Energy efficiency 251 Technology: general issues 88 History of engineering and technology 80 Energy utilization 72 energy efficiency 64 Research & information: general 41 Sustainable development 25 Energy efficient 24 Energy 23 Energy-consumption 20 History of engineering & technology 20 Physics 15 Internet of things 14 Research and information: general 14 Sensor nodes 14 Economic and social effects 13 Greenhouse gases 13 Carbon dioxide 12 Climate change 12 Energy savings 12 Energy-savings 12 Gas emissions 12 Economics 11 Energy technology & engineering 11 Sustainability 11 Energy performance 10 Investments 10 Optimisations 10 Performance 10 Biomass 9