An inverter controlling method for a power system that includes a DC power source and a plurality of inverters (PS1-PS3) connected in parallel to the power source, the method comprising the steps of: measuring output power of the power source at a plurality of periodic points in time; calculating a moving average at each periodic point in time based on the measured output power; calculating a power difference between the moving average and the measured output power at said each periodic point in time; comparing the absolute value of the power difference at said each periodic point in time to a predetermined reference value; calculating an adjusting value at said each periodic point in time by utilizing the power difference at said each periodic point in time, wherein the adjusting value is zero if the absolute value of the power difference at said each periodic point in time is smaller than the predetermined reference value, the adjusting value is positive if the absolute value of the power difference at said each periodic point in time is not smaller than the predetermined reference value and the power difference at said each periodic point in time is positive, the adjusting value is negative if the absolute value of the power difference at said each periodic point in time is no smaller than the predetermined reference value and the power difference at said each periodic point in time is negative; calculating a moving average revised value at said each periodic point in time by adding the adjusting value at said each periodic point in time to the moving average at said each periodic point in time; calculating a regression curve by utilizing moving average revised values at a current periodic point in time and past periodic points in time; estimating output power at a next periodic point in time based on the regression curve; and determining, based on the estimated output power, how many inverters to be actuated. measuring output power of the power source at a plurality of periodic points in time; calculating a moving average at each periodic point in time based on the measured output power; calculating a power difference between the moving average and the measured output power at said each periodic point in time; comparing the absolute value of the power difference at said each periodic point in time to a predetermined reference value; calculating an adjusting value at said each periodic point in time by utilizing the power difference at said each periodic point in time, wherein the adjusting value is zero if the absolute value of the power difference at said each periodic point in time is smaller than the predetermined reference value, the adjusting value is positive if the absolute value of the power difference at said each periodic point in time is not smaller than the predetermined reference value and the power difference at said each periodic point in time is positive, the adjusting value is negative if the absolute value of the power difference at said each periodic point in time is no smaller than the predetermined reference value and the power difference at said each periodic point in time is negative; the adjusting value is zero if the absolute value of the power difference at said each periodic point in time is smaller than the predetermined reference value, the adjusting value is positive if the absolute value of the power difference at said each periodic point in time is not smaller than the predetermined reference value and the power difference at said each periodic point in time is positive, the adjusting value is negative if the absolute value of the power difference at said each periodic point in time is no smaller than the predetermined reference value and the power difference at said each periodic point in time is negative; calculating a moving average revised value at said each periodic point in time by adding the adjusting value at said each periodic point in time to the moving average at said each periodic point in time; calculating a regression curve by utilizing moving average revised values at a current periodic point in time and past periodic points in time; estimating output power at a next periodic point in time based on the regression curve; and determining, based on the estimated output power, how many inverters to be actuated.