IEEE 802.15.4-2015 is the third revision of IEEE 802.15.4 Standard for Low-Rate Wireless Networks. The standard presents Time Slotted Channel Hopping (TSCH) Medium Access Control (MAC) protocol which provides high reliability and low power consumption to various industrial applications. Despite the effectiveness and the importance of the TSCH protocol, the standard leaves out of its scope in defining how the schedule is built and maintained. In this work, we focus on scheduling in IEEE 802.15.4-2015 TSCH networks from the energy efficiency perspective in a centralized manner where the gateway makes frequency allocations and time slot assignment. At first, we derive an energy consumption model of a TSCH node to determine the network lifetime. Afterwards, we formulate the scheduling problem as energy efficiency maximization problem which is a nonlinear integer programming (NLP) and NP-hard. Motivated by the high computational complexity of the problem, we propose an low-complexity energy efficient scheduler (EES) which works in polynomial time by allocating the available frequencies to the node that attains the highest energy efficiency at this frequency. We made a comparison with the round robin scheduler (RRS) and analyse both schedulers in terms of success probability and energy efficiency. Performance evaluation indicates that EES performs better in terms of energy efficiency while at the same time yielding a good throughput.