Energy Matters Forums

[Gordon-Loomberah]

Ever wondered which is the most energy efficient way to boil a cup of hot water?

I've done a comparison using several methods:

Induction cooktop with kettle +375ml water
Gas burner with same kettle +375ml water
Ceramic electric jug + 375ml water
Microwave oven with 375ml water in a large mug
Immersion heater in same large mug with 375ml water

Water temp was 12C
I dont have any sort of electric hotplate, so that is not included in the comparison, however, it is considered to be less efficient than induction heating.


I've used my Graphtec data logger to extract the details of energy used from the battery/PV panels, ie this is a gross measure of the power used in my off grid situation, including inverter inefficiencies.
The electric power used, I can define precisely, but for the gas burner I searched online and Primus Australia specify that the burners on the GB300 use 416g/hr of gas. The label on the cooktop says 750g/hr which is 37.26MJ/h, I assume that is for all 3 burners running simultaneously, since the burner near the gas inlet uses significantly more gas than the far end, with the middle gas consumption being intermediate. I used the middle burner and took the 416g/h as being not too far from reality.
I've converted MJ to Wh for this comparison.
I adjusted the element position in the jug and ran it tilted back, so the element was well covered by the small volume of water in it.

Results:
Immersion heater ....118sec____ 48.3Wh .... @1473W
Electric jug ............115sec____ 51.6Wh .....@ 1615W
Induction cooktop ...101sec____ 63Wh ...... @2246W
Microwave oven ......171sec____ 89.5Wh .... @1884W
Gas burner ............167sec____ ~270Wh ..... @~5700W

The ~40 year old immersion heater is the most efficient, closely followed by the (old) electric jug, then the new induction cooktop, then the microwave. Gas was by far the least efficient, and in my off-grid situation the only method producing CO2 and other combustion gases. The microwave heating was done in 3 stages with a finger dip test to see how hot it was, the loss of heat during the 2 short breaks resulted in an insignificant increase in the amount of time required to boil the water IMO.
Traces from the data logger below- other loads were minimised when testing, to prevent fridges etc starting during the water heating test.

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[cw nsw]

My electric jug - labelled 1850-2200W, takes 125 seconds for 750ml (minimum vol required)

[bpratt]

Just wondering how the results might differ based upon the standard size cup of 250ml.

Microwave might be the most efficient, as most jugs require a minimum amount of water inside them to operate, which might be 375ml or greater.

Of course if you're making more than one cuppa at a time, then the immersion heater/jug would be the better option.

[Helipos]

Although it may be a bit more than a cuppa, Gordon I suspect a new fandangled "refridgerative" (spelling and for want of a better word) water heater should beat them all.

(for the uninitiated) Basicly an aircon stuck on heating mode, heating up water.

Cam

[Gordon-Loomberah]

Just wondering how the results might differ based upon the standard size cup of 250ml.

250ml isn't enough for the jug, but may be just enough for the immersion heater, so it would still win out. I reckon the induction hob would still be well ahead of the microwave though. 375ml is our standard sized mug, maybe we are thirstier than the average person

Although it may be a bit more than a cuppa, Gordon I suspect a new fandangled "refridgerative" (spelling and for want of a better word) water heater should beat them all.

Yea, depending on the coefficient of performance, they would be ahead by a reasonable factor. However, unless they are using a different gas than what's in a heat pump hot water system, they wont bring the water to a boil... or are there heat pumps for boiling water in the kitchen available now

I wont be doing any extra testing right now though, the output from my 2.19kW array didnt make it above 5A @ 24V until after 9am due to the gloomy weather and is still under 7A, although it did hit 75A for a couple of seconds due to a hole in the clouds.

[John46]

Ever wondered which is the most energy efficient way to boil a cup of hot water?

Yes I have and thanks for your data - it is very interesting.
If it was 100% efficient the electricity used would be 38.4 Wh (according to my calculations).
This is not far below your immersion heater (48.3Wh)

375ml water at 12°C
4.1868 is the specific heat water, joules to raise 1gm by 1 degree.
Joules = 4.1868 * 375 * (100-12)
To convert MJ to kWh, divide by 3.6.

I wonder whether you could hook an standard immersion heater up to a DC solar array ?
That should be more efficient.

[Gordon-Loomberah]

If it was 100% efficient the electricity used would be 38.4 Wh (according to my calculations).
This is not far below your immersion heater (48.3Wh)

If the 425g of ceramic cup is taken into account- inital temp of ~15C, specific heat unknown, but probably under half that of water, and it wont go all the way up to 98C (BP of water here) at the instant the water starts to boil due to poor conductivity, then much of the remaining 10Wh is accounted for.

I wonder whether you could hook an standard immersion heater up to a DC solar array ?
That should be more efficient.

Easy enough to do, just have a string of ~6 panels to produce ~230V @ 6A, and being a resistive heater, it will work just fine, with no inverter losses. Be careful with the high DC voltage though

[John46]

I was thinking of experimenting with a just a couple of 190W panels, for heating a small amounts of water slowly and continuously.
Unfortunately with immersion heaters, the power is proportional to Volts squared.
So a 2000W kettle element would only be around 100W with a 48 DC volt system.
I suppose with a really well insulated kettle or tank, it would take 1/2 an hour to boil the 375ml.

Thinking about it, I guess solar water heating is best done the conventional way, with hot water panels or evacuated tubes.

[Gordon-Loomberah]

Thinking about it, I guess solar water heating is best done the conventional way, with hot water panels or evacuated tubes.

Yes, a bit hard to connect to a cup, but I reckon a single evacuated tube would do a decent job of heating a cup of water. The metal bulb on the end of them certainly gets hot enough to cause a burn to your hand just a few seconds after the tube is exposed to the sun.

[Cherokee Solar]

Hey Gordon,

Don't want to be a smart alec , but I use the local timber to heat water, so I wonder how much energy that would use? The wood stove has an 8kW water jacket at the back of the fire box which heats all of the water in the house + the hydronics (if I switch the pump on). If I had to use PV to heat the house there'd be tears for sure .

I'm going to try some thermoelectric devices on the firebox over the next few weeks so I'll let you know how they go. It should be interesting as I don't have much sun, but there's 100's of years of timber here. I've usually got about 5 years supply cut up on the ground seasoning. I'm not kidding. All of the rain over the past year and a bit resulted in trees that have grown up another 4 to 5 metres and are starting to over shadow the solar panels which is a real pain. Cutting them down is painful too.

Regards

Chris