Definition of PER

1 by the means or agency of : through <per bearer>
2 with respect to every member of a specified group : for each
3 according to —often used with as <per instructions> <as per usual>

See per defined for kids »
1 : by means of
2 : to or for each <$10 per day>
3 : as indicated by

Usage Discussion of PER
Per occurs most frequently in business contexts; its use outside such contexts is often criticized but is quite widespread, especially in sense 2. Its most common and natural nonbusiness uses always involve figures, usually in relation to price <$150 per performance>, automobiles <32 miles per gallon> <55 miles per hour>, or sports <averages 15 points and 9 rebounds per game>.

Examples of PER
1. The speed limit is 35 miles per hour.
2. The car gets 32 miles per gallon.
3. He averages 15 points per game.
4. Per your advice, I accepted their offer.

Origin of PER
Latin, through, by means of, by — more at for
First Known Use: 14th century

‘Per’ As Applied to Energy Efficiency
Energy efficiency applies to both one specific end use and also to many. Lumens per watt refers to one theoretical lamp. “Per” means “for each” or “with respect to every member of a specified group.” So, one compact fluorescent lamp is as efficient as a hundred compact fluorescent lamps. From an energy efficiency point of view, it doesn’t matter how many individual end uses of energy there are. In contrast with energy efficiency promoters, everyone else knows that the more end uses of energy there are, the greater the total energy usage is likely to be. Two operating energy efficient lamps double the energy used by one.