Every electrical appliance carries, somewhere, a number in watts (W) — this number is the starting point for understanding everything related to that appliance's electricity consumption and its cost on your bill.
Where do you find the power (watt) value?
An appliance's rated power is usually printed on a metal plate or sticker on the back or bottom of the appliance, or on a specifications plate near the power cord. You may find the value directly in watts (such as 1500W), or you may need to calculate it from the voltage and current listed (such as 230V and 6.5A), using the basic relationship P = V × I that we explained in Basics of Voltage, Current, and Power.
Rated power isn't always the actual instantaneous consumption
The value printed is usually the maximum power the appliance consumes when operating at full capacity. Many appliances don't operate at this level continuously:
- Refrigerators and air conditioners: operate in on/off cycles (the compressor runs for a while then stops), so actual consumption over the day is less than the rated power multiplied by 24 hours.
- Appliances with multiple modes: (an AC unit with different speeds, a washing machine with different programs) consume variable power depending on the selected mode.
- Simple heating appliances: (kettle, iron, heater) consume close to their rated power throughout the actual operating period.
From power to consumption in kilowatt-hours
To convert power (watts) into actual energy consumption (kilowatt-hours, the unit on your bill), you need to multiply the power in kilowatts by the actual number of operating hours — as we explained in The Difference Between Volts, Amps, Watts, and Kilowatt-hours. This is the basis on which any serious comparison between the running costs of different appliances is built.
For anyone who wants accurate measurement instead of estimating from the label, small inexpensive devices (plug-in power meters) are available that connect between the socket and the appliance, displaying instantaneous power and energy consumed over a period directly — very useful for appliances with variable consumption such as refrigerators and air conditioners.
Sample answer: Because the rated power printed on the appliance is usually the maximum power it consumes when operating at full capacity, while appliances like refrigerators and air conditioners operate in on/off cycles (the compressor runs for a period then stops to maintain the required temperature). Therefore, actual daily consumption is less than directly multiplying the rated power by 24 hours.
Calculating a refrigerator's daily consumption by directly multiplying its rated power (such as 150 watts) by 24 hours, resulting in an overestimate. The refrigerator doesn't operate at full power for all 24 hours due to its on/off cycles, so actual consumption is less than this raw estimate.
Want to master home electrical wiring and installations?
Follow trainer Fahad Refai's Electrical Wiring & Safety courses — practical, step-by-step guidance from the basics to safely installing distribution boards and protection devices.
Browse Fahad Refai's Courses