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 = I^{2}R 
In terms of voltage and resistance  P =
V^{2}
/
R 
In terms of velocity  P = F.v 
In terms of workdone  P =
mgh
/
t 
Contents
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 
Calculations
Example 1:
Other Power formulas
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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 kilowatthours to joules.
Summary of all formulas: