Energy efficiency research focuses on delivering the same useful energy services using less input energy, thereby reducing costs, fuel use and environmental impacts. It spans buildings, industry, transport and electricity systems, and combines technology, behavior and policy.

In buildings, studies show major savings from better insulation, airtightness, high performance windows, heat pumps, efficient lighting, and smart controls. Retrofitting existing buildings often offers the largest and cheapest efficiency gains. Research emphasizes life cycle analysis, showing that upfront investment can be repaid through lower energy bills and improved comfort.

In industry, efficiency measures include waste heat recovery, improved process control, high efficiency motors and drives, and process integration that reuses heat and materials. Advanced techniques like combined heat and power and electrification of heat can cut both energy use and emissions. Analyses highlight that many cost effective options remain unrealized due to information gaps, split incentives and capital constraints.

In transport, efficiency improvements come from better engines, lighter materials, aerodynamics, low rolling resistance tires, and especially vehicle electrification. Mode shifts to public transport, cycling and rail freight further increase system wide efficiency.

On the system level, research assesses how efficiency can reduce peak demand, defer grid investments and support integration of variable renewables. Scenarios consistently find that efficiency is one of the fastest and least expensive ways to cut greenhouse gas emissions and strengthen energy security.

Overall, the literature concludes that energy efficiency is a central pillar of sustainable energy transitions, with multiple co benefits for health, air quality and economic resilience, provided that supportive policies and financing are in place.