Why energy meter reads energy while wattmeter does not. An energy meter has a registration constant of 100rev/kwh if the meter is connected to a load

An energy meter is fitted with some type of registration mechanism whereby all the instantaneous reading of power are summed over a definite period of time.ENERGY = POWER X TIME So, an energy meter can read energy.Whereas, a wattmeter indicates the value of power at a particular instant when it is read and hence it can not read energy.
Registration constant= 100 rev/Kwh I = 20 A, V = 230 V, cos φ = 0.8
Energy in 5 hours = VI Cos φ x 5 / 1000
= 20 x 230 x 0.8 x 5 / 1000
= 18.4 Kwh
The number of revolution of the meter should make it is correct
= 18.4 x 100
= 1840 revolution.
Number of revolution actually made
= 1800 revolution
(% error = 1840 - 1800 x 100 )/ 1840
= 0.0217%
It would be better from the customer point of view % age error is very less.