LED technology has evolved at a fast pace within the last years; offering already LED devices with luminous efficacy of 150 lumens per watt or even more. Based on studies about LED light output dependence on forward current and (junction) temperature, the optimal model for simulating the light output of monochromatic LEDs has been developed. Available solutions in literature are analyzed and modified to relate their parameters to LED properties, which could be predicted with a SPICE-like electrical model of LEDs. Experimental blue and red high power LEDs are measured using the state-of-the-art measurement systems - T3Ster and TERALED in a combined measurement setup and optimization algorithms are used for calculating additional fitting parameters for the developed model. The modeled results are compared against the experimental measurements and an excellent agreement is found between the simulated and measured output of monochromatic LEDs.