String inverters are the most commonly used inverters around the world. In a rooftop photovoltaic (PV) system, usually few modules of the same output voltage and power are connected together in series, or as a “string.” Several such strings are then connected in parallel at the inverter input. The string inverter needs to accommodate these inputs at predefined voltage and power levels, which means proper solar inverter sizing is crucial.

For example, given a rooftop PV system that has 4 strings, each with 4 modules producing 250W, the total output of the system is 4000 (250 * 4 * 4 = 4,000) watts. One may think it makes sense going for a 4000 W inverter, but this is not the case. The string inverter size is always optimized by oversizing calculations. A PV to inverter power ratio of 1.15 to 1.25 is considered optimal, while 1.2 is taken as the industry standard. This means to calculate the perfect inverter size, it is always better to choose an inverter with input DC watts rating 1.2 times the output of the PV arrays.

Even though the total rated power output of the PV arrays may be 4000 W, the output will never be that high thanks to many degradation factors. The output voltage is also affected as a result of this and this is lower than the rated voltage. Therefore, if an inverter is rated to work from 250-480 V optimally, it will still need 300 V to start working effectively. After operating for a while, the operating voltage across the inverter may slip down to 250 V and operate effectively.

This ensures that the inverter is utilized to its full potential, at its claimed efficiency, much earlier in the day. Occasionally in the case of normally sized inverters, earlier in the day the inverter struggles to start working. By oversizing, the inverter reaches its maximum efficiency earlier in the day and will increase the lifespan of the inverter. At higher altitudes, because of higher irradiance and ground reflectance, the inverter needs to be oversized even more, thus the PV-to-inverter ratio needs to be smaller, around 0.9-1.1.

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