Many meters have pulse outputs, examples include single part and 3 part power meters, Gas meters, Water meters. The pulse output is also a flashing light-emitting diode or a shift relay (usually solid-state) or each. Pulse Power plans should give you an idea of how energy can be monitored with the use of pulse output.
In the case of AN electricity meter, a pulse output corresponds to an outlined quantity of energy passing through the meter (kWh/Wh). For single-phase, domestic electricity meters (e.g. the Elster A100c) every pulse sometimes equals one Wh (1000 pulses per kWh).
Calculating Energy For the A100c meter, every pulse represents 1/1000th of a kWh, i.e. 1 Wh, of energy passing through the meter.
Calculating Power 3600 seconds per hour = 3600J per pulse i.e. one Wh = 3600J thus, instant power P = 3600 / T wherever T is that the time between the falling fringe of every pulse.
Optical pulse counting: Flashing LEDs