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How To Calculate Electricity Consumption Of Your Home Electrical And Electronic Devices

Updated on August 30, 2013
An electricity distribution system.
An electricity distribution system.

Electrical Efficiency And The Carbon Footprint

Most people nowadays are aware of the carbon footprint of everyday life; every single aspect of our day routine, such as driving to work, having a glass of milk for breakfast, watching TV or using our smartphones, they all have an impact on the environment.

Thing is, we cannot do much about our dependency on fossil fuels until technology makes harnessing of renewable energy sources more efficient and less expensive.

The electricity consumption (and therefore, the dreaded electricity bill) of a modern home is directly dependable on the electricity efficiency of the various electrical and electronic devices.
Every single thing which needs to be plugged to an electricity outlet, such as the washing machine, the AC units and your computer, they all have a certain efficiency rating. In engineering, electrical efficiency is defined as useful power output divided by the total electrical power consumed; put simply, this rating will indicate how effective a device is by comparing the electrical needs with the actual output.

More or less, every house has particular "electrical hogs" - devices which consume too much electricity. The usual culprits of a high electricity bill are the air-condition units, the electrical stove and other related equipment. Nevertheless, it doesn't mean that devices of lower consumption such as laptops are actually "electrically efficient".

Incandescent light bulbs are outdated and should not be used anymore in any household due to their extremely low efficiency and high consumption.
Incandescent light bulbs are outdated and should not be used anymore in any household due to their extremely low efficiency and high consumption.

Cutting Expenses Off The Electricity Bill

As stated before, for most homes in the world nowadays the electricity bill is the most dreaded of all bills; with the rise of price of electricity and the increasing number of electronics in a typical modern house, the expense is a serious one.

But you might wonder: how could someone lower the electricity bill without decreasing the usage of the various devices in an inconvenient way?

It might not be easy for everyone. Besides, choosing new, energy efficient machines brings up an initial expense which is too high and sometimes not even worth. Replacing an inefficient device is not easy for every household, unless it is for low-cost units such as light bulbs.

However, it is not bad to be able to distinguish an efficient device form an inefficient one; knowing which of your electrical devices is an energy hog makes it easier for you to decide if you need to replace it with a more modern, more efficient one. As a result, in the long run, you will have a knowledge over which devices need to be replaced and which ones can last for more years.

The Device's Power Tag

A power tag at the back of a TV. 130W for a period of 3 hours per day will make up for a 0.39 KWh of electrical consumption per day.
A power tag at the back of a TV. 130W for a period of 3 hours per day will make up for a 0.39 KWh of electrical consumption per day.

The Electrical Consumption

In order to proceed with some calculations regarding the electrical consumption of our devices, we first need to mention a basic electrical unit, the KWh (kilowatt-hour). This unit shows how many KW (1000 Watts) of electricity a device consumes per hour.

As you understand, this unit is able to take account of time as a major element of the calculation. After all, we might be more accustomed with Watt as a power unit, but it cannot be used when calculating electrical consumption.

Let's see how the formula works:

1 KWh = 1 KW for one hour = 1000 W for one hour

Now let's see an example:
We have a modern LCD TV unit with a power tag at the back which indicates that the device consumes 100 W during normal operation. The TV stays on for about 4 hours per day. So, for each day, we have a consumption as follows:

100 W multiplied by 4 hours equals to 400 Wh equals to 0.4 KWh.

Such calculations can be made with all devices of the house for which we have some information regarding the power during operation. The power tags usually placed at the back or bottom of the device are what we will be using. Mind however that they are not always accurate because the consumption can vary depending on the settings. Typically, the power tags indicate the maximum power dissipation; this number will be higher than standard operation, but in any way, it is better to calculate the consumption at the worst case scenario.

When you have a list of devices and their consumption per day depending on the hours of operation and you also know the price of KWh at your region, you can have a fairly accurate calculation of the electricity expense of your house at maximum.

Example Table Of Calculations

(click column header to sort results)
Device  
Number Of Devices  
Number Of Hours Each Per Day  
Watt Rating Each  
Consumption In KWh  
Light Bulbs
10
2
20
0.4
TV Units
2
3
110
0.66
AC Units
3
2
2500
15
Electric Stove
1
2
1500
3
Desktop Computer
2
3
300
1.8
Laptop Computer
1
2
120
0.24
Washing Machine
1
1
500
0.5
Totals
 
 
 
21.6
(Number of devices) x (Watt rating of each) x (Hours of use per day) = Daily Consumption

The Actual Expense

The total KWh consumption of the house devices, as shown in the previous example, approximate as they might be, are still a good indication of the house energy consumption.

Do not forget that our aim is not to pinpoint the consumption of the house to the last Wh, but merely to identify the energy hogs of our house and determine the possible profit (if any) of a replacement of the oldest devices with new, more efficient ones.

In any way, when you multiply the total electrical consumption in KWh with the current KWh price in your region you can get estimate of the expense of electricity for the above listed devices and duration of use.

Energy Meter

A low price energy meter, suitable to measure the electrical consumption of the devices plugged in it.
A low price energy meter, suitable to measure the electrical consumption of the devices plugged in it.

How To Get More Reliable Results

When someone calculates the electrical consumption of a device by using the Watt rating and the approximate amount of time during which the device is operated, he cannot expect to have very accurate results. This is because, the devices rarely consume exactly as much as the power tag indicates.

More accurate results can be achieved if an energy meter is used. These devices are connected on the electrical box and keep track of the total consumption of the house. These are reliable devices which can give exact consumption and thus calculate the expense for electricity, however there is a catch - they tend to be very expensive and need to be installed by qualified technician.

Another method is the kind of small energy meters which can be placed between the device to be checked and the wall socket. These electronic devices include an LCD screen and buttons, through which one can set the KWh price. When this is set and plugged in a wall socket, you can connect a device on it (for example plug your laptop on the energy meter) and while the laptop is running, the energy meter will keep track of the exact consumption. These inexpensive devices are able to provide daily, weekly, monthly or even annual reports, thus letting you know of the exact consumption of the device you have plugged in them. They do have some limitations - for instance, there is always a maximum of wattage the device can measure, so do not expect to connect many devices at once through extension adapters.

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