electricity in areas where grid power is unavailable or as a direct replacement to expensive grid power. The system size can vary from supplying power to run a fridge and a TV to supplying a complete island or remote community with electricity. In larger system there will often be a diesel generator integrated to assist in periods of higher than usual demand or to act as a backup for extended periods of bad weather. Other renewable like wind and hydro can also be integrated if the environment allows for this.

Compared to older off-grid technologies, modern off-grid systems are very easy to maintain, are fully automated and can even be online monitored and controlled. Modern Gel and AGM batteries are efficient and maintenance free which means there is no topping up with water or acid. The integrated generator can be fully managed by the inverter / charger which is the heart of your system.

A further benefit of modern off-grid systems is the quality of the power they produce. This power quality is in many cases better then what you would get from your average grid and will extend the lifespan of your appliances.

Gem Energy has over 35 years of in-house experience in off-grid design and works directly with leading manufacturers like Exide, SMA, Selectronic, OutBack Power, Imark and AERL. We cater from small to large systems which supply complete stations or even islands with power. Our engineers have been working on landmark systems like Lady Elliot and the Tuvalu system which was funded by the UN and SMA who are well known and respected throughout the solar industry.

Designing an off-grid system requires a high level of skill and knowledge as well as a good understanding of the customers energy needs. All of our customers are unique so all our systems are custom designed and tailored to suit each individual customer’s needs. Only by doing this can we guarantee that each system will perform perfectly.

As battery systems are starting to get a bit of traction in Australia there are plenty of new suppliers entering the market claiming they can do off-grid but seeing some of the systems we have quoted against we are amazed by the lack of correct understanding of off-grid solar. Incorrectly designing solar power systems can lead to all sorts of problems for the end user and the installer. Many systems have simply been put together with ‘kits’ or off the shelf systems with the incorrect use of the necessary formulas and values needed to correctly design a system. The unfortunate thing is that by the time this will become apparent the system has been installed.

To give you a better understanding of how off-grid solar works we have explained a few things below. This will also help you when you compare our quote with quotes from other companies.

Looking at the design of an off-grid system from a professional point of view, it is recommended to have at least 1.5 – 2 days autonomy in your battery bank. This is IF there is a backup generator present. Without a generator this “autonomy” time is recommended to be increased to 2-3 days. This means that with low or no sunlight during bad weather you will still have enough power to run all your appliances as normal without the need of the generator coming on. The autonomy time is the daily usage times. To calculate the size of the battery bank you take your daily kWh usage and multiply it by the amount of days you would like to have autonomy. For example, if you use 10kWh per day and would like 2 days autonomy you would need to have a battery bank which has 20kWh of usable storage.

A600 DOD chartThe usable storage of your battery bank is the amount of kWh you can take out of your batteries to a set point of discharge (DOD = depth of discharge). The depth of discharge is the major factor which will determine how many years your batteries will last before they slowly start to store less and less power. The recommended DOD state is between 30%-40% which means we are only taking out 30%-40% of the total energy stored in the batteries. By Using Sonnenschein A602 batteries for example, you  would have at least 4500-5500 life cycles without degradation. After this the battery will start to slowly store less power. It does not mean that the battery will die from one day to the other. If a good battery management unit and charger like the Selectronic is used then you will also be able to increase the life cycles and the overall health of the battery bank. To reach a better return on investment (at least in the first instance) we can on request design systems which will take up to 50% out of the batteries.

The last thing we would like to explain in regards to the battery bank is the so called “C” rating. The C rating is the total storage capacity in Amps of a battery. Most batteries are labelled and sold with their C100 rating. However, the C100 rating only applies if the batteries are getting discharged over a period of 100 hours. In an off-grid scenario this would not be the reality and the correct rating would be the C10 rating (discharge rate over 10 hours). If batteries are discharged faster then they have less storage, hence it is important to apply the right “C” rating when doing the calculation for the usable storage. Furthermore, It is common industry standard to allow for at least 10% conversion losses from AC to DC and DC back to AC. Both the correct C rating and the conversion losses are the two things lots of solar companies do not factor in as it lets the battery bank look smaller and / or lots of companies do not have the knowledge of designing an off-grid system correctly. It also reduces the supposed maximum value of the proposal and therefore the ambitious figure is used. This misleading information can mean that you can see a big difference in price between systems. In the end it is the customer who finds out the hard way if the company who sold the system know what they were doing or just tried to make a quick sale.

Depending on how much research you have done so far in regards to off-grid solar, you may or may not know the difference between AC coupled and DC coupled systems. In a DC coupled system the energy flows from the solar panels through an MPPT charge controller and then straight in to the battery. An AC inverter is then connected to battery and takes the stored power from the battery bank when needed in order to supply 240v AC power. In AC coupled system there is an AC inverter (almost like a standard grid connected inverter) installed which will turn the power from the solar panels directly into usable AC power that will get supplied directly to the electrical loads in the house or business without having to go through the batteries. This dramatically reduces the amount of usage the batteries will receive. A bi-directional inverter or so called Inverter-Charger like the Selectronic SP-PRO is then used to charge and discharge the batteries. This bi-directional charger also controls the AC inverters and manages the entire off-grid system. Depending on the generator, it can also automatically start and stop the generator as needed.

Having an AC coupled system has the benefit of less conversion losses and power does not need to flow though the battery bank first. Only at night you would need to draw power from the battery bank. A DC coupled solar system would have less conversion losses when charging the batteries as the power flows form the DC panels straight in to the batteries.  To get the best out of both methods we often design systems with both, AC inverters and DC chargers. Therefore the DC side can fill up the batteries for the night time usage with minimum losses and the AC side will be covering your loads during the day with minimum losses. This method has been proven to be very efficient and gives as an advantage over our competitors. It can also lead to more than a 10% efficiency gain out of your system.

Some of our components explained in detail


Sonnenschein BatteriesFor most of our systems we use Exide Sonnenschein Gel batteries which are maintenance free Gel batteries, especially designed for solar applications and made in Germany. Depending on the model, the expected life cycles are 2800-5500 or 7 – 15 years before they start to slowly deteriorate. This means they don’t just stop working, they will just store lightly less energy after this period. Depending on the exact usage, we have seen this type of battery stay in good conditions for 15 years plus. For Applications where we expect high discharge and fast charge rates we usually recommend the American Absolute battery. These batteries are AGM batteries which are heavy duty and also maintenance free. The life span is marginally less than that of the Gel Sonnenschein batteries but they have other features that make up for this. One of these feature is the mounting rack the batteries come supplied in which saves floor space. We can also BAE batteries which have very simular characteristic to the Exide Sonnenschien range as well as Lithium batteries which would last up to 20 years; however the Lithium batteries would be 3 times the price of Gel batteries at this time. If you wish us to quote on BAE, Lithium or any other type and brand of batteries the please let us know.

Inverter / Charger

We generally use OutBack Power equipment for smaller systems up to 5kw. Above this we tend to use SMA or the Australian made Selectronic products which we have found to be superior to SMA.

SPProThe Selectronic SP-PRO is the heart of the system. This unit will be the heart and brain of the off-grid system and will control the AC inverters as well as charging the batteries and supplying power from the batteries to your house if needed. The unit can also be connected to a diesel or petrol generator which can be automatically started if the batteries run low and there is no charge coming from the solar panels.  The unit will also log any of your consumption which then enables us to fine tune the performance of your system with over 120 different settings, (This is way beyond any other competitor). The unit is 100% developed and made in Australia in Melbourne by Selectronic which has been making inverters and power products since 1964. Selectronic knows and understands the Australian market and has experience with the rough climate. A further feature of the unit is that it produces very clean power. This will be very beneficial to your electronics such as TV’s, Kettle’s, Fridges, everything will last longer. One of our previous clients who is on a SWER line in Central Queensland had to buy up to 3 kettles and 2 microwaves per year prior to installing a hybrid system like this. Since the installation 2 years ago he has not yet had to replace any of his appliances. The last thing I would like to point out about the Selectronic unit is that it is made in Australia and Selectronic is exporting this unit in to the German solar market which to me proves that they must have done something right.

OutBack Power

The OutBack Power has been designed and is made in North Amerika. This product has been proven to be very reliable in the harsh Australian climate and are well known throughout the industry. The outback equipment has less functions then the Selectronic product and is easier to setup and maintain which is one of the reasons we recommend this for smaller systems. It still can be connected to a computer for monitoring and a generator can also be integrated. A further feature is that the units are stackable so you can expand your system capacity as your household grows. As OutBack only supports DC coupled generation, it is recommended to keep the system size under 10kw PV.


Imark is another great Australian brand who design and manufacture there products in Melbourne. Imark was established in 1977 by Owen Smart & Greg Lloyd and has since been manufacturing electronics. Imark’s quality and relatability is due to their continuous investment into research and development. Due to their outstanding quality and features you will find that many of our off-grid systems include Imark MPPT Chargers or Shunt regulators with an outstanding efficiency of 98-99%.

AERL DC Charger

The AERL DC charger is designed and made in Brisbane by the Australian Energy Research Laboratories.  This charger has won multiple national awards for its efficiency and we have seen firsthand the outstanding performance and quality of the product. The LCD touch screen interface makes it easy to check on the production and is industry leading. Again, just like all of our other Australian brands if service or warranty work needs to be done it’s only a phone call away.

How it all works:

Selectronic InverterAs per picture on the left, the Selectronic SP-PRO unit is connected to the battery bank, your house (loads) as well as the generator which can be started automatically by the SP-PRO unit if the batteries are low on charge and there is no sun. The SP-PRO will charge the batteries and take stored energy out of the batteries if needed at night of even during the day when a cloud comes over. On the load side of the SP-PRO we will have all your loads connected. This load side is usually connected to your normal main switchboard which we can supply if needed. The AC Solar inverters will then be connected to your main switchboard as they would be with any other grid connected solar system. The only difference is that the Kaco and SP-PRO will talk to each other via a RS485 network cable.

The SP-PRO will now know how much power you are currently using, how much it needs to charge the batteries and how much power is coming from the solar inverters. If your load is more than the solar inverters can currently produce then the SP-PRO unit will take the extra energy needed form the batteries first. If the batteries are to low then it will turn on the generator if needed.

If all your batteries are charged and the solar inverters producing more power than your loads are then the SP-PRO will tell the Kaco solar inverter to produce less power. If this happens during the day you can see this on the display of the Kaco inverters and you can then turn on extra loads.

In general, the system is fully automated and can easily be monitored locally. Online monitoring is also possible but requires an ADSL or any other type of hard wired internet connection.

The standard process for us is that we will be monitoring the system closely over the first 4-6 weeks so we can make adjustments to the system and fine tune it to your individual needs.


We are fully accredited Selectronic integrator which benefits all of our customers as there standard 5 year warranty is automatically extended to 7 years.

We are also preferred Q.Cells and Phono Solar Partners and are trained REC Professionals.

Our Engineers also have years of experience in SMA off-grid equipment.

Not to mention that we are Electrical contractors and CEC members.


One of our dedicated in-house engineers will be allocated to your project and will design and will be available for you 7 days a week from the quoting process though to the handover of the system itself. The installation itself will be completed by one of our highly trained Electricians.  As mentioned above, the training includes manufacture specific training on their equipment and includes a certification process which increases the warranty on the individual products. Besides the below installations from Armagh Downs in Jericho QLD that was installed by Glenn Saunders, our other head Electrician Peter Libretto has been highly acclaimed for his work on Lady Elliott Island (29kW off-grid) and Tuvalu (46kW off-grid financed by the UN).


Selectronic SP-PRO: http://www.selectronic.com.au/

Imark DC Charger: http://www.imark.com.au/

AERL DC Charger: http://www.aerl.com.au/

Phono Solar Panels: http://www.gemenergy.com.au/phono-solar-panels/

Sonnenschein Batteries: http://www.exide.com/

Q.Cells Solar Panels: http://www.gemenergy.com.au/q-cells-solar-panels/

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