Energy can be transferred from one object to another or from one form to another but the total energy remains the same. Work is the change of energy due to an external force. Conservation of energy is that the change in energy is equal to work. The unit of energy and work is joules or J . In a car collision on a level road, the momentum is conserved, but kinetic energy is not conserved, as work is done on the car body by scrunching body parts.
Power is the rate at which energy is transferred.
A horsepower is a unit of power and is equal to 746 watt. Note the unit of energy for electricity use is kwh, which is kilowatt times hours.