A typical PV system design starts with a target array size, rated in STC watts. From this, an inverter is chosen. The ratio of the inverter’s size, in maximum AC watts output, to the size of the array, in DC STC watts, is known as the array-to-inverter ratio. For example, a 4 kW PV array with a 4 kW inverter has a 1:1 or 100% ratio. A 5 kW array with a 4 kW inverter has a 125% ratio (5 kW DC ÷ 4 kW AC).
Most arrays only spend a small portion of their life operating at their full STC rating, and array output power is typically lower than the rated wattage. (Voltage declines at temperatures greater than 25ºC; and irradiance is often below 1,000 W/m2, resulting in lower-than-STC power.) Because of this, the rated array power can be somewhat larger than the inverter’s without worrying about losing power during normal operation.
So what ratio should you aim for? Many inverter manufacturers list a maximum array STC power in their specifications—this number should not be exceeded, as the National Electrical Code requires following the manufacturer’s instructions. For example, SMA America lists the maximum array STC wattage as 125% of the inverter output; Fronius lists a lower ratio of 115%. Design practices for different climates and incentive programs lead to different design ratios.
Arrays located at high elevations (which often have higher than 1,000 W/m2 irradiance) and/or arrays that routinely experience daytime cell temperatures below 77°F (which, depending on the mounting method, means ambient air temperature is around 32°F or lower) can operate close to, or even above, STC ratings. When the array wattage is higher than the inverter output capability, the inverter will “clip” the power, shifting the operating point off of the maximum power point (MPP) on the IV curve, thus lowering the array’s power output. Over time, this can result in significant energy loss. In this type of climate and conditions, a ratio closer to 100% keeps energy loss to a minimum.
Arrays in warm climates will usually operate below STC ratings and are not likely to experience power clipping if the array-to-inverter ratio is within the manufacturer’s specifications. But hot climates have another potential problem: high inverter temperature. A hot inverter (simply due to the ambient air temperature) operating at, or close to, its maximum capacity (which causes more heating) will move off of the MPP of the IV curve in an attempt to keep the internal electronics from overheating, which causes additional power loss. This situation can be avoided by locating the inverter in a well-ventilated, conditioned space.
Some incentive programs define system size by inverter rated watts, decreasing payouts as the inverter power rating increases. For example, in a feed-in-tariff program in Ontario, Canada, rooftop PV arrays connected to inverters rated less than 250 kW earn a higher FIT payment than systems with larger inverter capacities. A high array-to-inverter ratio could potentially keep a larger array with a smaller inverter in a higher-paying tariff category, sweetening the financial equation.