Choosing the right Off Grid Solar Inverter

    The Solar Inverter is an integral part of the entire power system for both Grdi Connect and Off Grid solar solutions. The difference in function between Grid Connect and Off Grid solar inverters are significant, as such we will only be discussing Off Grid Solar inverters and their typical prices here.

    Before we begin I want to just say, Grid Connect inverters cannot be used as the inverter for an Off Grid Solar solution.... so ltes leave the rid Connect solar inverters behind.

    If you want to understand what the inverter does and how it fits into the overall Off Grid solar solutions, see the intro into Off Grid Solar Design.

    Decision #1:

    Do you go with Pure Sine wave or modified Sine Wave?

    What does this even mean? The picture below shows the difference in the output of both types of solar inverters. A modified sine wave solar inverter tries to make the AC output look like true AC mains power by producing it in steps. While a pure sine wave solar inverter produces output exactly as mains power looks (well its usually even better than mains power).

    modified sine wave and pure sine wave outputs from solar inverters


    The choice is not really a choice. Modified Sine wave off grid solar inverters are very cheap, but they are not very good if you want to power lights and electronic equipment like computers and the like. We do not stock modified sine wave inverters because as far as I am concerned it is old technology and their is no real place for them.

    I remember a farmer asking me one day why his brand new inverter would run his water pump in the house, but it wouldnt run his fluro lights... response "the inverter was a modified sine wave inverter."

    So the moral to this little story is you want your Off Grid Solar inverter to be Pure Sine. 


     Decision #2:

    Does your Solar Inverter need to act as a battery charger as well?

    There is usually a bit of confusion around equipment terminology when you first start looking at these things. An Off Grid Solar Inverter has the job of converting DC power coming from batteries (or some other storage source) into AC power (like the power we get from mains electricity).

    In recent times the inverter manufacturers have added the ability for solar inverters to act not only as an inverter (converting DC to AC electricity), but they can also act as a battery charger. If an inverter acts like a battery charger then it means there is a way to plug a generator into it.

    Why is it handy to have your inverter act as a battery charger? The main reason is cost and convenience, you wont need to buy a dedicated battery charger.

    No Battery Charging..

    These off grid inverters are a lot cheaper than the others because they are simpler. They also tend to be a lot smaller in size, again because of the reduction in complexity. The Cotek brand of inverters are an excellent Pure Sine Wave Off grid Inverter that do not have a battery charging function.


    With Battery Charging..

    All of our Off Grid solar designs by default will use a solar inverter that has a battery charge function built in. The correct term for the solar inverters with a battery charge function are "Inverter/Chargers". These Inverter/Chargers are highly intelligent machines; you plug a generator into the Solar inverter and turn it on. The solar inverter will automatically detect the generator and start charging batteries as well as supplying power to the house. The solar inverter/chargers we use can even be told how much power to take from the generator. So if you have a 6kVA generator but for some reason only want to use 3kVA for battery charging. This can be done.
    When it comes to Solar Inverter/Chargers we use Outback inverter/Chargers, they are built for rugged climates like here in Australia, and they are almost unbreakable.



     Decision #3:

    Do you want your Solar Inverter to Auto Start a Generator?

    WHAT!!.. why would that need to happen?

    This feature is the key to system autonomy. Lets go through an example:

    "You have a 3kW Off Grid Solar inverter and it works beautifully.. Its a cold winters night (snowing outside) and you are entertaining. Unbeknown to you your batteries have almost been drained because the Juke box has been hammering all night."

    What do you want to happen next?

    Option 1: The Solar inverter shuts down due to its low battery cut off and the lights (and music) all go out. You have to head over to the shed in your best going out clothes to start the generator so that power is restored.

    Option 2: Your Off Grid Solar inverter has an Generator Auto Start feature, and the generator is wired in, so the inverter tells the generator to start up and run. The generator starts, the inverter switches over to the generator, which leaves the generator to start charging your batteries and keep power on at the house. Best of all, this all happened automatically. 

    All of our Outback Inverter/Chargers have a generator auto start feature.


     Decision #4:

     Do you need built in redundancy?

    I promise this is the last decision.. its also the hardest to understand.

    Let me ask you a question? Its 6pm on a cold winters night, its tea time, the kids are in the shower and you are in the kitchen preparing for the evening meal. How important is it to have electricity in your house? I would say very important.

    Redundancy is a way to minimise the risk of equipment failure. If your entire premises is being run by a single 6kW Off Grid solar inverter and it decides to blow up (because even the very best equipment will sometimes fail) what are you going to do? Wouldnt it be better to have two 3kW inverters working in tandem, when one fails the other just automatically takes over the job.

    This technique of "redundancy" reduces the risk of Off Grid power outage to being so small it becomes insignificant. The Outback Inverter/Charges have a feature called parallel operation and it works like this:

    If I have two 3kW Outback Inverter/Chargers (as an example) wired together for redundancy the following happens:

      • One inverter will do all the work most of the time assuming you dont need any more than 3kW of electricity.
      • If the house requires more than 3kW, the first inverter tells the second one to start up and help out.. giving the house a total of 6kW of available power.
      • If the first inverter dies for some reason, the second inverter can take over the role of supplying power to the house.

    All of our Outback Inverter/Chargers have a parallel operating mode which provides the safety of redundancy.



    Well thats about it when it comes to the key decisions that need to be made when buying an Off Grid Solar Inverter. The only other decision is about quality; There is no such thing as a good quality "Cheap" inverter. When it comes to good quality Solar Inverters the price is directly related to the amount of functionality available in the component, thats why the Cotek Inverters are cheaper than the Outbacks, they are both good quality, but one has a lot more features.

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