PlotWatt Labs: Boiling Four Cups of Water
Posted by Luke Fishback
In the last post, we had a real head-scratcher from a real tea-drinker. Francis wanted four cups of tea in an hour, and he wondered whether it’s smarter to boil all of that water at once in his electric kettle, re-heating it as necessary, or just to boil one cup of water from the tap each time.
What’s the difference?
Advocates for the bulk-boil camp argue that there’s an energy cost of heating up the kettle and the water. If you boil all of the water at once, the greater thermal mass of water will retain more heat, and you won’t be cooling off the kettle with your frigid tap water each time.
Those in the “one-at-a-time” camp thought that a single cup boil would be more efficient because the kettle may not be well insulated. Also, if you boil more water than you drink, any gains you make in efficiency would be lost by heating unneeded water.
I was a staunch advocate of the single-cup method, reasoning that maintaining water at high temperatures just makes the heat radiate, conduct and convect faster. It’s a lot like the beach ball analogy we used when looking at programmable thermostats – the more you inflate a pin-pricked beach ball, the faster the air leaks out. Reducing the pressure (or heat difference, in this case) reduces the energy loss.
Enough conjecture, let’s get to the experiment.
To the lab!
I set up the tea kettle with a plug-load monitor to keep track of energy use, and then placed the kettle on a scale. Those of us who like the metric system know that one milliliter of water weighs about one gram, so I could keep track of my water volume without pouring it in and out of a measuring cup and increasing the heat loss.
I did the single-cup experiment at first by pouring one cup (between 239 and 242 grams in my trials) of cold tap-water into the kettle and pushing the button. The kettle boiled and I recorded how much water remained after some evaporation during boiling. I then emptied the water into my wife’s teacup, waited 15 minutes, and got fresh water from the tap for the next run.
For the bulk-boiling experiment, I filled the kettle with 965 grams of water (the total of my four single cups) and hit boil. I then weighed the kettle, poured about 240 grams into a tea cup, and waited 15 minutes before re-boiling the remaining 3 cups. I repeated the process until all the water was gone.
Crunching the numbers
The results? Here’s the tea-kettle’s energy use over time for my two experiments:
First, you’ll notice that the kettle behaves very predictably, coming up to 1,370 watts, remaining on while the water boils, and then turning back off. You’ll see my four individual cups across the top, and the bulk boil’s four cups across the bottom.
In the single cup experiment, the boil time (and therefore energy use) for each cup was about the same. The tap water started between 65 and 70 degrees F and boiled in an average of 1 minute 44 seconds, consuming an average of 39.3 watt hours.
The total energy consumed by boiling four cups, one at a time was 157 watt hours. If you’re paying 10 cents per kWh, that tea cost you just shy of two cents.
Boiling four cups all at once has a different profile: there’s a much longer initial spike (5 minutes 13 seconds) and then the subsequent re-heats take a little less time since the water is already hot (between 142 and 180 degrees F with the hotter temps corresponding to the higher volume of water). Here are all the times for each experiment:
The total energy consumed by boiling the four cups by the bulk method was 190 watt hours, about 21% more energy than the single cup method. Where is all of that extra energy going?
Single-cup win, but why?
A big piece of the energy loss is due to evaporation. By boiling each cup individually, I lost 52 grams of water (between 12 and 15 per cup) due to evaporation. Those steam molecules take energy with them to the tune of 2257 joules per gram (the heat of vaporization). Since a watt is 1 joule per second, each gram of water that evaporates takes with it 0.63 watt hours, accounting for over 20% of the total energy input to boiling, and that’s without even opening the lid on the kettle.
Bulk-boiled water had longer to sit at hotter temperatures, and it lost 68 grams of water, or just over 22% of its energy to evaporation. In fact, my last cup of tea from this experiment was a scant 177 grams, or 3/4 of a cup. In effect, I’d have had to boil MORE water initially if I wanted to get close to my full final cup of tea.
So bulk boiling took 33 more Watt-hours than boiling one at a time. Thirty percent of that was due to evaporation, and the other 70 percent was likely due to radiation, conduction, and convection caused by holding hot (140-180 degree) water in a room that is 71 degrees.
Now therefore, we the jury find that…
The winner of this test is boiling one cup at a time, just-in-time.
I should probably point out that the difference amounts to about 3 tenths of a cent. In the end, you may value the quick re-heat of the bulk boiled water, or the convenience of not running to the faucet for each cup. If that’s the case, go dig for change in the couch, and you can probably cover the cost of your slightly less-efficient ways.
Or better yet, get a thermos and make your tea in bulk. Then, when you get a craving, your piping-hot tea is just a click and pour away.