Heat up hot water by Gas or Electricity?

Mstty

Raxiel said:

macman said:

Using hot water, because the bulk of the heating is done by (cheaper) gas. However, you will be using more water, if metered, so factor that in. 
Given that the cost of boiling a 3kW kettle for 3 mins is about 5p, the saving is going to be trivial, probably less than 2p.

This thread has piqued my curiosity enough that I've finally decided to figure out exactly what it takes to run a kettle.

1 Cup of water is about 0.25 litres, and it takes 250 calories to raise that much water by 1°C

The temperature of incoming water varies throughout the year, but lets give it a pretty conservative lower bound of 4°C, so that's ▲96°C to raise it to boiling (in reality the bulk of the water in a kettle is only close to boiling but we'll ignore that for this exercise).

So thats 250 x 96 = 24,000 Calories, or 24kCal (not the food type)

24kCal = 100,416 Joules or 100,416 Watt Seconds, 28 Watt Hours or 0.028kWh

At the most expensive April 23 EPG single rate of 33.11p/kWh that means a perfectly insulated kettle would cost 0.9p to boil one cup of very cold water.

A typical cheap plastic jug kettle isn't that well insulated, but I doubt the losses in the time it takes to boil (a 3kW jug should be about 40s per cup) would extend the heating time to push it over 1p/cup.

It's been done to death before 

https://forums.moneysavingexpert.com/discussion/6354340/the-kettle-debate/p1

Raxiel

Mstty said:

Raxiel said:

macman said:

Using hot water, because the bulk of the heating is done by (cheaper) gas. However, you will be using more water, if metered, so factor that in. 
Given that the cost of boiling a 3kW kettle for 3 mins is about 5p, the saving is going to be trivial, probably less than 2p.

This thread has piqued my curiosity enough that I've finally decided to figure out exactly what it takes to run a kettle.

1 Cup of water is about 0.25 litres, and it takes 250 calories to raise that much water by 1°C

The temperature of incoming water varies throughout the year, but lets give it a pretty conservative lower bound of 4°C, so that's ▲96°C to raise it to boiling (in reality the bulk of the water in a kettle is only close to boiling but we'll ignore that for this exercise).

So thats 250 x 96 = 24,000 Calories, or 24kCal (not the food type)

24kCal = 100,416 Joules or 100,416 Watt Seconds, 28 Watt Hours or 0.028kWh

At the most expensive April 23 EPG single rate of 33.11p/kWh that means a perfectly insulated kettle would cost 0.9p to boil one cup of very cold water.

A typical cheap plastic jug kettle isn't that well insulated, but I doubt the losses in the time it takes to boil (a 3kW jug should be about 40s per cup) would extend the heating time to push it over 1p/cup.

It's been done to death before 

https://forums.moneysavingexpert.com/discussion/6354340/the-kettle-debate/p1

I see a debate about different models of kettles and how long they take (and the journalistic standards of the daily heil). I'm just stating the scientific fact of how much energy it takes to heat one cup. Perhaps someone did that in the thread but I wasn't going to read all 15 pages.

Mstty

Raxiel said:

Mstty said:

Raxiel said:

macman said:

Using hot water, because the bulk of the heating is done by (cheaper) gas. However, you will be using more water, if metered, so factor that in. 
Given that the cost of boiling a 3kW kettle for 3 mins is about 5p, the saving is going to be trivial, probably less than 2p.

This thread has piqued my curiosity enough that I've finally decided to figure out exactly what it takes to run a kettle.

1 Cup of water is about 0.25 litres, and it takes 250 calories to raise that much water by 1°C

The temperature of incoming water varies throughout the year, but lets give it a pretty conservative lower bound of 4°C, so that's ▲96°C to raise it to boiling (in reality the bulk of the water in a kettle is only close to boiling but we'll ignore that for this exercise).

So thats 250 x 96 = 24,000 Calories, or 24kCal (not the food type)

24kCal = 100,416 Joules or 100,416 Watt Seconds, 28 Watt Hours or 0.028kWh

At the most expensive April 23 EPG single rate of 33.11p/kWh that means a perfectly insulated kettle would cost 0.9p to boil one cup of very cold water.

A typical cheap plastic jug kettle isn't that well insulated, but I doubt the losses in the time it takes to boil (a 3kW jug should be about 40s per cup) would extend the heating time to push it over 1p/cup.

It's been done to death before 

https://forums.moneysavingexpert.com/discussion/6354340/the-kettle-debate/p1

I see a debate about different models of kettles and how long they take (and the journalistic standards of the daily heil). I'm just stating the scientific fact of how much energy it takes to heat one cup. Perhaps someone did that in the thread but I wasn't going to read all 15 pages.

And that helps the OP in what way?

Maybe start a brand new kettle thread we haven't had one for some time or use "search" lots of them👍 this will just send a thread not about kettles down the endless rabbit hole🤣

armith

Raxiel said:

Mstty said:

Raxiel said:

macman said:

Using hot water, because the bulk of the heating is done by (cheaper) gas. However, you will be using more water, if metered, so factor that in. 
Given that the cost of boiling a 3kW kettle for 3 mins is about 5p, the saving is going to be trivial, probably less than 2p.

This thread has piqued my curiosity enough that I've finally decided to figure out exactly what it takes to run a kettle.

1 Cup of water is about 0.25 litres, and it takes 250 calories to raise that much water by 1°C

The temperature of incoming water varies throughout the year, but lets give it a pretty conservative lower bound of 4°C, so that's ▲96°C to raise it to boiling (in reality the bulk of the water in a kettle is only close to boiling but we'll ignore that for this exercise).

So thats 250 x 96 = 24,000 Calories, or 24kCal (not the food type)

24kCal = 100,416 Joules or 100,416 Watt Seconds, 28 Watt Hours or 0.028kWh

At the most expensive April 23 EPG single rate of 33.11p/kWh that means a perfectly insulated kettle would cost 0.9p to boil one cup of very cold water.

A typical cheap plastic jug kettle isn't that well insulated, but I doubt the losses in the time it takes to boil (a 3kW jug should be about 40s per cup) would extend the heating time to push it over 1p/cup.

It's been done to death before 

https://forums.moneysavingexpert.com/discussion/6354340/the-kettle-debate/p1

I see a debate about different models of kettles and how long they take (and the journalistic standards of the daily heil). I'm just stating the scientific fact of how much energy it takes to heat one cup. Perhaps someone did that in the thread but I wasn't going to read all 15 pages.

Well I appreciated your input anyway  "Ensure that you only boil what you are going to use immediately".

The cost of the energy required to boil the water is about the same as the cost of the tea bag (and the journey the tea in the tea bag takes is the real environmental issue!).

matelodave

Everyone seems to forget that when they've run off several litres of cold water before the tap gets hot there is the same  amount of lovely hot waters sitting in the pipework which then cools down until you run it off again.

It takes four litres of water before our hot tap gets warm, that's half a kettleful of wasted cold water plus the same of wasted hot water sitting in the pipes.

Likewise, when a combi boiler fires up, it uses 28+kw of gas, most of which goes straight out of the flue until the water can manage to absorb it. Combis are far less efficient for short draw offs of hot water than they are when idling away at a lower temperature providing heating.

Even when using a system boiler to heat a hot water tank there's a loss off efficiency because of the hot water circulating in the plumbing between the boiler and the tank. Even a 90% efficient boiler wont transfer all of the energy to the tank because some will be lost in the pipework.

At least with a kettle or immersion heater, all the energy goes straight into the water and is only lost by the surface area of the kettle or insulated water tank rather than via several metres or more of pipework which may be under the floor or even in a loft.

Scot_39

armith said:

On a slightly related theme - is it better/cheaper/worse/horribly-expensive to boil a kettle using hot water or cold? It takes about 3 minutes to boil the kettle from the cold tap - but only a minute to boil it from the hot, with the water having run from the tap for about 30 seconds. This is with a combi boiler - I wouldn't do it from an immersion heater tank.

In winter its debatable - as the hot water in the pipes (unused between the boiler and the tap) maybe adding heat to the rooms.

But in summer that water - in the pipe - is going to be wasting energy.

It takes a while for even the water from a hot tank to raise to sufficient e.g. for washing hands or face.  So all that cold water is wasted - and probably partially heated below a usable temperature - wasting energy - and if metered for water - that costs money too.

In my place probably about 1/2 a basin full - before can put the llug in - for a full basin to be warm enough - and the tank maybe only 4m away as the crow flies from the basin.  It's a non trivial volume of water.

So it's not going to be the same for everyone.

But in reality - it doesn't come down to costs - but waste as above and health - for not using the hot water.

From a heath perspective - arguably it depends far more on your heating and hot water configuration - and what you are going to use water for.

Certainly if a hot water tank / fed from a cold water tank in loft - as per old systems - it's generally advised to err on side of  caution - and "assume hot water taps not fit for human consumption" - even if boiled - that may even arguably be too short a period.

And stored water and hot water pick up trace elements more - so bad when lead pipes etc.

Which is why the cold water in the kitchen in most houses - if not the rest of the house these days - is not normally tank fed - even in houses built / plumbed decades ago with those tanks.  As a source of safe potable water.

I personally would always use cold if for me / drinks or for cooking.  

But thats based on growing up in houses with cold water tanks and immersion / gas het hot water tanks. 

Others with combis might disagree entirely.

marie_hc

Tgrewal said:

Due to recently getting an electric car, I switched my energy supply to Octopus. I am on the Intelligent Octopus (IO) tariff for my electric and Octopus Flexible tariff for my gas.

Every day in the early hours of each morning, I heat up my hot water (Megaflo hot water tank) for 2 hours via the 30kW gas boiler. The off-peak electricity rate (23:30 - 05:30) is 10p kWh and the gas rate is 10.41p kWh. 

Stupid question but if I were to heat up my hot water for 2 hours via the electrical immersion heater during the off-peak hours, would it cost me the same in electricity as it would in heating up the water via gas for 2 hours, or would it roughly be the same since both, the gas and electricity per kWh rate is roughly the same?

Is this 2 hours at separate times through out the day?   I heat my water up on gas boiler for 30 minutes tank is hot. 

Why is 2 hours significant?

marie_hc

Mstty said:

markin said:

The boiler will at be around 90-98% efficiency so electricity will be cheaper, also not burning gas would be greener even if costs were dead on, if you care about that.

Is this factually correct as I keep reading the actual average is 85% for gas boilers (perhaps this is all ages combined) but would love to know the real figure as it does have a bearing when calculating cost.

I can tell you I used immersion for whole month my electric went through the roof  I have gone back to gas for hot water 

Scot_39

marie_hc said:

Mstty said:

markin said:

The boiler will at be around 90-98% efficiency so electricity will be cheaper, also not burning gas would be greener even if costs were dead on, if you care about that.

Is this factually correct as I keep reading the actual average is 85% for gas boilers (perhaps this is all ages combined) but would love to know the real figure as it does have a bearing when calculating cost.

I can tell you I used immersion for whole month my electric went through the roof  I have gone back to gas for hot water 

You didn't say by who much cf your gas dropping though.

And was that Combi on demand or tank heating.

And were you heating to the same level.

The thermostat on your immersion heater and the one on your gas boiler may have been different.

And as an adide in summer - I only run 1x or 2x a day when can be bothered switching it off - rather than 3x in cold winiter months - off my E10.

And also depends on your tariff.

Some people were getting 6-7p night rate over last 3 months on E7  - more than enough to beat gas - on a straight like for like energy calculation.

My E10 - and others on E7 - saw double digit price percenatage rises in Jan - in my case to 22.6p - so gas would have beaten hands down.

And against flat rate c34p - even more so.

Mstty

marie_hc said:

Mstty said:

markin said:

The boiler will at be around 90-98% efficiency so electricity will be cheaper, also not burning gas would be greener even if costs were dead on, if you care about that.

Is this factually correct as I keep reading the actual average is 85% for gas boilers (perhaps this is all ages combined) but would love to know the real figure as it does have a bearing when calculating cost.

I can tell you I used immersion for whole month my electric went through the roof  I have gone back to gas for hot water 

This is where threads go off topic. I am responding to the OP who has a 10p kWh rate on electricity they can utilise over night which is cheaper than gas at 10.4p kwh. Is this the same situation as you and if so you are doing something wrong.

So in very simple terms it should be cheaper to heat with electricity over gas ignoring both the efficiency of gas is lower but conversely electricity immersion heaters may not produce as much heat in the same 1-2hour period so you have to factor in some tank temperature losses.

In this instance a quick fag packet calculation has them coming out almost equal but it would be good for the OP to test this and report back.