The useful energy, nominator in the formulas for the SPF and the PER includes the energy flows "hot storage to heat distribution" (QHS.HD), "solar collectors to heat distribution" (QSC.HD), "hot backups to heat distribution" (QHB.HD), heat pump to heating distribution (QHP.HD) and direct electrical heating (QEL.HD). The denominator, e.g. to calculate the PER is the sum of all electrical and thermal energy inputs (after subtracted solar inputs) of devices related to space heating (Qel.SH+ Qth,SH), converted into Primary Energy.