A guide to solar inverters for grid tied solar systems
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Our summary of what you need to know about solar inverters
If you know your average power bill and your utility enter it below and the solar panel calculator (which includes rates for each utility) can work out how many kilowatt hours of power this equates to per month. From your address the solar panel calculator also knows how much solar irradiation there is in your area (how many kilowatt hours per year will be generated per kilowatt of solar panels) and so with this information it will tell you the size of system you need in DC peak capacity.
The choice of solar inverter is probably the most important choice you make when buying solar panels for your home;
Whilst most inverters convert power from AC to DC at a similar efficiency when they are working, there is an enormous variance in the reliability (failure rates) of inverters.
There is also a large difference in the way you are treated as an inverter customer if there is a fault between the best and worst inverter companies. A good inverter company will ship a new inverter to you as soon as the inverter reads specific error messages. This means your installer only has to go to your house once to swap it over and your system is back up quickly. Others will require you to take the faulty inverter off the wall and ship it to them so they can either fix or replace it. This generally takes weeks and means more work at your end and a longer system outage.
Always pay the extra for a better quality inverter... in fact, of all my solar tips I give people this is the one I think is most important.
On the issue of "string inverters v's microinverters v's power optimizers" I would say this: If my roof was unshaded and your roof is such that all the panels in a string can be in the same direction and at the same pitch, then I would buy a string inverter. Why? The less electronics you have on a job the less that can fail and I don't like the idea of having 20 inverters on a house unless there is an output advantage. My gut feeling is there are thousands of failed micro inverters out there on roofs where because only one or two panels have failed the customer doesn't. I note that the biggest micro-inverter companies charge extra for panel level monitoring. My suspicion is the reason they do this is so they get less warranty issues if customers don't know that one or two of their panels are out. Installers have panel level monitoring but I would not trust an installer to tell me some of my micro-inverters have failed because its warranty work they then have to do, probably without charge. At least with a string inverter if it fails you notice because your whole array is down.
In cases where shading is an issue, or when panels have to be in many different orientations then there are real advantages to micro inverters and so in these cases I would use them or power optimizers. Power optimizers seek to maximize the output of each panel (as do microinverters) but they are different because they manipulate the DC voltage of each panel to maximize power yield without converting the power to AC as micro inverters do. The downside is that you still need an inverter on the wall at the bottom with power optimizers but the upside is that you have less electronics under each panel so one could assume less is likely to go wrong.
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What does a solar inverter do?
The primary function of a solar inverter is to convert DC electricity produced by solar panels into AC electricity. AC electricity is used by appliances within your home.
The second function of a solar inverter is to maximize the yield of power that you get from your solar panels. To do this it can manipulate the voltage and amperage of DC solar output to maximize the amount of power it produces for each given level of solar irradiation on that array of solar panels. With a string inverter it will manipulate the voltage and amperage of the string output and in the case of a micro-inverter it will manipulate the amperage and voltage for each panel to maximize output.
What is an MPP tracker and how do they work?
A solar panel will generate different voltages depending on the conditions that the panel is operating in namely the amount of solar irradiation available and the temperature the cells are operating at.
For any given voltage, the solar panel will also produce a current (Amps). The amount of Amps that are produced for any given voltage is determined by a graph called an IV curve, which can be found on any solar panel's specification sheet and typically looks like this:
[graph page 33]
Now the one formula you need to remember to understand this is:
What we are trying to do is maximize power. To do this we need to find the point on this curve where V multiplied by I gives us the greatest number.
[graph page 34]
(PS I stole these graphs of my friend Fins website, www.solarquotes.com.au as I was too lazy to redo them. Fin's blog on MPP trackers is more technical than this summary and is a good read).
You will always find one point where the Voltage multiplied by its corresponding Current is higher than anywhere else on the curve.
This is called the solar panel's Maximum Power Point (MPP).
In the example above the MPP is somewhere between where the blue line touches the red line and where the green line touches it.
For the example above that would be about 33V and 6A.
The MPPT forces the inverter to work at 33V by varying the resistance of the inverter input using power electronics. The higher the resistance, the higher the voltage across the solar panel.
If your solar panels are mounted across multiple roof spaces that face different directions or are at different tilt angles you will need multiple MPPT's. This is because each string will be receiving different amounts of solar irradiation due to their different directions or tilts. Therefore they will have different shaped IV curves, and different max power points. So you need an MPPT for each string to get maximum performance from your system as a whole.
What is the most efficient solar inverter?
Inverter companies produce specification sheets for their inverters and test certificates from certified labs that have tested a sample unit at standard test conditions.
Most inverters of the same type report similar efficiencies. For string inverters commonly used on residential homes most report maximum efficiency around 97-98%.
What this means is that at the inverter level in standard test conditions it will turn 1000 watt of DC power into between 970-980 watts of AC power.
Efficiency of solar inverters is not such a big issue because it tends to be that they all have similar efficiency in terms of simple power conversion.
What is the real difference between brands of solar inverters?
As mentioned above there is little difference between brands of inverters in terms of the efficiency at which they do their core functions (MPPT tracking and converting power to AC).
The real difference in brands of inverters is in how reliable the inverter is at performing this function without failing and also how you as a customer are treated when you do have a failure.
To repeat what I have said above I would always pay more for a better quality inverter. If I had an unshaded array all on one roof I am buying an SMA string inverter every time. It is not that SMA inverters don't fail, they might fail one in 50 at my best guess (a best guess based on having installed about 5,000 of them) it is just that when they do they have a new one at your door in a few days, no questions asked (once certain error codes appear).