Energy conservation, power and efficiency

Choose a system and account for energy entering, leaving and changing form. When only gravity transfers between kinetic and gravitational potential energy, Ek+ Ep is constant. With friction, mechanical energy can decrease while total energy is conserved through internal-energy increases and other transfers. Do not silently discard a heating term or assume every object starts from rest.

Average power is P=tE​, or the rate of doing work. Efficiency is useful output divided by total input, with matching energy units or matching power units. Multiply by 100 for a percentage. A Sankey-style balance can be solved arithmetically: input equals all outputs in a steady device without net storage. Define the useful output for the stated purpose.

Watch out: It has transferred into outputs less useful for the chosen purpose, commonly heating. Total energy is still conserved.