Numerical calculation has been used to assess the effect of using PCM in fully mixed water accumulation tank. Packed bed sys-tem of spheres with diameter of 40 mm have been considered as option to increase energy storage density. Random close pack-ing factor of 63% have been chosen, since author believes that if PCM spheres would be introduced on the site, it would be dif-ficult to use tools like shakers to reduce the volume of voids. Change of specific heat for PCM before and after phase change has been taken into account in energy equations. Four types of PCM are used, described by their melting temperature in degrees Celsius – 65, 55, 35 and 25. Multiple type PCM system is found to provide better solution compared to both water alone and single type PCM systems, since it can work at lower temperatures and provide higher energy density compared to water alone system.