Sunday, 10 August 2008

Tea-making with gas and electricity

B3ta recently asked whether it was cheaper to make a cup of tea using an electric kettle or a gas one. My gut feeling on this was that gas was cheaper, because gas is a lot cheaper than electricity per unit energy. I've done an experiment to see which is actually better.

Starting with gas, I put a mugful of water in a steel saucepan and heated it over a low flame. I can measure the amount of gas used using my gas meter. I've completely shut off my central heating boiler, including the pilot light, so no other gas is used during the experiment.

This took 0.0092 cubic metres of gas to bring it to the boil. I'm calculating the energy and cost based on my last gas bill, which explains how to convert from cubic metres to kWh - ultimately by multiplying by 11.0786. So I've used 0.102 kWh. I pay 2.88p per kWh for gas. reckons I could get gas for 2.86p from another supplier, so I'll use that. That makes a total of 0.29p.

Next I set up an electric kettle with a plug-in power meter. I've got to use half a litre of water to cover the element and make up the 'minimum' mark on the kettle, which is slightly more than the mugful I boiled in the saucepan. To boil this took 0.06kWh. The cheapest electricity I could find on USwitch was 11.21p/kWh, so this cost me 0.67p. Even given the inaccurate measurements in this experiment, gas is the clear winner.

But what if I've got friends round? I did a test with a large teapot full of water as well. This time, the saucepan took .281kWh for 0.82p, and the kettle took .140kwH for 1.57p. Still nearly double the price of the gas option.

Boiling speed is another factor - the gas saucepan takes a lot longer to boil than the electric kettle. I did some more experiments, using the gas turned up higher than I normally would. I expected this would waste gas, because a lot of heat escape round the side of the saucepan. However, it was barely any extra. There's probably a 'sweet spot' for the gas flow - too high would waste gas, and too low would take so long to boil that heat gets a chance to escape from the saucepan. Still, I couldn't get it as quick as the kettle. The kettle isn't a fast boil one - it's about 2.2kW. Even so, in all the experiments I measured time for, the kettle is twice as quick. So for the extra third of a penny it costs to boil the electric kettle, and the convenience of having an automatic off switch, I may well stick with that for the time being.

You'll notice that the kettle uses less energy despite costing more. Does this mean the kettle is more efficient? Not necessarily. The kettle is more efficient in itself than the stove and saucepan, but a lot of the electricity comes from gas in the first place. About 36% of our electricity comes from gas-fired power stations, and the best stations are about 58% efficient. Another 37% comes from coal, but I haven't found efficiency figures for those yet. If our electricity came entirely from gas, the kettle would use about the same amount of energy as the stove. To do a fair comparison you would also need to know how efficient the national electricity grid is, and how much energy is used in pumping gas around the country.

The data in full:

FuelQuantitySpeedTime (s)Volume of gaskWhcost/kWh (p)Cost (p)
Gas1 cupslow 0.00920.1022.860.29
Gas1 cupfast1920.01000.1112.860.32
Gas1 teapotslow13760.02540.2812.860.8
Gas1 teapotfast5060.02580.2862.860.82
Electric1 cupn/a  0.06011.210.67
Electric1 teapotn/a213 0.14011.211.57


Daniel said...

Very Good Work Sir!

Woodsy said...

My missus is addicted to tea...we're going to the Caribbean on the back of this! Thank you, fine sir!

adam said...

Move into the 21st century! How does a mug of water in the microwave compare on cost and speed?

Very interesting results so far...

Eff said...

I wonder, what would the data be using one of the Tefal "quick cup" kettles?

They claim it takes 3 seconds to deliver the hot water to make tea, however, the water isn't actually boiling; but more than hot enough for a cup of tea.

Would you look at that method as well?


cowjam said...

As the one who posed the question in the b3ta newsletter, I thank you wholeheartedly.

Amethyst; tea should be made using boiling water. Those quick cup kettles should be used for instant coffee instead.

Tom said...

Stunning investigative journalism here.
As a penniless student layabout type I'm always interested in ways I can shave fractions of pennies off my utility bills - my hat goes off to you sir!
I do have beef with eff/amethyst though:
"They claim it takes 3 seconds to deliver the hot water to make tea, however, the water isn't actually boiling; but more than hot enough for a cup of tea."
Are you an American by any chance? We all know that water used for making Tea should be added to the leaves/bag as soon after boiling as possible. Just isn't right otherwise... My scorn is poured upon you!

Tom said...

Very intereting and useful analysis. Something you didn't mention however is the contribution of the 'wasted' heat of the gas method to the temperature of your house. In winter, and if you heat your house with gas, there is effectively zero wasted energy (and money) when using gas to heat water - it all ends up heating your house, thus making the gas method even cheaper. Obviously in the summer this is not the case and your analysis remains accurate.

Peter Eaton said...

Hmmm..good work, but you used a saucepan for the gas, and a kettle, for the electric.You should use a kettle for the gas expt. too, they are more efficient than the saucepan, because of the higher vapour pressure above the liquid/gas interface.
I, too would be interested in the microwave experiment, apparently some people do this, i believe it will take LOADS of energy to get a boiling cup that way...

srimech said...

Peter, I think you're right about having a proper gas kettle, but since this experiment confirmed my hypothesis, I didn't see the need to go out and buy one.

I might do the microwave experiment this weekend. I don't see how it could be any more efficient than a kettle though...

Joinedup said...

You've not allowed for night rate / economy 7 tariff electricity, in my area this runs to 16.53p per kWh between 7AM and midnight (GMT) and 4.08p per kWh between midnight and 7AM.

If you have a dual rate meter you could save money by confining your tea making to the economy 7 hours... or perhaps by bulk boiling and filling up thermos flasks, though this would admittedly be more suitable for instant coffee or hot chocolate making.

Jayxxx said...

This is all a bit Unscientific. You nowhere compare the energy to heat Exactly Equal Volumes of water. Mug-fulls and kettle-fulls (or half-fulls) are all very well - but..... I suggest you go and repeat your experiments, or get your local 6th form college to set this as a physics assignment.

Peter Eaton said...

@Jayxxx: It's not unscientific to use a "Mugfull" of water, as long as it's the same mug, and filled to the same level eachtime.

OK, to your average Joe, saying "we used 186mL of water" *sounds* more scientific than "we used a mugfull of water", but in reality it's the same thing.

The NAME of the units used does not make them unscientific.

Mark said...

I had no idea how expensive boiling a kettle was!

Alice Thomas said...
This comment has been removed by a blog administrator.
plumbing supplies said...

Sometimes I drink coffee too, but I find that for the optimum relaxed mental clarity required for good prgramming, tea can't be beat.

Shank said...

Great post sharing information related to compare gas and electricity

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