Energy
Energy, or work, is the integral of instantaneous power. Observing the passive
sign convention, energy absorbed by a component in the time interval from
t1 to t2 is
(2-2)
If v(t) is in volts and i(t) is in amperes, power has units of watts and energy has
units of joules.
Average Power
Periodic voltage and current functions produce a periodic instantaneous power
function. Average power is the time average of p(t) over one or more periods.
Average power P is computed from
(2-3)
where T is the period of the power waveform. Combining Eqs. (2-3) and (2-2),
power is also computed from energy per period.
(2-4)
Average power is sometimes called real power or active power, especially in ac
circuits. The term power usually means average power. The total average power
absorbed in a circuit equals the total average power supplied.