Undersized Inverter

[mnlara]

I came along a PV system of 21 Sunmodule 260 watt panels in 3 strings with one SMA sunnyboy 5000tl-us(240) inverter. the system is 5460 STC, why the use of the undersized inverter.

 

[SolarPro]

Several reasons. One, the STC rating is a factory test condition. It assumes an irradiance of 1,000 watts per square meter and a cell temperature of 25?C (?77?F). In the real world, continuous irradiance levels are usually lower and cell temperatures are higher, both of which result in reduced PV output power. Two, the DC input power to the inverter is further reduced by derate factors like wire losses, array soiling and module mismatch. Lastly, weighted inverter efficiencies are typically in the 97% to 94% range, so your PV-to-inverter capacity ratio has to be greater than 1:1 in order for the inverter to output 100% of its rated power.

PV USA Test Conditions (PTC) were developed to be more indicative of module performance in the field. They assume a 20?C air temperature, which will put the cell temperature up around 45?C. The PTC rating for the Sunmodule SW 260 is 235 W. So the PTC rating of this array is 21 x 235, or 4,935 watts. That's an ideal fit with a 5,000 watt inverter.

In residential applications, PV capacity at STC is typically sized at 110% to 120% of the inverter capacity. In utility-scale applications, it's not uncommon for the PV-to-inverter capacity to be in the 120% to 140% range.

 

[jaggedben]

In addition to the factors SolarPro mentioned, consider that the PV array will only output its max power a very small percentage of the time. The rest of the time its output is below the rating of the inverter. Spending more money on a larger inverter to take advantage of the very small additional amount of energy is not cost effective if the larger inverter costs more than the extra kWhs are worth. This is why the approach for residential systems is generally to choose the largest size inverter that is smaller than the PV array power, not one that is larger. With this approach you are 'wasting' a very small amount of money on PV power that won't get used. Alternatively, if you choose a larger inverter then you are wasting 100% of the money spent on whatever extra inverter power never gets used.