Everything You Need To Know About Solar (Grid Connect) Inverters

Please note: As you can see from the date of this post – it was written in 2010 – it was accurate then, but now it is out of date – most inverters in Australia are now ‘transformer-less’.

In my “Dummies Guide” to Solar Power I provide a very brief (some may say shallow!) overview of what a grid connected solar inverter does and why.

For those of you who want to know more details about how inverters really work, including the nerdgasm inducing concept of “Maximum Power Point Tracking” (which is all about maximising the power from your panels), then this blog post is for you!

Inverters have 2 main roles in life:

  • Converting constant, Direct Current (DC) electricity from the panels into oscillating, Alternating Current (AC) electricity for your power points.
  • Transforming the low(ish) voltage from your panels to the high(ish) voltage needed by your appliances.

But they also have to:

  • Keep you safe – i.e., the grid connected inverter should shut down if there is a power blackout or a fault with your Solar Power system.
  • Export electricity to the grid (and get paid for it) when your solar panels are producing more power than you are using.
  • and (ideally) do some funky electronic stuff to maximise the power from your panels if, for example,one panel is giving more output than its neighbour

Let’s go through how that box of electronics bolted to the wall manages to do all these things:

a)         Converting DC electricity to AC electricity

Solar panels produce direct current (DC) electricity. This is the type of electricity used in car batteries. However, the electricity we use in our homes for lighting and power is 240 volt Alternating Current (AC) electricity.

The transformation of DC electricity to AC electricity is achieved by use of very efficient electronic switches to alternate the flow of the DC electricity produced from solar panels.

That is, switch one opens and switch 2 is closes and the current flows one way across the circuit. Then switch 1 closes and switch 2 opens and the current runs the opposite way across a circuit. Thus the DC electricity is converted to AC electricity

b)         Transforming the voltage of the electricity produced by solar panels

The transformation of the voltage from the solar panels (23 to 38 volts) to match the voltage of the electricity grid (240 volts) is achieved by using a transformer.

Put simply, transformers have 2 sets of coiled wire on each side of a circuit. The current of electricity flowing through the first coil causes a current to flow through the second coil.

The voltage across each coil is related to the number of coils in each set of coils. The higher the number of coils the higher the voltage. Thus, in a grid connected inverter, there are more coils in the second set of coils than in the first set of coils and this increases the voltage of  electricity produced. The number of coils in the second set is adjusted so the output voltage matches the voltage of the electricity grid (240 volts). See diagram below.

Inverter Circuit

A very simplified inverter switching circuit

c)     Maximising the electricity you get from your solar panels

In order to get the most electricity out of your solar panels the grid connected inverter uses a maximum power point tracker (MMPT).

Each cell in a solar panel has a maximum power point (MPP). This is the point at which the maximum power (i.e. electricity) can be extracted from a cell. The MPP is determined by the voltage and current of the cell, the cell temperature and the amount of sunshine hitting the cell. The MPP for a given cell can vary by as much as 25%.

Thus, to get the maximum power out of a cell we can use a MPP tracker (MMPT) to vary the current and voltage of the cell (using software logic or circuitry controls) so the cell  is operating at its MPP.

Most current grid connected inverters have one MPPT that will seek to get the MPP of a system as a whole. Since individual cells will each have a different MPP (due to differences in manufacturing, differences in shading, etc.) then some cells will not be operating at their MPP. Therefore, there is a loss of efficiency.

Some inverters do have more than one MMPT. These are called Multi String Inverters. This can be useful if you have a system where you might have strings of panels in different situations (e.g. a string facing north and a string  facing west). As each panel will have a different MPP depending on where they are facing, each MMPT can find the MPP for each string of panels. This will increase the overall efficiency of the system.

Multi string inverters may also be useful if you want to connect different panels types provided your inverter can handle the increase power from the solar panels (e.g. if you want to upgrade but your original panels have been superseded).

d)    Safety of your grid connected inverter

All grid connected inverters are required to have certain safety features to protect you, the grid technicians, your electrical appliances and your inverter. These are:

  • A DC isolation switch isolates the inverter from the solar panels. This prevents damage to the inverter or household appliances if there is a fault with the solar panels.
  • An AC isolation switch isolates the inverter from the grid to protect the inverter in case of a fault in the grid.
  • A safety switch that will turn off the inverter and isolate it from the grid in case of a blackout. If this did not happen your Solar System could send electricity into the grid. This, in turn would be a danger to any mains grid technicians that come to check the grid.

e) Exporting Electricity to the grid (and measuring the power output from your panels)

If you want to get paid for the electricity coming out of your panels then you need to do two things: measure the electricity, and export it to the grid (if you don’t use it all in your home).  If you are on a Gross Feed In Tariff then you need a “Gross” Meter that measures all the electricity coming out of those panels whether you export it or not. If you are on a Net Feed In Tariff then you need a meter that measures the difference between what you generate and what you use, because you are only gonna get paid for the exported electricity.

Warning: Make sure that any quote you get includes a new meter and connection of the meter to the grid. It amazes me that anyone can quote a system as “fully installed” without doing this, but a lot of  companies are doing just that. Kind of useless to get a solar power system for your home that isn’t connected to the grid. The worst case scenario is that they connect it to your existing meter which doesn’t know the difference between what you have generated and what you have used, then you will actually be charged for the solar power you generate (not credited) !  Ouch.

About Finn Peacock

I'm a Chartered Electrical Engineer, Solar and Energy Efficiency nut, dad, and founder of SolarQuotes.com.au. My last "real job" was working for the CSIRO in their renewable energy division.

Comments

  1. Hi Finn,

    Great website! Just finished reading your info on inverters and MPPT’s. You say Multi String Inverters have two MPPT’s and as a result improve the overall efficiency of the system.

    I’ve just had installed a triple stacked system (3 individual inverters). Would having 3 MPPT’s make my system even more efficient?

  2. I have a 2.1 kw system.The voltage going to the isolater switch is approx 24 % less than that transferred to the inverter/grid.Why is this loss of voltage ?

  3. How does it work if you have panels on one side of the house and you want panels later on another side of the house? Do you need to upgrade to a two track inverter or can you use two inverters to grid connect? Do the two systems need to be the same size? Will the two systems work against each other?

  4. Hi Finn,

    I run an electrical contracting firm and recently been approached with an inquiry to do electrical installation for a grid connected solar PV plant.
    I am interested to know the following specific technical specifications about utility grade grid-connected solar PV plant (I am talking here about may be a 1 /2 MW generation).
    1) I understand that a group of panels are first suitably connected in series (strings) and each one attached to a string inverter that will produce AC power output.
    What next? I want to know how these strings are integrated to develop large power output?
    What will be the V, A power output rating that will go out of the PV plant and connect to the grid?
    Can you please share a schematic / one line diagram that describes this?

    2) Are there any technical documentations / links / pdf that can provide me above information as well as applicable safety rules / stds to be followed while working in solar PV field?

  5. Gilbert Griffith says

    I have a problem with the inverter cycling on and off by itself.
    The input voltage from the panels is normally 570v no load, and when under load it sits on about 430-440vDC input.
    Watching the display as it switches on the input goes down as low as 418vDC and the output goes up from a typical 246vAC on the mains to 257 or 258vAC output of the inverter, coinciding with an output of around 8 amps, at which point it switches off for about 30 sec before starting again.
    I was wondering if this is caused by a bad connection of high resistance somewhere in the solar array or isolators, or something in the inverter that is allowing the output voltage to go high enough to trigger some sort of safety cut out. The inverter is a Diehl Platinum 4300 S.
    I don’t even know if there is a safety feature such as this, but if it’s not programmed right I assume that this could happen, especially if there are a lot of customers feeding into a badly regulated phase on the AC supply lines.

    • It may happen if the out voltage goes high due to another inverter nearby on the same phase. Try to connect the inverter to another phase. Or try to program your inverter to a higher AC disconnect voltage if distribution company permits.

  6. Gilbert Griffith says

    Found some info about these inverters here,
    http://www.go-greenenergy.co.uk/products/datasheets/Diehl%20Test%20Report.pdf

  7. Gilbert Griffith says

    Or maybe a high resistance connection from the house to the mains supply?

  8. Gilbert Griffith says

    Problem solved. High resistance connection on private pole fuse box on the neutral line. The cable clamp was a bit loose and had overheated melting the plastic support. Power company tested it Saturday and didn’t find anything. I could smell burnt bakelite on Monday morning and they came and it was replaced.
    I suggest that if you are having this type of problem, a simple test with a voltmeter on the mains then switch on a couple of kilowatts of load like a kettle or oven) and take note of any voltage drop.
    Anything over 2 volts or so is worth investigating. Mine was 10 volts drop with solar system disconnected and switching on two ovens at once. After the repair it is only 1 volt.

  9. Mark Severn says

    I’m looking out for an Inverter/controller for a solar installation in an area that is characterized by brown outs or sagging voltage. The property owner would like to get a solar system that would top up the mains voltage and maintain a constant 230Vac ie the installation is using both mains and solar power at the same time. Is there a controller on the market that is suitable for that sort of application?

    • I’d recommend getting a separate voltage regulator/power conditioning box. There are some pretty good ones around on the Aussie market that perform just this function.

  10. This article is wrong in many ways and in fact dangerously so!

    Inverters in Australian are NOT transformer coupled. (Only in the US)
    Hence there is not transformer isolation between Panels and the Grid.

    This is extremely important issue regards installation practices and Inverter selection!

    PV voltages and other stuff woefully misleading as well.

    • Gary. This article is dated 2010. It was accurate when it was written. I’ve added a note at the top – saying it’s an old article and out of date.

  11. Daniel Kolinjim says

    I have a off grid solar system that has 3 x 250W panels, a 3 KW inverter and 2 x 200Ah batteries. I need to run some equipment during the day without having to use the power from the batteries. At nigh I can use the power from the batteries. Can some one advise how to do the connections so that I can realise my needs?

  12. Hi Finn & Readers,
    very informative place to learn from all these posts. Thanks all.

    We’ve had a 3kW system since 2011, originally getting the Brisbane 44c, now lifted by Mains Supplier to 60c / kWh from about late 2016.

    Over the last 5 years, we’ve cooked 2 Inverters !! ( we think from poor mounting and lack of ventilation and overheating) ….all that’s another long story, for later on.

    My Question :
    1.My Wife and I had a “strong” debate about whether a normal functioning Solar System will supply the House as a default, when the Mains goes out ?
    My Suburb seems to get power cuts sometimes, …if my cheap Chinese Inverters “tried” to supply the House through the day, ( during a Mains outage)..would this stress them too much and cause them to fail ?……
    2. that’s “if” Grid -Tie Inverters actually had this ability ??,
    3. or do they shut off the Solar Supply when the Mains goes down too ? If Inverters had the ability ? to supply some Solar power to the House, during the day, with the Mains off, how would a home owner know the mains was off?…a good LCD display ? with comprehensive info on 240v direction flow ? IN / OUT / Solar / Mains etc ( not much info on a Cheap Inverter screen) …now checking out the SMA range.
    always keen to keep learning
    regards.

    • Ronald Brakels says

      Hello Brett, Ronald here.

      A normal solar system will not produce any electricity during a blackout. As soon as the inverter detects that the grid is down it shuts itself off. All inverters that are permitted to be installed on-grid do this. It is a safety precaution to protect line workers that may be attempting to fix the grid.

      With a multimode inverter (also known as a hybrid inverter) it is possible to disconnect the house from the grid (a switch will usually be installed to automatically do this during a blackout) and run the house using solar power from the panels. But without also having batteries and/or a generator this is difficult to manage as the amount of solar power produced will change through the day and a cloud can greatly reduce output.

  13. Hi,
    I have Solax X1 Mini inverter in a grid tied inverter-solar rooftop system.I like to know how to set up or change the inverter parameters for uninterrupted supply preferably at a constant voltage ,say 230 or 240 V+/-10%.I often find inverter shows a grid volt fault error shutting down the solar power export.How to resolve such issues ? Is it possible to adjust the invert parameters to resolve the issue?If so,what parameters are to be adjusted?
    Thanks.

    • Ronald Brakels says

      Hello Karan.

      The voltage settings that can be used by solar inverters are determined by the Distributed Network Service Provider (DNSP) in your region. Your inverter should already be set with the limits they require and it is not permitted to have an inverter connected to the grid that goes beyond them.

      If your inverter is shutting down or reducing output to the grid because of local grid over voltage you can complain to your DNSP and they may do something about it. If your neighbors have similar problems it would probably be a good idea to ask them to complain as well.

      Some inverters will give many messages that they shut down or reduced output, but the interruptions were only for short periods of time, or the reductions weren’t large and didn’t last long, so you may be fortunate and not losing much solar electricity.

  14. Hi,

    In theory only, if you connected a grid tie inverter through into a powerpoint from a solar system that’s making more power than the current electricity usage of a home on an old style meterbox, could the meter spin backwards getting a one kw for one kw ratio/discount from your power company? Haha.

    Also not sure if the house/powerpoint wiring would burnout?

    Maybe not practicle in reality may it work in theory?

    What’s your opinion in theory? (obviously don’t try it out, lol)

    Will help settle a debate!

    • Ronald Brakels says

      In Germany it is apparently legal to plug a small solar system, that is designed for this specific purpose, into a home or flat power point and benefit from the meter running backwards when its output is higher than the household loads.

      When a house with an old style analog meter gets solar in Australia, the analog meter will spin backwards if the system is turned on or tested before the old meter is replaced, if the solar system is producing more power than the home is using. It would do the same if the current from the solar inverter was fed into a normal power point, but too much would be bad and it would perhaps set the house on fire. But provided the power was kept within what the wiring could handle, an analog meter could be made to spin backwards if household loads were low enough.

      You are, of course, very correct that this is not something that would be a good idea to actually do.

  15. Peter Gorring says

    I have a small 1.5kW solar array and inverter. The smart meter has only the same two wires (single phase) that supplies power from the grid that I am billed for and also measures the power I am generating back into the grid. The only 2 ways I know this can be done is increase the voltage or the frequency of the generating force. this is of course clamped by the grid as a battery would.
    I have never been happy with the generated amount stated by the supplier as it is always much less than the readout on the inverter.
    Question is the generated amount stated by the power supplier an accurate measurement of just an estimate?

    • Ronald Brakels says

      Hello Peter

      Your electricity retailer only knows how much solar electricity you have exported to the grid. This will be what is generated by your solar system minus what your household uses itself. Normally an inverter will only show the total amount of solar electricity generated and not now much is exported. Hence the discrepancy. Your meter should be over 99% accurate and it is unlikely to be faulty.

  16. Sujeewa Abeytunga says

    I am going to install 3.5 KW solar system.My inverter having two MPPT inputs.I am going to install 10 x 310 W solar cells for this inverter. The inverter starting voltage is 150V – 530 V.
    so If I use both MPPT inputs the voltage at each MPPT will be ~ 150 V.
    Is this problem to start the Inverter?

    • Ronald Brakels says

      Hello Sujeewa

      A starting voltage of 150 volts is high for an inverter. Usually it is 80 or 90 volts. I would check that is the starting voltage and not the “Full Power MPPT Voltage Range”.

      If the starting voltage is 150 volts then you are likely to have a problem with 2 strings of 5 panels each. Options would be to put the panels on one string if they can all share the same orientation or add more panels to each string.

      If you can contact the manufacturer of the inverter they should be able to advise you.

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