Energy is the ability to do work or cause change. It can take many forms, such as heat, light, sound, and motion.
There are several types of energy, including kinetic (energy of motion), potential (stored energy), thermal (heat energy), electrical, chemical, nuclear, and electromagnetic (energy of light and other forms of radiation).
The standard unit of measurement for energy is the joule (J). Other commonly used units include calories (cal) and kilowatt-hours (kWh).
Energy can be transferred from one system to another, such as from a battery to a light bulb, or transformed from one type to another, such as from electrical energy to thermal energy in a toaster.
Renewable energy sources, such as solar, wind, hydro, and geothermal energy, can be replenished and will not run out. Non-renewable energy sources, such as fossil fuels (coal, oil, and natural gas) and nuclear energy, will eventually be depleted and cannot be easily replaced.