Two fundamental types of energy efficiencies are derived from the first and second laws of thermodynamics: first-law efficiency and second-law efficiency. First-law efficiency deals with the amount of energy without any consideration of the quality or availability of the energy.
It is calculated as the ratio of the actual amount of energy delivered where it is needed to the amount of energy supplied to meet that need. Expressions for efficiencies are given as fractions; multiplying the fraction by 100 converts it to a percentage. As an example, consider a furnace system that keeps a home at a desired temperature of 18°C (65°F) when the outside temperature is 0°C (32°F).
The furnace, which burns natural gas, delivers 1 unit of heat energy to the house for every 1.5 units of energy extracted from burning the fuel. That means it has a first-law efficiency of 1 divided by 1.5, or 67%. The “unit” of energy for our furnace is arbitrary for the purpose of discussion; we also could use the British thermal unit (Btu) or some other units. When the tire swing finally stops, all the initial gravitational potential energy has been transformed by way of friction to heat energy. Energy is conserved in the tire swing.