What consumes less electricity: a kettle or an immersion heater?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

28 December 2008

Judy Nyanjau asks a straightforward question thus: “Which electrical element uses more power; an immersion heater or an electric kettle?”

Well, Judy, the answer is also straightforward: the writing is on the “wall”! The power rating is always written on the appliance. You will find a figure like 2,000W, or 2,000watts, etc. This indicates how much electrical energy it consumes per second.

Watts are different from volts (V). All appliances that can be plugged into a wall socket in Kenya are rated at 240V! Some have a slightly lower rating of 220V and exposes them to a small risk of blowing up.

Voltage measures how “hard” the power is “pumped” through the appliance. Imagine electricity flowing like water in a pipe; if it was flowing from a raised tank, voltage would be the height above the ground and the watts would be the amount of energy generated at the end of the pipe.

Now, if we compare a 2,000W kettle to a 2,000W immersion heater, we might find that the former consumes slightly less electricity that the latter. This is due to the effect of heat losses.

A good kettle will have a white plastic casing and a lid to cover the water. The colour white is a very poor radiator of heat and plastic is a bad conductor. Thus energy loss is minimised.

Consequently, when you boil water in the kettle, its outer body remains (very) warm while the liquid inside is scalding hot. This indicates that very little heat is lost to the surroundings thus most of the electrical energy is retained as heat in the water.

The water heated by an immersion heater, on the other hand, is usually poured in an ordinary plastic bucket. The sides of the container get very hot indicating that a lot of heat is being lost to the surroundings. To compensate for this loss, the heater consumes additional electricity.

As a result, the 2,000W immersion heater will take longer to boil, say, two litres of water than a 2,000W kettle. Thus the former consumes more electricity…since it will remain connected for longer!

Remember, the energy consumed is the power rating of the appliance (in thousands of watts) multiplied by the duration it is operated (in hours). This is how KPLC gets the kilo-watt-hours (kWh) quoted on your bill.

Therefore, if you want to compare, say, a 1,500W immersion heater to a 2,400W kettle, you would have to measure how long it takes for each appliance to boil an equal amount of water and then multiply the time by the power ratings of the respective devices…if you use different quantities of water, the calculation becomes a little more complicated, though workable.

Still, another issue arises from that question: why are some appliances designed to operate at 240V while others are rated 220V? Well, that is a story for another day.