Power Formula [P = W/t and P = VI]

Generally, the power is defined as the amount of work done per unit. In electrical terms, it can be though as the energy supplied by the current in a certain unit of time.

Four basic formulas can be used to calculate electrical power under different circumstances.

Formulas to calculate Power
General formula P =
W / t
In terms of voltage and current P = VI
In terms of current and resistance P = I2R
In terms of voltage and resistance P =
V2 / R
In terms of velocity P = F.v
In terms of workdone P =
mgh / t

Power Formula Calculator

Enter energy and time to obtain the power.


Derivation of P = VI

P = W/t … (1)

From the formula of voltage, we know that V = W/Q or alternatively:

W = QV Putting W = QV in equation 1

P = QV/t

Since current can be mathematically expressed as I = Q/t

We can replace it with the formula of Power.

P = V.I .. (2)

Equation 2 is the derivation of power in terms of electrical engineering.

Velocity & Wattage

Power can also be expressed in terms of velocity. P = Fv

Unit of power

Power is measured in terms of watts. One watt itself is the one joule of work done in one second. Mathematically,

1 watt = 1 joule per second

The power of electrical components is often labeled on them. The approximate wattage rating of some electronic devices is mentioned here:

Devices Watt ratings
1 Ton AC 1200
1.5 Ton AC 1500
Fan 70
Heater 2000
Iron 2000
Laptop 100
Mobile phone 6
Toaster 850


Example 1:

Other Power formulas

kVA to Amps: A formula used for converting kVA to amps.

Amps to kVA: A formula used for converting amperes to kVA

Amps to kW: A formula used for converting amps to kW in AC, Single, two and three phase dc circuits.

kWh to joules: A formula which is used for converting kilowatt-hours to joules.

Summary of all formulas:


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