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The sizing of grid-connected PV systems does not usually take into account the site-dependent peculiarities of the solar irradiation distribution characteristics. Because of the perceived notion that PV systems hardly ever have a DC output equal to or above their STC-rated nominal power, inverters are usually sized with a nominal AC output power some 30% (sometimes even more) below the PV array nominal power. This practice might lead to considerable energy losses, especially in the case of PV technologies with low temperature coefficients of power and at sites with warm climates and an energy distribution of sunlight shifted to higher irradiation levels. We discuss the consequences of the time resolution of the irradiation data set used to determine a particular site irradiation distribution on inverter sizing. Most of the available irradiation data measured at individual sites are hourly averages, which lead to filtering of irradiation peaks, which might result in inaccurate performance estimations. When calculating inverter yearly efficiencies using both hourly averages and one-minute averages of the solar irradiation, we can show that the actual losses due to inverter undersizing increase with increased time resolution, revealing that hourly averages hide important irradiation peaks.