交流電的用電量計算

In dc circuits, the current I and voltage V are always in phase, but this is not the same in ac.
In ac circuit, voltage Vac and current Iac can be at some phase angle with each other, i.e. either Vac leads Iac or vice versa.
Hence, the general equation for power in ac [Pac] is,
Pac = Vac.Iac.cos(a)
where a is the phas angle between Vac and Iac.
It is only in some special circumstances that Vac and Iac are in phase in which a = 0 and cos(a) = 1
Notice that as the phase angle [a] is always zero for dc, the equation reduces to the power equation in dc, P = V.I

首先，P=V * I，
where unit of Power-P is W (Watt), unit of Voltage-V is V(Volt), and unit of Current-I is A (Ampere):

計算DC, say Vdc = 9V (always constant ------------------) , I = 0.5A
so P= V * I = 9 * 0.5 = 4.5W

計算AC similarily, say Vac=220 V( always root-mean-square value of wave /\/\/\/\/\/\/\/\/\ ), I = 10A
so P= V * I = 2200W

同樣是用 Ohm law(歐姆定律)計算
V=IR
I=V/R
P=VI