The unit of electrical power is the watt (W), named after James Watt (1736-1819). One watt of power equals the work done in one second by one volt of potential difference in moving one coulomb of charge.
Remember that one coulomb per second is an ampere. Therefore, power in watts equals the product of amperes times volts.
Power in watts = volts x amperes
P = E x I
Example: A toaster takes 5 A from the 240V power line. How much power is used?
P = E x I = 240 V x 5 A
P = 1200 Watts
Example: How much current flows in the filament of a household 75 watt light bulb connected to the normal 240 Volt supply?
You know P (power) and E (volts). You need to transpose P=EI for I and you get:
I = P/E
Therefore:
I = 75/240
I = 0.3125 Amperes
This amount of current is best expressed in milliamperes. To convert amperes to milliamperes multiply by 1000 or think of it as moving the decimal point 3 places to the right, which is the same thing. This gives:
312.5 mA
Power in watts can also be calculated from:
P = I2R, read, “power equals I squared R”.
P = E2/R, read, “power equals E squared divided by R”.
Watts and Horsepower Units.
746 W = 1 horsepower.
This relationship can be remembered more easily as 1 horsepower equals approximately 3/4 kilowatt. One kilowatt = 1000 W.
