At Infinite Energy, we will always customise and optimise the design of a customer’s solar system (we don’t just sell standard x kW systems). We do so on the following basis:
Surprisingly for many customers the recommended system has more maximum panel output (kW of DC), than maximum inverter capacity (kW of AC).
Firstly, quality inverters are designed to be able to handle “oversizing”. For example SMA inverters can take much more in DC capacity than their nameplate AC maximum capacity as long as the voltage limits on the inverter are not exceeded. For example it is quite normal to connect 6 kW worth of solar panels to a 5 kW inverter (again, as long as it sits within the inverters parameters such as voltage).
There are a number of reasons:
1. The maximum power output of a solar panel is measured at what are called Standard Test Conditions. STC involve 1000 watts per square meter of light falling on the panel at a cell temperate of 25 degrees. There are a number of reasons why these conditions do not eventuate often (if at all) particularly in Australia:
- For 1000 watts / M² of light to hit a panel it generally needs to be in the middle of the day (when solar irradiance is highest) and the panel needs to be perpendicular to the sun. This may or may not be possible given the pitch and orientation of your roof. Furthermore, if this does occur, in Australia given it’s warmer climate your panel’s cell temperature is likely to be higher than 25 degrees at that point. Therefore it would need to be a very cool, sunny day with your panels perfectly perpendicular to the sun for this to happen.
- You may have your solar panels facing different directions – therefore the theoretical peak of each array would not coincide with each other.
Therefore, it is rare (if at all) your solar panels will produce at their maximum STC rated output.
2. Even if the maximum DC output of your solar panels exceeds the maximum output, then a good quality inverter will “clip” the output to match the maximum AC capacity. It does this by making itself less efficient by adjusting the DC voltage to a non-optimal level (ie reverse maximum power point tracking).
What you will see is a “flat top” effect in the inverters production curve.
The upside of this is that a well-designed system’s output at all other times of the day (other than when “clipping” might occur), will be higher.
If you examine a solar system’s production profile it is like a bell curve with lower production in the mornings and evenings with the peak occurring in the middle of the day. For most if not all of the day the inverter’s maximum capacity is not used.
3. Given that Feed-In Tariffs in each state generally cap eligibility at 5kw of inverter capacity (AC), oversizing the array is a way of maximising the benefits of a solar system whilst retaining a FIT.
4. Given the above, downsizing the inverter slightly and spending more on additional panels may provide more production and a better financial outcome than simply matching max DC of the panels to max AC of the inverter.
For example, when considering “ a 3 kW system” it may be better to downsize your inverter to 2.5 kW and purchase a couple of extra panels to bring your solar panels capacity to 3.5 kW (particularly if you’re splitting the panels across two roof directions).
The Clean Energy Council Solar PV Design Guidelines recommend that you can oversize your array by up to 33% (DC to AC capacity). There are situations whereby you can exceed this as well.
For example a 5 kW inverter may be able handle 6.66 kW (as long as it sits within the inverter’s technical specifications).
If you would like to discuss Infinite Energy designing the best possible solar power system for your home contact us 1300 074 669 or firstname.lastname@example.org