# Energy

### What is Energy?

In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object. In other words Energy is the ability to do work. Energy is a conserved quantity; the law of conservation of energy states that energy can be converted in form, but not created or destroyed.
The energy (W) lost or gained by any system is therefore determined by $$W = Power \times time \text{ (wattseconds, Ws, or joules)}$$ Since power is measured in watts (or joules per second) and time in seconds, the unit of energy is the watt-second or joule.
The watt-second, however, is too small a quantity for most practical purposes, so the watt-hour (Wh) and the kilowatt-hour (kWh) are defined, as follows: $$\text{Energy (Wh)} = \text{power (W)} \times \text{time (h)}$$ $$Energy (kWh) = \text{power (W)} \times \text{time (h)} \times 1000$$ Note that the energy in kilowatt-hours is simply the energy in watthours divided by 1000. To develop some sense for the kilowatt-hour energy level, consider that 1 kWh is the energy dissipated by a 100 W bulb in 10 h.

### What are Different forms of Energy?

• Heat (thermal)
• Motion (kinetic)
• Electrical
• Chemical
• Nuclear energy
• Gravitational
People use energy for everything from walking to sending astronauts into space.
There are two types of energy:
• Stored (potential) energy
• Working (kinetic) energy
For example, the food a person eats contains chemical energy, and a person's body stores this energy until he or she uses it as kinetic energy during work or play.

#### Renewable or Nonrenewable Energy

Renewable energy
There are five main renewable energy sources:
• Solar energy from the sun
• Geothermal energy from heat inside the earth
• Wind energy
• Biomass from plants
• Hydropower from flowing water
Nonrenewable energy
• Petroleum products
• Hydrocarbon gas liquids
• Natural gas
• Coal
• Nuclear energy