How many cups of tea/coffee do you have a day?

[dangermouse]

I'm not a big tea drinker, but for certain types of tea (I'm told) and certainly for making coffee, it's actually not required or optimal to heat the water to 100C. For making filter coffee, about 85C is best, although it depends on the blend.

Assuming you use an electric kettle with 100% efficiency:

To heat 250ml of water from 20C to 100 takes 25.25 Wh.

Specific heat capacity of water is 4.184. IE 4.184 Joules per ml per C. A Joule is a Watt second, so 3600 Joules is a Watt hour.
We're raising the water temperature by 80C (assuming water comes out of the tap at 20C).
And we assume a mug holds 250ml.
4.184 x 80 x 250 = 83,680 J
Divide by 3600 to give Wh = 23.25 Wh.

Or, for 1 kWh, you can boil 43 cups. 1000 / 23.25 = 43.

If we heat the water to 85C instead, the calculation becomes:
4.184 x 65 x 250 = 67,990 J
67,990 / 3600 = 18.89 Wh
1000 / 18.89 = 53 cups per kWh.

Lets say you drink 10 cups a day (we are British after all). Heating the water to 100C gives an energy usage of approx 1kWh per 4 days. Heating to 85C means you get 5 days of tea for 1kWh.

Or putting it another way, that's either 6.98 kWh per month, or 5.66 kWh per month. Saving 1.31 kWh per month.

If it's free energy from PV etc, who cares? If you pay for energy, that could save you 50p a month. So the payback period on a £50 temperature controlled electric kettle is 8-9 years.

I only posted this because I just bought a new kettle - I wanted one with lower power usage for when I get battery storage (this is the first tiny step in my master plan) - and I wondered how much it would actually save me! Probably not worth replacing a working kettle, but if you need a new one anyway, something to consider?

I apologise if tea drinkers find this offensive

[Caesium]

I find this sort of debate similar to those of light bulbs of old.

Yes old light bulbs used to use 60W rather than the 6W or so LEDs do now, but that energy all became heat that I didn't then have to heat my home with. So I could turn my heating down. In winter when the lights are on more, thats useful energy.

Isn't it the same with a kettle? If I boil water to 100c then all that energy has to end up in my home at some point, warming it up, so its not wasted

[NikoV6]

Boil once and put in a thermos! Tea water for all day then!

[dangermouse]

I find this sort of debate similar to those of light bulbs of old.

Yes old light bulbs used to use 60W rather than the 6W or so LEDs do now, but that energy all became heat that I didn't then have to heat my home with. So I could turn my heating down. In winter when the lights are on more, thats useful energy.

Isn't it the same with a kettle? If I boil water to 100c then all that energy has to end up in my home at some point, warming it up, so its not wasted

Not in summer, but it's a very good point! Making my kettle purchase even less justifiable!

[Mr Gus]

Boil once and put in a thermos! Tea water for all day then!

Ditto, as chewed over at "st eleswhere" ..many of us simply resorted to big flasks (mine was a thermos 1.8 litre, glass jobby, free from clearing a local festival) ..replacement lid £5 a decade later.
We use sigg hot & cold these days, when out & about.

thermos is fine for a handful of hours for tea before it affects flavour, ..coffee can have any old water grot & still taste like the orrible instant it aways is.

for real coffee, ive already banged that into a thread, thermal (full size) coffee maker pot, good for 4+ hrs, & so little air contamination that is ok a few days later reheated in the microwave.

[Mr Gus]

When (years ago) wife ..non tea drinker, got it into her head that "we" needed temp controlled kttle, thsy were extortionate in terms of pricing, ..would likely never have paid for itself, ..i leave mugs to brew n stew, so only upper temp matters to me, when the kettle died it was replaced by a modern fast boil via decent brand for £6 as an open box return, flawless quiet operation, & vinegar keeps it clean.

if it was sous vide, id be less contrary temp wise, but tea means boil, bag, stir to get started & leave, typically 3 mugs in quick succession.

(still careful not to waste tap water for a brew) easier in summerwhen dogs are thirsty, but just been told to stop bottling run off as weather cools) ...grr! ..instantpot gets cleaned with it, plants etc, but too much excess currently.

[richbee]

Boil once and put in a thermos! Tea water for all day then!

Not quite sure about the benefit of this - if you boil 3x as much water for a thermos, it uses 3x as much energy to heat it up.
Assuming you only boil the right amount for your cuppa (which might be a big assumption), it doesn't change the amount of energy used boiling it 3x instead of 1x.
I have been wondering about buying a 2.2kW kettle to replace my 3kw current one - as my single battery Pv system can only output 2.5kW from the battery, so will always use energy from the grid unless there is a substantial amount of sun in addition to battery usage.

[Oldgreybeard]

One luxury item I fitted when building this house was a boiling water tap. I was in two minds about it, as it didn't seem to me to be something compatible with my goal of building an extremely low energy home. What has really surprised me is how little energy it uses. Compared with the kettle at our old house the boiling water tap uses around 60% to 70% of the energy, for slightly more use, as we use it to cook things like rice and peas that barely need anything more than being covered with boiling water and left to cook themselves.

I can only assume that the energy saving comes from the good insulation around the under-plinth boiler unit plus the lack of wastage, the thing dispenses boiling water almost instantly, so a mug of tea only uses about a mug of boiling water. I monitored ours for the first couple of years, and noticed that its "keep hot" heater didn't use anywhere near as much power as the initial "bring to the boil" heater, so most of the time, even in winter, there's enough PV generation to cover the keep hot heater power demand.

Doesn't meet the requirement for variable temperature dispensed hot water, though. Our's dispenses water at slightly over 100°C, I think the boiler runs at an internal temperature of about 105°C. That's probably the only downside, as it does mean that the tap tends to spit a bit, as the water coming out is really boiling as it leaves the tap.

[nowty]

When I saw the thread, I assumed it would be about this,
https://www.bbc.co.uk/news/uk-england-london-63201578

[Oliver90owner]

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I have been wondering about buying a 2.2kW kettle to replace my 3kw current one - as my single battery Pv system can only output 2.5kW from the battery, so will always use energy from the grid unless there is a substantial amount of sun in addition to battery usage.

Buy a 1kW kettle? That is what I have. Easily filled with one mug-worth of water.

Actually, using gas is much cheaper, when the solar is low or zero. I’m afraid cost is important to me and daytime leccy is x4 of gas.