Energy-efficient technology is technology that provides a desired service while using less energy than a relevant alternative for delivering the same or an equivalent service. The key concept is energy intensity: the amount of energy required per unit of useful output or service. An efficient electric motor, for example, converts a greater proportion of supplied electrical energy into mechanical output, while efficient insulation reduces the energy required to maintain a building at a desired temperature.
Energy efficiency is distinct from energy conservation, which generally involves reducing energy use through behaviour or reduced activity. Replacing inefficient lighting with LEDs is an efficiency improvement because the same lighting service can be provided with less electricity, whereas turning lights off because a room is unoccupied is conservation. Energy-efficient technologies exist in buildings, transport, industry, computing, appliances, power systems and many other sectors. Their benefits can include lower operating costs, reduced fuel consumption, lower emissions and reduced demand for energy infrastructure.
Efficient technologies work by reducing losses or improving conversion processes. An incandescent lamp converts much of its electrical input into heat rather than visible light, whereas an LED produces light much more efficiently through electroluminescence. Electric motors lose energy through electrical resistance, magnetic losses, mechanical friction and other mechanisms, so improved motor design and control can reduce those losses. Power electronics can efficiently convert voltage and current between forms required by equipment.
Insulation reduces heat transfer through conduction, while efficient windows reduce unwanted conductive and radiative heat exchange. Heat pumps use a refrigeration cycle to move heat from one location to another rather than generating all useful heat directly from an energy source. The efficiency of a technology therefore depends on the physical process involved and the losses that occur at each stage. Engineering improvements often target the largest loss mechanisms first.
Buildings provide many important applications for energy-efficient technology. High-performance insulation, airtight construction, efficient windows, heat-recovery ventilation, heat pumps, efficient lighting and intelligent building controls can reduce heating, cooling and electricity demand. Building-management systems can adjust heating, cooling and lighting according to occupancy, schedules, weather and indoor conditions. Variable-speed drives can reduce motor energy consumption by allowing pumps and fans to operate at appropriate speeds rather than continuously running at full output.
Heat recovery can capture energy from exhaust air or industrial waste streams and transfer it to incoming air or another useful process. Building efficiency is highly dependent on the interaction between technologies and the building envelope. Installing an efficient heating system in a poorly insulated building can leave substantial avoidable heat losses. Energy efficiency is therefore often a systems-engineering problem rather than a matter of selecting a single efficient appliance.
Industrial energy efficiency can involve motors, pumps, compressors, boilers, furnaces, process heating, refrigeration, compressed-air systems and production controls. Industrial motors may operate for thousands of hours, so small percentage improvements in efficiency can produce significant lifetime energy savings. Pumps and fans can be particularly inefficient when throttling is used to control flow instead of varying motor speed. Waste-heat recovery can capture thermal energy from exhaust gases or hot process streams for use elsewhere.
Combined heat and power systems can use fuel more efficiently when there is a suitable simultaneous demand for both electricity and useful heat. Process optimisation can also reduce energy indirectly by reducing defective products, idle operation and unnecessary material processing. Industrial efficiency measures must account for process requirements because reducing energy use at one stage can sometimes increase energy use elsewhere. The correct metric is therefore usually total energy required per unit of useful production rather than the efficiency of one isolated component.
Transport efficiency concerns the amount of energy required to move people or goods over a given distance. Vehicle mass, aerodynamics, rolling resistance, drivetrain efficiency, traffic conditions and driving patterns all influence energy consumption. Electric motors are highly efficient at converting electrical energy into mechanical work, while regenerative braking allows some kinetic energy that would otherwise become heat in the brakes to be recovered into a vehicle battery.
Public transport can reduce energy use per passenger where high occupancy is achieved, while rail systems can be particularly efficient for suitable routes. Freight efficiency can involve route optimisation, vehicle loading, aerodynamic improvements and modal choices. Aviation, shipping and heavy-duty transport have different technical constraints, so efficiency measures vary by sector. Energy-efficient technology should therefore be evaluated against the service delivered, such as passenger-kilometres or tonne-kilometres, rather than simply comparing the energy consumption of individual vehicles.
Computing and electronics contain numerous energy-efficiency technologies because semiconductor devices, processors, displays, storage systems and data centres consume substantial amounts of electricity. Modern processors use techniques such as dynamic voltage and frequency scaling, power gating and specialised accelerators to reduce energy consumption while maintaining performance. Data centres can improve efficiency through server virtualisation, efficient power supplies, airflow management, liquid cooling, workload scheduling and higher utilisation of computing resources. Storage systems can use lower-power technologies and intelligent access patterns.
Displays use efficient LED or other advanced backlighting and panel technologies, while mobile devices employ aggressive power-management strategies to extend battery life. A common metric for data centres is power usage effectiveness, or PUE, which compares total facility energy use with the energy used by computing equipment. Efficient computing therefore involves both the semiconductor level and the infrastructure level.
Energy-efficient technology does not automatically reduce total energy consumption by the same proportion because of rebound effects and increased usage. If an appliance becomes cheaper to operate, users may use it more frequently, partially offsetting the efficiency gain. Economic growth can also increase the total number of devices or services even as each individual device becomes more efficient. System-level effects therefore matter.
Efficiency can still provide major benefits because it reduces energy required for each unit of service and can lower peak demand, fuel use and emissions. However, energy efficiency is distinct from decarbonisation: a highly efficient device powered by a carbon-intensive energy source can still cause emissions, while an inefficient device powered entirely by zero-emission energy can have low operational emissions but high energy demand. Combining efficiency with cleaner energy sources can address both energy consumption and emissions.
Measuring energy efficiency requires defining the useful output and system boundary. A refrigerator should be assessed according to useful cooling delivered relative to energy consumed, while a motor may be assessed according to mechanical output per electrical input. Seasonal performance matters for systems whose operating conditions vary, such as heat pumps and air conditioners. Standby consumption, partial-load efficiency and control behaviour can substantially affect real-world performance.
Standards and energy labels can provide comparative information, but laboratory ratings do not necessarily reproduce every user's operating conditions. Maintenance is also important because clogged filters, poor lubrication, incorrect controls or degraded components can reduce efficiency over time. Energy-efficient technology is therefore most effective when correctly sized, installed, controlled and maintained. Its broader significance lies in reducing the energy required to deliver useful services, which can lower costs, reduce infrastructure requirements and decrease environmental impacts when the energy supply itself has associated emissions.
