Efficiency = (Useful Work done)/(Total Work done).
This is usually expressed as a percentage.
Example:
For every 200KJ of energy released from coal in a power plant, only 60KJ is converted to electrical energy. Therefore, Efficiency = 60/200 =30%.
Rate and efficiency are two different things, but can be related.
Rate : How many units per period are being completed
For example, a car assembly line might be set at a rate of '5 cars an hour'...