Method for computer-aided ascertainment of the impedance (Z G) of an electrical power supply system, wherein, at a connection point at which an electrical power generating installation is connected to the power supply system, the electrical voltage (V PCC), the active power (P PCC) and the reactive power (Q PCC) are measured at respective successive instants, with an impedance value (Z G) for the power supply system being ascertained by: a) ascertaining, at a respective present instant, whether the absolute-value change (|ΔV PCC |) in the measured voltage (V PCC) between the preceding and present instants and the absolute-value change (|ΔQ PCC |) in the measured reactive power (Q PCC) between the preceding and present instants satisfy a first criterion, wherein the first criterion is satisfied when the absolute-value change (|ΔV PCC |) in the measured voltage (V PCC) is greater than a first threshold value and the absolute-value change (|ΔQ PCC |) in the measured reactive power (Q PCC) is greater than a second threshold value; b) only if the absolute-value changes (|ΔV PCC |, |ΔQ PCC |) in the measured voltage (V PCC) and the measured reactive power (Q PCC) satisfy the first criterion, ascertaining whether the changes (ΔV PCC, ΔQ PCC) in the measured voltage (V PCC) and the measured reactive power (Q PCC) satisfy a second criterion, wherein the second criterion is satisfied when the changes (ΔV PCC, ΔQ PCC) in the measured voltage (V PCC) and the measured reactive power (Q PCC) are caused by the power generating installation, wherein the second criterion is checked such that the arithmetic sign of the change (ΔV PCC) in the measured voltage (V PCC) is compared with the arithmetic sign of the change (ΔQ PCC) in the measured reactive power (Q PCC), wherein the second criterion is satisfied when the two arithmetic signs match; c) only if the changes (|ΔV PCC |, |ΔQ PCC |) in the measured voltage (V PCC) and the measured reactive power (Q PCC) satisfy the second criterion, estimating an impedance value (Z GE) on the basis of a computation rule, which is independent of the phase of the measured voltage (V PCC) and is based on the assumption of a constant phase of the measured voltage (V PCC), and determining an estimation error (errZ GE) for the estimated impedance value (Z GE), wherein additionally the change (ΔP PCC) in the active power (ΔP PCC) between the preceding and present instants is ascertained, wherein the estimation error (errZ GE) is determined on the basis of a predetermined relation that indicates the estimation error (errZ GE) on the basis of the measured active power (P PCC), the absolute-value change (|ΔP PCC |) in the measured active power (P PCC) and the absolute-value change (|ΔQ PCC |) in the measured reactive power (Q PCC); d) the estimated impedance value (Z GE) representing the ascertained impedance value (Z G) only if the estimation error (errZ GE) is below a predetermined error threshold (errZ G). a) ascertaining, at a respective present instant, whether the absolute-value change (|ΔV PCC |) in the measured voltage (V PCC) between the preceding and present instants and the absolute-value change (|ΔQ PCC |) in the measured reactive power (Q PCC) between the preceding and present instants satisfy a first criterion, wherein the first criterion is satisfied when the absolute-value change (|ΔV PCC |) in the measured voltage (V PCC) is greater than a first threshold value and the absolute-value change (|ΔQ PCC |) in the measured reactive power (Q PCC) is greater than a second threshold value; b) only if the absolute-value changes (|ΔV PCC |, |ΔQ PCC |) in the measured voltage (V PCC) and the measured reactive power (Q PCC) satisfy the first criterion, ascertaining whether the changes (ΔV PCC, ΔQ PCC) in the measured voltage (V PCC) and the measured reactive power (Q PCC) satisfy a second criterion, wherein the second criterion is satisfied when the changes (ΔV PCC, ΔQ PCC) in the measured voltage (V PCC) and the measured reactive power (Q PCC) are caused by the power generating installation, wherein the second criterion is checked such that the arithmetic sign of the change (ΔV PCC) in the measured voltage (V PCC) is compared with the arithmetic sign of the change (ΔQ PCC) in the measured reactive power (Q PCC), wherein the second criterion is satisfied when the two arithmetic signs match; c) only if the changes (|ΔV PCC |, |ΔQ PCC |) in the measured voltage (V PCC) and the measured reactive power (Q PCC) satisfy the second criterion, estimating an impedance value (Z GE) on the basis of a computation rule, which is independent of the phase of the measured voltage (V PCC) and is based on the assumption of a constant phase of the measured voltage (V PCC), and determining an estimation error (errZ GE) for the estimated impedance value (Z GE), wherein additionally the change (ΔP PCC) in the active power (ΔP PCC) between the preceding and present instants is ascertained, wherein the estimation error (errZ GE) is determined on the basis of a predetermined relation that indicates the estimation error (errZ GE) on the basis of the measured active power (P PCC), the absolute-value change (|ΔP PCC |) in the measured active power (P PCC) and the absolute-value change (|ΔQ PCC |) in the measured reactive power (Q PCC); d) the estimated impedance value (Z GE) representing the ascertained impedance value (Z G) only if the estimation error (errZ GE) is below a predetermined error threshold (errZ G).