OVERSIZING
Oversizing an array is good solar design
Learn why your solar panel rating is larger than your inverter rating.
What is clipping?
Clipping refers to the situation where the AC power output of an inverter is limited due to the peak rating of the inverter, even though additional power may still be available from the solar panels. This phenomenon occurs with both string inverter and microinverter systems. In practice, clipping results in the top of the daily AC output curve being flattened off on a bright sunny day.
You may think that clipping reduces the output of your system. Yes, it does, but the loss is inconsequential, compared to the financial gain from oversizing the system.
Why is my solar panel rating larger than my inverter rating?
This common question has a simple answer. In real world conditions, solar panels rarely produce power at their rated output due to thermal and other system losses. Solar panel power depends on temperature and the amount of available sunlight falling on the panel. The image with the scattered points shows the real sampled DC measurements of a solar panel over time. In this case the solar panel is connected to a micro inverter. Most of the time, the solar panel output power is well below the micro inverter DC input limit (blue slanted line). When the input power of the micro inverter limit is reached, the micro inverter is said to be clipping. If the DC input limit is never reached, the inverter never clips and is underutilised.
Oversizing always gives more yield than the losses from clipping?
As you can see from the scatter diagram, clipping hardly ever occurs as the output of the solar panel is nearly always below its rated (theoretical) output. Most of the time the inverter is not being utilised to its full capacity. The measurement of inverter utilisation is known as capacity factor and is defined as the ratio between actual and maximum energy production (think of the inverter running at full output all the time where it would have a capacity factor of 1.0). A higher capacity factor indicates higher use of inverter rated capacity. Increasing the power output of the solar panel relative to the power limit of an inverter always increases the capacity factor and greatly improves annual power output of the system.
Professional solar designers recommend over-sizing solar panel DC peak power to inverter AC power rating to increase the capacity factor. Increasing the DC:AC ratio increases energy yield, even though there may be some loss of energy harvest due to inverter clipping at certain times of the year. However, the increased energy yield is always larger than the loss due to clipping, even at very high DC:AC ratios as shown in the column graph which plots energy increase due to oversizing versus energy loss due to clipping.
Getting greater energy harvest from your system
When larger power solar panels are paired with inverters of a lower wattage a good solar design engineer will size the ratio between the two so that raw clipping is less than 1%.
It is important to remember that other factors also reduce solar panel output. E.g., a system with 330W solar panels will never produce 330W of power into the property. There are always losses e.g., due to panel orientation, panel degradation over time, panel mismatch, DC wiring, connections, soiling, shading and inverter inefficiency. Typically, these add another 5-15% of loss thus reducing the 330W theoretical output of the solar panel down to 280-315W. This gives a net clipping loss that is even lower.
The orange table shows the impact of clipping and the increase in energy harvest when various size solar panels are paired with 240W micro inverters.*
* The table comes from simulation modelling with 8% system losses, 0° azimuth, 25° tilt and using 240W micro inverters.
The sweet spot for oversizing
You can see that even large sized solar panels only show very minor clipping losses over the course of a year. There is actually very little time that the inverter is saturated and limited to its nameplate rating. For most of the time, i.e., earlier or later in the day, or when it is cloudy, the inverter is able to capture all the output from the solar panel.
The sweet spot in terms of cost-benefit is to overload an inverter by 20 - 50%.
Contact McNae Group for your solar installation. We have professional engineering expertise that ensures your system will be optimally sized so that you can reap the maximum benefit from the sun.
The financial benefits of oversizing
If, for example, with reference to the above table, we assume that a micro inverter is clipping for 0.4% of the year, and energy from the solar electric system is being used to offset energy purchased at $0.28 per kWh then the value of all of the energy lost due to clipping over a year is $0.56 for each micro inverter. For a typical 12 panel system this equates to a financial loss of $6.72 in the first year. In subsequent years as the panels age, the annual losses would reduce.
So, although the clipping shown in the image above may look concerning to an owner, it is in fact inconsequential to the overall performance of the system.
More importantly, selecting a larger inverter to ensure no clipping occurs at all would increase the capital cost of the system by an amount significantly more than the value of the extra energy that could be harvested – meaning that the overall return on investment for the solar system is worse for a system with no clipping than for a system with a small amount of clipping as in our example above. There is obviously no benefit in selecting inverters that increase the capital cost of a system by say $35 per panel (in the case of micro inverters) in order to harvest an extra $0.56 of electricity from each panel per year. This extra investment would take 62 years to pay back!
Further proof
If you want further proof that oversizing will cause virtually no loss of generation you can do your own calculation by using the PVWatts online solar generation calculator…
Enter the solar panel array size, enter azimuth as 0 degrees (if its north facing). Critically, in the 'Advanced Parameters', enter the DC->AC Ratio e.g., enter 1.33 if the array is 33% bigger than the inverters AC output.
PVWatts will calculate the annual solar generation as AC energy produced from the system.
Go back and do the same calculation with a DC->AC ratio of 1.0, to reflect an inverter the same size as the array.
PVWatts will calculate the annual solar generation again.
Compare the values from step 3 and step 5. You will find that they are only different by a very small amount – usually less than 1%.