Watts (W)

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Frequently Asked Questions


In electricity, the watt is the power unit of a system delivering or absorbing an intensity of 1 ampere under a voltage of 1 volt. Instantaneous power is the product of the voltage by the current: P (Watt) = U (Volt) x I (Ampere)


The kilowatt-hour (kW⋅h or kW h or kWh or kW-hrs) is a unit of energy. The kWh is commonly used as a billing unit by electric utilities.

Why calculate the power consumption of a device?

There are a few reasons to calculate the consumption of an electrical device, but one of the most interesting is to save on your electricity bill.

For example, when you want to buy a device, (example, a television), and you hesitate between several models, compare the consumption, it may be more interesting to take a model that is a little more expensive, but which consumes less, on the long term you will save money.

As another example, many of us leave devices plugged in or turned on at night or when they're not in use, calculating the electrical cost of these practices can make you realize that you have a better interest in turning them off and unplugging them.

How to manually calculate the cost of electricity for a device?

You need to know the power of the device (in watts), the duration of use and the price of electricity that your supplier charges you.
With this data, you can calculate the consumption of your appliance for 1 hour of operation (kwh) and multiply the consumption of your appliance by the duration of operation.

For example, to calculate the consumption of a bulb with a power of 40 watts (W) which will be used 2 hours per day for 1 week.

Calculation of kwh :

kwh = W / 1000

for 1 week (7 days) :
              kwh    * hours * days
total kwh = W / 1000 *   2   *  7
More concretely :
total kwh = 40 / 1000 * 2 * 7
total kwh = 0.56

If the energy supplier charges $ 0.25 per kwh, we can estimate the cost of using our bulb:
cost = total kwh * price ($)
cost = 0.56 * 0.25
cost = 0.14 $
The electricity cost of our bulb is 0.14 $.

Devices without power indication (W)

Some devices do not give information about the power consumed and other devices only give an indication of the maximum power consumed (eg computer power supplies).

For devices not indicating the power consumed:
It is often indicated the voltage (V) and the amperage (A) required, for example the charger of my phone indicates:

input: 240V - MAX 0.5A
from its information, we can calculate the power consumed:
Watt (power) = Volt x Ampere
in the case of my phone charger, the power is 120 watts maximum (the "MAX 0.5A"), which brings us to the second case.

For devices indicating a "maximum" (W or A)
Some devices do not have a fixed and definitive power, this is the case of mains adapters (usb charger ...), power supplies for computers and other electrical equipment, for these devices an indication is given on maximum power.
For example, a computer with 500w power supply, does not mean that the computer consumes 500w, but the power supply can provide 500w power, if all of your computer components consumes 300w, the power supply will consume 300w and not 500w.
To know the real consumption of your device, the easiest way is to use a Wattmeter, it is a tool which measures the power of your device while it is in use, some models can provide additional information (kwh, ampere, frequency ...), I recommend that you use a model that plugs into a power outlet and into which you plug your electrical device.

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