The energy losses in networks over time span T can be calculated by decomposing the load graph on time spans and a lot of periodical measurements must be done. Using the maximum load losses time (utilization time of power losses UTL)), it is possible to obtain load losses more swiftly. However, to calculate UTL, the series of measurements are necessary as well. To accelerate determination of the UTL, the one-off index is applied, which is known as maximum load time (MLT), as far as approximate interdependence exists between MLT and UTL. MLT can be obtained from the readings of energy counter during time span T. The known mathematical expression of said interdependence allows noticeable error. In two alternative formulas, it is necessary to apply two one-off extreme indices: the maximum Smax and minimum Smin power of the consumers over time span T, but load duration graph must be decomposed in two parts: the base part and delta part. If UTL formula is obtained using only two mentioned extreme indices, then this UTL can be applied only for rough losses estimation of those consumers which UTL is prior known to be more than 0,35 – 0,4. The UTL formula of three imdices (the integral and two extreme ones) gives the most accurate appreciation of load losses in all consumer cross section. Obtaining this formula, the fact has been taken into account that load losses are square function of the current. The UTL of delta part was searched as the square of the first degree polynomial of delta part MLT and the coefficients of the polynomial are obtained from the delta part model. By UTL calculation, it is necessary to use relative values: relating the current time t to considered time span T and the rest quantities – to maximum power value Smax.