Going green

Electric kettles use power. LOTS of power.

I have an electric kettle. It’s pretty fast to boil water, but wow, does it burn up electricity.

Here you can see our live electricity meter thingy on the fridge. On the left, a couple of lights are on, as well as the fridge and I think a computer was on as well. On the right — the kettle is boiling as well.

Power consumption - On the right is when the kettle was turned on

Conclusion: Put just the water in the kettle that you need; don’t fill it up every time if you’re only making a cup of tea or two. It’s not just quicker, but it uses a lot less power.

Update Wednesday 9:30pm: Some information from an Origin Energy insider (they are my power provider, and supplied the meter), in reply to the post and some of the comments.

The comments on your blog are correct. Almost all electrical appliances should have a power consumption label in an inconspicuous place that will tell you the maximum power consumption. Kettles are normally around 2200W. It’s interesting to see how much energy heating and cooling systems use – even ducted gas heating and evaporative cooling which require powerful fans to propel air through the house.

Your energy monitor will continue to work if you leave Origin BUT it may not show your energy costs accurately. It will stop showing costs completely after (I think) six months, because they didn’t want out of date energy costs being shown indefinitely. Your electricity consumption in kW will continue to be displayed indefinitely.

If you do a factory reset though it will lose the connection with your smart meter and Origin will have no ability to reconnect it, as only the current electricity retailer can do this. However, if you are in the United Energy or Jemena distribution areas you can use their ‘Energy Easy’ portal to reconnect it (you should be in the UE area?) You would love the Energy Easy portal anyway as you can do all kinds of number crunching on your electricity consumption (but not see it in real time).

By Daniel Bowen

Transport blogger / campaigner and spokesperson for the Public Transport Users Association / professional geek.
Bunurong land, Melbourne, Australia.
Opinions on this blog are all mine.

24 replies on “Electric kettles use power. LOTS of power.”

Useful tips (common sense, but we need to be reminded!)
I wonder how much energy is used when a kettle is boiled on the stove? I’m sure someone has the stats somewhere

That reading is not correct. The kettle cannot take more than 2.4 kW (continuously) from the mains supply. Most kettles take between 2.2 kW and 2.4 kW (i.e. they’re not likely to use less than 2.2), so either something else is running as well, or the reading is wrong.

But your conclusion is right. A kettle’s power consumption is proportional to the amount of water in it, so you don’t save energy by loading it up with more water when you only need a little bit.

That’s an interesting power meter. Due to the fake competitive environment in which our electricity is made and sold, I can’t get a meter like that in my area.

Also remember that another product of the electricity cartel setup is that the power meter (in the meter box or on the fridge) never displays a correct instantaneous reading of power consumption. All it can provide is an average over the most recent time period (of variable length proportional to the instantaneous power consumption). This is because the meters only provide a flash of a light to indicate another Watt-hour has been consumed. The meters are perfectly capable of displaying the present level of consumption in Watts, but this feature has been disabled by those in charge.

@Philip, thanks for that – good feedback! I’ll see if I can measure it again, perhaps a little more scientifically. (Note the earlier post, which was the same kettle, reckoned it was 2.2 kW.)

Yes, the meter is provided by Origin, with a wireless connection to the Smartmeter. If I switch providers, it will apparently stop working.

What Philip said. More specifically, electric kettles are rated to draw 10 amps at ‘normal’ domestic voltage (230V to 250V depending how you interpret the standard). This normally results in maximum power consumption around 2.4kW, but this will vary if the voltage happens to be higher or lower at your premises, because the bog-standard electric kettle has a resistive element and draws current proportional to voltage.

So, a typical design resistance for a kettle would be 240V / 10A = 24 ohms, and at this voltage level the consumption is 240V x 10A = 2.4kW. But if your actual home voltage is 250V (this is not uncommon), then the kettle actually draws 250V / 24ohm = 10.42A and the power consumption is 250V x 10.42A = 2.6kW. Conversely, if your voltage is 230V then the current is only 230 / 24 = 9.58A and the power consumption is 230 x 9.58 = 2.2kW. For resistive elements the power consumption actually scales with the square of the voltage.

(NB for pedants, I’m assuming the magnetic inductance of the element is negligible, so no reactive power. That’s a whole ‘nother can of worms.)

So your electric kettle is probably rated to draw 2.4kW but could draw between 2.2kW and 2.6kW depending on the supply voltage at your house. (In fact the allowable variation is a bit wider even than this, but it’s less common to see more extreme values.) However, the 2.94kW inferred from your meter reading is still probably too high, and may be due to some other appliance drawing variable power (maybe your fridge happened to cycle on just before you took that second reading).

The other thing to keep in mind is that boiling less water in the kettle does use less power, but not because it reduces the current drawn by the kettle – this is essentially constant in most kettles. Rather, it reduces the time required to boil the water for a given power input. So strictly speaking, it’s the total energy consumption that’s proportional to the amount of water, not the power consumption.

I reckon gas would be less efficient in terms of the ratio of (heat generated in the house) to (heat put into the water), because the gas flame heats up the air around it, as well as the water in the kettle. The electric kettle puts most of its heat into the water.

However, an electric kettle relies on the conversion of coal into heat, which then heats water to make steam, which turns a turbine to run a generator and make electricity, some of the power of which is lost during transmission. Some of the heat from the coal goes into the atmosphere, some condenses the steam back into water for re-use. So while virtually 100% of the electricity going through the kettle ends up as heat in the water, this is not all of the potential heat energy that was in the coal. But this is rather complicated and uses a lot of assumptions, so it’s probably better for the householder to use what they prefer!

While practically all of the electrical energy that the kettle uses goes into the water it costs more to use electricity to boil water than natural gas if you have a gas stove. The gas flame will add some of it’s heat to the room in addition to the water in the pot so it’s not as efficient at energy transfer to the water as an electric kettle but since gas is much cheaper it will cost less to boil water here this way.

I have a plug in energy meter that displays the actual voltage and it varies as high as 250 and as low as 220 but it is usually about 239 to 242. It will lower a bit if I use my kettle, heater, or microwave and go back up when I turn these appliances off. It also varies with the time of day and if other people in my building or neighborhood are using power. My 1800 watt heater is on and it is reading 237 at the moment.

In the US I have only ever seen an electric kettle for sale in a store. I have never seen one in use in anyone’s house ever in the 40 years I lived there. Americans don’t drink much tea and when we do need boiling water we boil it on the stove in a pot or whistleing kettle or in the microwave. Most people in the US do have an electric drip coffee maker which is rarely seen here. 120 volt appliances that plug into an ordinary household outlet (powerpoint) in the US can only use a maximum of 1500 watts (12 amps at 120 volts) so a 1500 watt kettle would not be quicker than the stove in the US anyway. Electric stoves in the US use 240 volts on their own special circut and they can generate much more heat than a plug in appliance.

It might use a lot of power but it’s only on for 3 mins or so. Assuming 1kWh = $0.25, it will cost you 2.75 cents each time you boil water. (2.2kWh * 3/60mins) * 0.25 :)

As per a previous comment, maybe the kettle uses a lot of electricity, but it only does so for a few minutes. But what about the “cheap” (to buy) little fan heater we’ve got? It’s rated at 2.0 kW – and it’s tempting to leave it on for hours at a time!

Let’s calculate!

Specific heat capacity of water is 4.186kJ per kg per degree Celsius, and density of liquid water is near enough to 1kg per litre. So to raise one litre of water from 20 to 100 degrees Celsius requires 4.186 x 80 = 335kJ, that is to say 335kW-seconds. Dividing that by 2.4kW from the kettle, it should take around 140 seconds or a bit over 2 minutes to boil a litre of water if the kettle is 100% efficient. Allowing for heat losses it’ll realistically take 2.5 to 3 minutes.

3 minutes is 1/20 of an hour, so 2.4kW for 3 minutes is equivalent to 2.4 / 20 = 0.12kWh. At Ryan’s 25 cents per kWh that’s 3 cents’ worth of electricity. Of course that scales with volume, so boiling 2 litres takes 5-6 minutes and costs about 6 cents.

I’ve noticed though that many kettles on the market thwart the principle of boiling only as much water as you need by overly restricting how much water you can boil at a time. My kettle at home goes from 0.5 to 2 litres which is pretty flexible, but I’ve seen some that don’t appear to let you boil less than a full litre at a time. A big waste if you only need a cup or two at once.

I’m unsure whether it costs less to boil water on a gas stove: I believe it used to, but gas prices have risen a lot recently. But even with the greater heat loss, it’s almost certainly more efficient when considering the losses in generating and transmitting electricity from fossil fuels only to use it for heating at the end.

And electric radiators are the spawn of the devil. Get a reverse-cycle air conditioner, or at least a gas furnace. :-)

” It’s not just quicker, but it uses a lot less power.”

Actually, if you only partly fill your kettle, it will still use the same amount of power as if it was full. It will use the same amount of power, but only for half as long.

” I wonder how much energy is used when a kettle is boiled on the stove? ”

All the heat from a kettle element goes into the water in the kettle. If you are using a kettle on top of a stove ( gas or electric ), there will inevitable be more heat wasted which is not contributing to boiling your water.

Heat “wasted” by heating water on a gas stove, is only wasted in summer.

In winter that wasted heat is still warming up your house.

@Enno – but as others have pointed out there are transmission losses for any electricity you use, so the gas stove is probably the more environmentally friendly option, (especially in Victoria with our filthy power generation).

@Tony Morton – How does a kettle thwart your attempt to boil less than half a litre? Will it actually not engage or are you just talking about the water metre on the side?

@Robert – That video is fantastic. I was trying to explain to some friends recently as to why you can’t just add plenty of renewables (except hydro) without some kind of turbines you can turn on and off rapidly or some very efficient (at turning on) energy storage solutions and that video highlights the issue very well. Thanks!

@Julian – Underfilling can damage the kettle, and overfilling can have messy consequences: I have experience of both. Unfortunately, kettles and their elements come in all shapes and sizes, and the only clue one has to the design tolerances of the appliance are the ‘Min’ and ‘Max’ water level indications.

It may be that the cheap kettle with the 1 litre ‘minimum’ can actually boil a smaller volume without damage, but there’s nothing to indicate this.

So what your’e saying is that my electric kettle should NOT clock over 1kw/h in the 4 minutes it takes to boil a litre , my power company is trying to still convince me everything is normal with the meter. If my mathematics are correct that means I am pulling 15KW thru a 16 amp breaker without it tripping.???

A kettle’s power is NOT proportional to the amount of water it contains. The power is determined by the heating element. The higher the power of a kettle, the faster it will boil a given mass of water.

@Adrian, yes – but the point of the post was that only putting in as much water as you need means the kettle is running for less time.

No one has said which uses the less power to boil a litre of water, an electric kettle or a stove top kettle on an electric stove which is what I would like to know.

The electric kettle uses less electricity.

Start by just thinking about the water itself. It clearly takes the same amount of energy to boil a given quantity of water in the electric kettle and in the stove top kettle. So you can ignore this energy in doing the comparison.

So, then think about what else gets heated up when you boil the water.

For the electric kettle, the element will heat up, the body of the kettle will heat up, and there will be a bit of radiation from the kettle.

For the stove top kettle, it is necessary to heat up a larger element. And the trivet or glass that supports the kettle. And then the body of the kettle. The element will also heat up the air surrounding the kettle, and the body of the stove. All of these will also radiate heat. Worse, the heat is applied from the outside so the temperature gradient means that the element, trivet, and kettle need to be hotter than if the heat is applied inside the kettle in direct contact with the water – or operate for longer.

In other words, there is much more heat wasted with a stove top kettle. The heat came from electricity, and so the stove top kettle must use more electricity to boil a given quantity of water.

I have been obsessing about the energy use of appliances recently, as I’m designing an off-grid cottage that’ll be reliant on solar PV and batteries. I found this website handy:
(Ignore the ridiculously low US cents/kWh prices; Australia’s are typically 24 to 38 c/kWh)

I’ll have a gas cooktop (bottled LP gas), and use a traditional stovetop whistling kettle, to avoid the 2000w x 3 minutes x 5 times per day (0.5 kWh) electricity demand of an electric kettle.

I have a domestic plugin electricity usage measuring device, and have checked every appliance in my house to find and eliminate (or substitute) the energy hogs. The guzzlers: central gas ducted heating fan (winter), aircon (summer), clothes dryer, fridge, electric oven/cooktop, dishwasher, vacuum cleaner, washing machine, microwave oven, TV, water pressure pump (for rainwater tank supply).

The kettle I use in the UK is labelled 3 kW on the base. Some are lower than that.
(The mains voltage does vary around the UK more than it should do so the actual power will vary anyway).

More usefully, the element is flat and integrated into the base, so it is possible to boil a coffee-cup-ful of water if I want. OTOH a flat element is probably less efficient at transferring heat to the water than the traditional raised coil.

As I drink a lot of coffee, I use a drip coffee maker which is rated at a mere 700 W.

Once the LPG cooker is organised in this (new) house, I’ll have a choice of boiling water on a gas flame. In winter, I’m pretty sure that it will emit less CO2 to boil water that way. The heat transfer efficiency drops to about 40% but the waste heat enters the house.

In summer, with renewables soon providing say 30% of UK electricity, an electric kettle could be better overall, although I cannot see the overall effficiency being much better than 85-88% (for the kettle) x 35-40% (for the power stations and then the low voltage distribution losses of 12%).

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