World Energy Efficiency Day is celebrated on March 5, as part of the first meeting of world experts on energy efficiency, held on this day in 1998 in Wales, Austria. What is energy efficiency? Every day, people use energy for various purposes: heating, cooling, lighting, production, entertainment, etc. The ways in which energy is used significantly affect our environment and life, so it is very important not only whether we save energy, but also whether we use it in the most efficient way. It is important to distinguish between two terms, which have been used more and more lately, namely energy saving and energy efficiency. Saving energy means everything we do to not waste energy – these are simple steps that everyone can adopt as a way of behaving, from turning off the lights after leaving the room to recycling plastic or aluminum packaging. Energy efficiency is a term that refers to the use of technology that requires less energy – as a good example can be energy-saving LED bulbs that consume less energy instead of conventional bulbs while giving the same amount of light. An increasing number of cities in the world are supplied with energy from renewable sources Compared to 2015, the number of cities supplied with energy from renewable sources has more than doubled, relying mostly on clean energy instead of fossil fuels, some of which are Oslo, Nairobi, Brazil, etc. Clean energy is currently supplied to 43 cities worldwide, of which as many as 30 cities are located in Latin America, where hydropower is widespread. As for Europe, Reykjavik, the capital of Iceland, draws electricity from hydro and geothermal power plants, and by 2040, the plan is to free all cars and public transport from the use of fossil fuels. What activities can we take? The most common measures taken to reduce energy losses and increase energy efficiency are: 1. Replacement of non-renewable energy sources with renewable ones, 2. Replacing energy inefficient consumers with more efficient ones, 3. Insulation of space that is heated or cooled, 4. Replacement of dilapidated or inefficient carpentry of rooms that are heated or cooled, 5. Installation of measuring and regulating devices, 6. Replacement or installation of efficient heating, air conditioning or ventilation systems.