Energy conversion is changing energy from one form to another. A solar panel converts sunlight into electricity. A turbine converts the motion of steam into mechanical energy. A battery converts chemical energy into electrical energy. The process happens constantly, in nature and in machines.

Every conversion loses some energy as heat. That is the second law of thermodynamics. A car engine converts roughly 25 to 30 percent of gasoline's energy into motion. The rest leaves as heat through the radiator and exhaust. A power plant converts 35 to 60 percent of fuel energy into electricity, depending on the technology.

Engineers work to raise conversion efficiency. Combined-cycle gas plants capture waste heat to generate more electricity, reaching 60 percent or higher. Heat pumps move heat rather than create it, achieving effective efficiencies above 100 percent in favorable conditions. LED lights convert a much larger share of electricity into visible light than incandescent bulbs.

Common conversions

• Chemical to thermal (burning fuel)
• Thermal to mechanical (steam turbine)
• Mechanical to electrical (generator)
• Radiant to electrical (solar PV)
• Electrical to chemical (battery charging)