Because inverters cost more expensive per Watt than PV modules, a solar industry practice is to oversize the PV power installed with respect to the inverters nominal power; for example, by using a 1 MWdc inverter with a 1.3 MWdc PV installation. The inverter will then clip the top of the daily power curve and that amount of energy will be lost. This practice is known as ‘overbuild’ or AC-to-DC Ratio.
The Figure below shows the effect on a solar farm with six types of mounting systems. It shows the impacts of 10% overbuild. It is based on a hypothetical sunny summer day and obtained from EcoSmart simulation available here. The pink line indicates the upper limit set by the inverter. It has different effects, depending on the mounting devices of the PV system:
- With a fixed mounting (light yellow profile), the daily power profile is a sinusoid with a marked maximum around noon. In this case, no power is clipped.
- In a 2-axis tracker (green profile), the daily power profile is a wide curve and the power stays near its maximum for most of the day. As a result, the inverter will clip a significant amount of power.
- In a VAT system, the power profile is as wide as the 2-axis system but the power clipping is not very significant because the profile is depleted around noon.
The optimum DC-to-AC ratio must be selected on a case-by-case basis. The figure below shows how the AC power loss increases when DC-to-AC ratio increases. It is the average over 25 years taking into account the annual PV module derating. The DC-to-AC ratio does not affect significantly the final AC output. For 25% DC-to-AC excess, the AC loss is less than 1%. For 50% DC excess (for ex. 1.5 MW DC with 1 MW AC) the AC loss is less than 5%.
The PV modules performance decreases over time (see aging), while the inverter power stays constant. Therefore, the DC-to-AC ration decreases over time . As a result, the average clipping losses over the 25-year lifetime is less than the first year loss.