# Units of Power and Energy

Back to Energy

##### Energy and Power
• Power is the rate of doing work or, equivalently, the rate of using or producing energy.
• Power is Energy or Work divided by Time.
• Power can also be expressed as Force times Velocity,:
• Power = (F x D) / T = F x (D/T) = F x V.
• Sentences referring to energy and work:
• We used 500 Kilowatt-hours of electrical energy last month
• It takes 600 foot-pounds of work to lift a 100 pound weight 6 feet in the air
• Sentences referring to power:
• This 10 watt LED bulb uses electrical energy at the rate of 10 joules per second
• My portable electric generator has a power output of 1600 watts, that is, 1600 joules per second.
• Analogies for distinction between energy and power
• How far you ran (energy) vs how fast you ran (power)
• How much you ate (energy) vs how fast you ate (power)
• Energy is stuff.  Power is how fast you make or use the stuff.
##### Units of Energy
• Joule (J)
• force x distance (N x m)
• 1 joule = 1 newton x 1 meter
• mass x velocity2
• 1 joule = 1 kg x 1 (m/s)
• Foot-pound (ft-lb)
• force x distance = (lbf x ft)
• 1  ft-lb = 1 pound-force x 1 foot = 1.356 joules
• Kilowatt-hour (kW⋅h)
• power x time = (energy / time) x time = (kW x h)
• 1 kW⋅h = 1000 watts x (60 x 60) seconds = 3.6 million joules
• Electron volt (eV)
• volt x electric charge =  (energy / charge) x charge
##### Units of Power
• Watt (W )
• Energy / time (J/s)
• 1 W = 1 joule of work done per second
• 1 W = 1 joule of energy produced or used per second
• Horsepower (hp)
• Energy / time (ft-lb/minute)
• 1 hp = 33,000 foot-pounds of work per minute
• 1 hp = 745.7 watts