Solar panels of different power classes cannot be mixed in a string inverter-based solar system because the current of the string will be limited by the lowest current panel. This can lead to a significant loss of power.
The most flexible solution to overcome this issue is to use Module Level Power Electronics (MLPE) in the form of either microinverters or power optimisers. MLPEs allow each panel to work independently, providing the most power it can.
Ultimately, the decision of whether or not to use MLPEs depends on the specific needs of the solar system. If there are panels of different power classes, or if the system is located in an area with shading, then MLPEs are a good option. However, if the system is simple and straightforward, then a string inverter may be sufficient.
Can you use panels of different power classes in one solar system?
In Australia solar panels attract a rebate and overall have become very cheap. Many people can not be bothered to re-use old solar panels and many unfortunately end up on the tip. This is despite that face that a 175W panel in 2007 did cost over $1,800 per panel, and most likely still works fine.
When looking to design a solar system, you would usually use identical panels. However, there may be circumstances, like an off-grid system in the bush where you want to mix different kilowatt solar panels. While this can be done, it is not necessarily straightforward.
Why can you not mix panels?
Firstly, it’s worth understanding why solar panels of different power classes or specifications cannot be mixed. If we look at string inverter-based solar systems, a set of panels are connected in series or a daisy chain format together. Each panel has an operating voltage and current. Identical brand and model panels facing the same direction should have very similar voltage (V) and current (A). The power (W) that the panel creates is a simple multiplication of Volts (V) x Current (A) = Power (W). In the below example, this works out as 30v X 12A = 360w of power per panel.
What does this mean for your PV system?
To work efficiently in a string system the panels work together to generate full power. In the series as per the example below, each individual panel is operating at 12A and 30v.
The voltage of the panels is added through the string (7 X 30v = 210v) while the current (A) stays constant. So the power of the full string of panels would be:
- (7 X 30v) x 12A= 2520w of power.
When discussing electricity, it can be useful to think of it like a garden hose. The current (A) is the diameter of the hose. The voltage (V) is the speed of the water going through the hose and the power (W) is how much water comes out of the hose.
If we then look at the situation below, where we have a panel with a different power in the string. When the first 6 panels are operating at 30v X 12A = 360w per panel, the last panel on the right is operating at 30v X 8A= 240w
While 6 of the 7 panels are operating at 12A, the final panel at 8A acts like a ‘kink in the garden hose’, restricting the electricity from all the panels to 8A.
This, therefore, restricts the power of the string to 210v X 8A = 1680w, which is a drop in power for the whole string of 33.3%.
In a case where there are panels of different current characteristics, the complete series or string of panels will be limited in performance to the lowest current from all the panels. As such this can have a significant effect on the overall performance of the complete string.
So, what should you do?
To overcome the performance losses or issues with mixing different kilowatt solar panels in a string, the most flexible solution is the use of Module Level Power Electronics (MLPE) in the form of either microinverters or power optimisers.
The most popular of these are Enphase for microinverters or SolarEdge and Tigo for power optimisers. As per the very rough sketch below, by connecting the panels with an MLPE, you are effectively taking the panels out of a pure in-line series and allowing each panel to work independently, providing the most power it can.
Therefore, the smallest or least-performing panel does not restrict the performance of the bigger more powerful panels. While there are 3 different types of panels in the 7 panels shown below they are all able to provide all the power they are generating without being restricted by the others.
As the rebates for solar panels end in 2030, hopefully by this time older lower wattage perfectly working panels are being re-used in higher number than today. In such a circumstance, you can mix different kilowatt solar panels one solar system, as long as you get optimiser or micro-inverter technology.
The one word of warning if you consider going down this very sustainable path of re-using panels.
Visually such systems can look very unsightly and uneven, as panels over the years have constantly evolved and changed size and look. Also modern panels are more fire resistant than older models, so make sure the 2nd hand panels you purchase were CEC approved at the time and have the appropriate fire resistance rating.