Home appliances, Education, Healthcare, Transportation, Food industry are some applications of Smart city applications. IoT sensors play vital role to increase the energy efficiency of those IoT based smart city applications. For example, heating, lighting and cooling in home appliances can be automatically controlled and managed by IoT sensors. Lightweight protocols, scheduling optimization, predictive models, cloud based, low power transmitters and receivers, cognitive managing framework are the energy efficient solutions for smart cities. The authors has chosen washing machine, dryer, electric vehicles and dishwasher for demonstrations. The challenges for energy harvesting ins smart cities are receiver design, energy arrival rate, transmitters scheduling, multipath routing etc. A several future works can be done for the efficient energy management in IoT based smart cities.