Time Slotted Channel Hopping (TSCH) is the MAC layer amendment introduced in 2012 in IEEE 802.15.4 standard for supporting industrial applications. TSCH aims at providing low power and high reliability to industrial networks. The communication in TSCH is handled by a schedule, that indicates when a node will transmit the data. Each cell in a schedule is identified with the tuple (time slot, channel offset). A group of time slots constitutes a slot-frame. The number of time-slots with in a slot-frame is termed as slot-frame length. This paper investigates the effect of slot-frame length on energy consumption using 6TiSCH simulator and ANOVA method. A cubic regression equation best fits the simulation results and the predicted values were in good agreement with the simulation results. The slot-frame length optimization is done using Hill Climbing. The energy consumption can be improved significantly through optimised slot-frame length.