Over-sizing is become a popular theme in solar power plant design in India as there is an increased pressure to drive down costs in installations. Let us understand the technical benefits and dangers of over-sizing solar arrays.
1. Advantages of Oversizing Solar Arrays
1.1 Higher Yield
As all solar EPCs know, a solar array typically never generates at more than 80% of the nameplate capacity or kWp. Thus a 10 kWp solar array, at any given time in Indian conditions would rarely exceed 8 kWp. Thus if we can connect more than 10 kWp of solar panels to a system with a 10 kVA inverter, the net generation or yield of electricity will ofcourse be higher.
1.2 More Optimal or Lower System Costs
In an industry where Rs/ Wp is the main criteria for vendor selection, everyone is pushed to optimize cost vs generation. Specifically in smaller residential roof top systems, the inverter is sometimes as high as 27 Rs/Wp or more. In such cases, by oversizing within acceptable technical limits it is possible to reduce the cost of the system without impacting system efficiency.
2. Disadvantages of Oversizing Solar Arrays
2.1 Components age faster
When solar inverters are loaded at more than there ratings, the inverter is operating at its peak capacity for more percentage of the time- this can lead to overheating and shorter life-span of the components in the system. Hence the components will age faster. This it is extremely important to ensure that you are not voiding the inverter manufacturer’s warranty by oversizing the solar plant. Most reputed inverter manufacturers like Fronius and Delta will provided a written undertaking on what percentage of oversizing is supported under warranty.
2.2 Lower Efficiency
Oversized arrays typically have higher Vdc or string voltages. Refering to an inverter’s efficiency curve, we find that over-sizing an inverter to the higher limits of the MMPT voltage range may cause the system to always run at 0.5-2% lower efficiencies. It is however important to note that an inverter can be oversizing properly up to 20-30% by staying in the optimal voltage range for highest efficiency.
2.3 Power Clipping and Generation Loss
Under-sizing your inverter will mean that the maximum performance of your system will be dictated by the size of your inverter and regardless of how big the output of your solar panels is, the output will be cut-off by the inverter.
Let’s take and example where you oversize a 10 kW solar inverter by 150% with 15 kWp solar panels. In such a condition, at noon, 15 kWp of solar panels may be capable to produce 12 kW of power. However the inverter will only be able to process 10 kVA and 2kW/ 2kVA will be lost or clipped.
Additionaly most good quality inverters derate the inverter output in high ambient temperatures in order to manage temperature and increase the life of the inverter. If a system is over-sized by more than recommended design, then the power may be clipped for more percentage of the time.
Inverter and array sizing are one of the most important tools in the hands of a solar system installer to optimize cost and generation.
If the inverter is sized with the ‘average power received on-site’ then some money can be saved by choosing a smaller inverter (‘average power received’ being a calculation based on meteorological data and estimated solar panel efficiency at those conditions).
Sunkalp Energy recommends a 120-130% oversizing with most reputed solar inverter brands for operation in Northern India.