# Tea time

Discussion in 'General Science & Technology' started by foghorn, Apr 20, 2022.

1. ### foghornValued Senior Member

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The spec label on my small electric kettle shows wattage as 250-1000w.
So, what does it use? Minimum is one cup.
I know how to work out the price using wattage, but what wattage do I use?

3. ### SarkusHippomonstrosesquippedalo phobeValued Senior Member

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It's likely to be 1000w, or you'll be waiting for a long time for the water to boil.

Typically it takes c.0.1 kWh to boil 1 litre of water, not accounting for losses (heating the walls of the kettle etc).
So at 250w that would take 100/250 hours = 24 minutes.
At 1000w it would be 6 minutes.

In terms of the maths to reach that 0.1 kWh figure...

The definition of 1 calorie is the amount of energy required to raise 1g of water by 1C, at 1 atmosphere.
1 litre of water is 1 kg, so each degree-C rise in temp requires 1000 calories.
To take water from, say, 15C to 100C, would therefore require 85,000 calories.
1 calorie is roughly 4.2 joules, so this is now 357,000 J.
1 Watt is 1 joule/sec, so 1 Watt-hour is 3,600 Joules.
1 kWh is therefore 3,600,000 joules.

To boil the 1 litre of water therefore requires roughly 0.1 kWh (plus an amount for losses).

Hope that helps.

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5. ### Michael 345New year. PRESENT is 72 years oldlValued Senior Member

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Years ago when they just started to become the new appliance I bought a number of coffee capsules so I could buy the matching appliance ½ price

Only used those capsules. Now I buy the coffee I like, spoonful in the cup, press go button, 50ml hot water into coffee

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7. ### foghornValued Senior Member

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Thanks, that does help.
Does that scale down as such:
One cup (250ml)
To bring one cup to boil at 1000w will take (6mins/4) = 1.5 minutes?
To bring one cup to boil at 250w will take (24minutes/4) = 6 minutes?
Is that too simplistic?

8. ### DaveC426913Valued Senior Member

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It might apply to a closed system.
It does not take into account heat loss over time.
In practice, you want to minimize time elapsed, so that you minimize that heat loss.

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9. ### exchemistValued Senior Member

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More or less, though as Dave points out it won't scale exactly due to the greater heat loss over a longer time.

What baffles me is that your kettle apparently claims it can have a variable power. Is this something you can set yourself? What is the point of that? I'd have thought the faster the better, so max power is always what the user will choose.

10. ### DaveC426913Valued Senior Member

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Hair dryers had a similar feature. You don't always want highest wattage.

At least one use for a lower kettle setting would be to humidify a room - such as for someone with lung problems.

11. ### exchemistValued Senior Member

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Fairly obviously, the mode of use of a hair dryer is nothing like a kettle.

I doubt anyone sells a kettle on the basis of its alternative use as a room dehumidifier.

12. ### James RJust this guy, you know?Staff Member

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Some kettles have a feature to "keep your coffee warm", which I think means periodically re-heating the water after the initial boil. Maybe the 250 W setting is for that kind of thing. Personally, I don't see the point; seems very wasteful of energy, exacerbates global heating etc.

13. ### foghornValued Senior Member

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No, I can't set it.

I did think of the heat loss to other things besides the water, but wanted to know if I was roughly right.

Now that I've seen the two wattage rates on my kettle, I have noticed it on others, especially on the higher wattage kettles. One was 2520-3000 Watts. Take a stroll around a shop looking at the bases of electric kettles.

I have an idea what maybe going on here.
Maybe the electric element doesn't reach 1000W heating instantly. And so with a small volume of water (250ml), the heating on the build up to 1000W raises the temperature to near boiling and boils outright when the element reaches 1000W. And so the bulk of the heating up time was not at 1000W.
Something like that????
Ps . That picture is not my kettle.

14. ### SarkusHippomonstrosesquippedalo phobeValued Senior Member

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The picture above - is clearly for European / UK market - or anywhere else that uses 220-240V. In the UK a typical kitchen will be rated at 13A for appliances, so 13A * 230V = 2,990W max power for an appliance.
Assuming European countries (220V) use the same 13A standard, this would be c.2,860W max power. So the 2.5 - 3kW range looks appropriate (given losses and fluctuations in current) as being the max for the European / UK markets.

If you try to use it in the US without a suitable converter, it would try to draw c.double the current - so to achieve 3kW on 120V supply you'd need a current of 25A, which the plug/kettle/socket may not be able to withstand - hence it is not rated for that.

My guess about the smaller kettle is that it is usable with a wider range of voltages and currents off the bat, so might well be sold around the world rather than specific to one market.
If it is Japanese then they operate on 100V, so at 250W they'd need 2.5A, up to 10A for the 1kW. In the UK you're anywhere between 1 - 4 amps for the same.
If it's Taiwan, then they're 110V etc.

What does the sticker suggest?

If it's an expensive small kettle then it may have a sensor and adjust power consumption for the level of water, so that it always boils in roughly the same time? I'm not aware that kettles do that, but you never know.

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15. ### exchemistValued Senior Member

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Agreed: the variation between 2520 and 3000 W could, just about, be due to the max and min voltages in EU countries + UK.

But this does not account for a range of 250W-1kW as in the OP.

16. ### foghornValued Senior Member

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Looking around the web, I have noticed my model kettle is advertised as 1000W and not what is showing on my kettle's base label i.e 250-1000W.

17. ### exchemistValued Senior Member

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Does this kettle have a "keep warm" feature, by any chance? Looking on the web I see that quite a number of them do. I wonder if this may use a segment of the element only, or something. Perhaps if you post a picture of it, we can try to find an on-line description of it that explains the 250W label.

18. ### foghornValued Senior Member

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''Does this kettle have a "keep warm" feature'' Not for £19.

Russell Hobbs site.
https://en.russellhobbs.com/product/travel-kettle-23840-70
I think Sarkus was on to it about Europe and all.

Notice at the bottom of page it makes the point of '' Dual Voltage 120-240V''

It does not mention the spec label's 250-1000W, only shows 1000W.

19. ### exchemistValued Senior Member

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AHA! The penny has dropped. It's a sold as a "travelling" kettle. If it is used in a 120V country then, given the resistance of the element is fixed, the current it will draw will be half as much as at 240V. But the power dissipated in a resistance is i²R, so halving the current reduces the power dissipated to a quarter of its value at 240V. So 250W instead of 1000W.

In Europe the voltage is always 220-240V, so far as I am aware, but in N America it is usually 120V. So they are covering themselves for you taking it there.
However, as someone observed earlier in the thread, 250W is pretty hopeless for boiling water. So in practice, in a 120V country you would not want to use this kettle.

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21. ### foghornValued Senior Member

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Why am I making all this fuss over a kettle?
I'm checking my reckoning against the smart meter reading. Funny thing is the smart shows 2 pence and sometimes 3 pence a cup, even one of 1p.
And, I have to say, I wait a minute or two to watch it update the display after a cup has boiled. That's what started me on this trip.
I suppose the smart meter is not so punctual in updating the display, and when it does it picks a number out of the air between 1 and 3 ??? I suppose and hope a more 'accurate' usage reading is being sent to the electric company.

Last edited: Apr 25, 2022
22. ### foghornValued Senior Member

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When you pour that into a mug, would you get a sprinkling of cinders for a topping?

23. ### SarkusHippomonstrosesquippedalo phobeValued Senior Member

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THAT's the phrase I was looking for, but couldn't think of it!

Small kettle, small amount of water. If just 1 cup then would only take a few minutes.
And sometimes better to have a cup of tea than none at all, even if it takes quite a while!

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