Electrical Power Calculator
Table of contents
How to calculate electrical powerWhat is the power factor?Electrical power calculatorFAQsThis electrical power calculator helps you compute the power consumed by electrical devices. Do you want to know how to calculate electrical power? Are you wondering: what is the power factor? Just read the text below to find out.
How to calculate electrical power
Electrical power, just like mechanical power, is the amount of work performed in a unit of time. In electrical circuits, work is performed by an electric current. The power depends on the "amount of workers available in a unit of time" — the current $I$ and the energy of "a single worker" — the voltage $V$.
In a DC circuit, the power is:
where:
 $I\ \rm [A]$ — Current in amperes;
 $V\ \rm [V]$ — Voltage in volts; and
 $P\ [\rm W]$ — Power in watts.
In AC circuits, the equation is:
where the new symbol $\rm pf$ stands for power factor.
What is the power factor?
In AC currents, both the current and voltage vary periodically in time. The values $I$ or $V$ correspond to the root mean square (RMS).
RMS is a square root of the mean of squares of numbers (see the root mean square calculator for more). The commonly referred voltage of electrical outlets (230 V in the EU and Australia, 110 V in the USA and Canada, 100 V in Japan) is the RMS voltage.
In AC circuits, the current and voltage might not be in phase. The maximum value of the current might be ahead of or lag behind the maximum value of the voltage. This makes the transfer of power less effective. In the worst case, when the current and voltage are entirely out of sync, the transmitted power is zero.
The power factor tells us how synchronized the current is with the voltage. If they're in sync, the power factor is 1. Otherwise, it is less than one, reaching zero in the completely outofsync case.
The power factor depends on the device. For a device that is purely resistive, like an electric kettle or an electric heater, the power factor is 1. A device with inductive or capacitive elements puts the current and voltage out of the phase. This makes its power factor less than 1. Check out the power factor calculator to learn more.
Electrical power calculator
To compute the electrical power, you have to specify the current, voltage, and power factor of the item. For devices connected to electrical outlets, the voltage is just the voltage of the domestic power. The current drawn by the device can usually be found either marked on a plug or somewhere on the device.
The power factor is a bit trickier to find — unless you have a power quality analyzer at hand. Check this list for power factors of a few typical household devices:
 Lamps with a standard bulb: $\rm pf = 1$;
 Fluorescent lamps: $\rm pf = 0.93$;
 Common induction motor at half load: $\rm pf = 0.73$, at full load: $\rm pf = 0.85$;
 Electric oven (with resistive heating element): $\rm pf = 1.0$; and
 Inductive oven: $\rm pf = 0.85$.
The exact value of the power factor depends on the details of the construction, so take these values with a grain of salt.
Appliance wattage
You can also use our appliance wattage calculator to learn how to estimate your power consumption.
What is electrical power?
Electrical power is the rate of energy transfer within a circuit. It is measured in watts (W) and is usually denoted by the letter P. The electrical power at any given time is given by the current and voltage of the circuit.
What is the electrical power formula?
The electrical power is described in terms of the current (I
) and voltage (V
) of the system by the following formula:
P = I × V
In AC circuits, the electrical power formula is:
P = I × V × pf
where pf
is the power factor.
How do you calculate the electrical power dissipated by a resistor?
To calculate the electrical power dissipated by a resistor, follow these steps:

Write down the resistance
R
(in ohms) 
Write down the voltage of the circuit
V
(volts). Alternatively, write down the currentI
(amps). 
Substitute the
V
andR
values into the following equation to get the power in watts:P = V²/R
Or if you recorded the current
I
use:P = R × I²
What is the electrical power of a device working at 120 V and 10 A?
1200 W or 1.2 kW. To calculate the electrical power of a device in a circuit with voltage (V) = 120 V
and current (I) = 10 A
, you need to multiply the two parameters together:
P = I × V = 120 V × 10 A = 1200 W
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