The early summer cold-front that has been chilling Johannesburg for the last few days has me scratching my head. I measure my electricity consumption regularly and record it on a spreadsheet, and suddenly found that the household’s daily usage had jumped from about 15 kilowatt hours to about 25. Then we ran out of tea just a few days after we opened a box of 80 bags. Could there be a connection? Four children are at home studying for exams in the cold, and teacups have been piling up on their desks. With a thermometer, a stopwatch and a calculator, I set out to solve this mystery.

To raise a litre of water by one degree Celsius theoretically requires 0.0011 kilowatt hours, so heating my 1.7 litre kettle from tap temperature to boiling, 17 to 97 on my thermometer, should use 0.15 kWh. But since it took 3:48 to boil while using approximately 2750 watts, the actual consumption was more like 0.175.  (My Watts Up meter cannot handle appliances over 2000 watts, so I could not measure directly.) The inefficiency probably comes from the heat lost warming up the stainless-steel kettle itself—a plastic kettle might reduce those losses—and the delay between when the water reaches boiling and the kettle shuts off.

Of course, they don’t all share a pot of tea at once, so I used a spreadsheet to simulate two scenarios. In one, my children have suddenly become green angels, and measure out exactly one mug full of water into an empty kettle to boil. Under these ideal conditions, their additional 24 cups of tea a day would use just 0.59 kWh. The worst-case—and very common—scenario is to fill a kettle and boil it over and over again  until nearly empty before repeating the process. Under these circumstances, those same 24 cups of tea use nearly three time as much electricity, or 1.61 kWh. (Even after taking into account the warm water that is being re-boiled.)

My mystery is not entirely solved. The kettle could account for a sixth of the increase, but the rest must come from additional lights burning at desks, the refrigerator door opening and closing for study snacks, and those electronic devices that seem to take over study breaks.

Still, I’ve learned something useful. Changing kettle habits can make a significant impact on consumption. The Eco-Kettle is designed to make this simple, saving you from running back and forth to the sink to measure another cup of water. The water reservoir at the top of the kettle can release measured cups of water to the element at the bottom. It is available from a few sources in South Africa for R450 and up. Six people drinking 4 cups of tea a day with a full-kettle habit to break could probably save enough to pay for the expense in a couple of years.  I’ve seen a few complaints on the internet about durability, however, and I wonder if a simpler solution wouldn’t suffice. I plan to put a water jug next to the kettle and ask my family to measure out the water in their teacup before pouring it into the kettle. (Our kettle has a flat bottom. If yours has a raised element, it may need extra water to cover the element.) It should save them time, too. My stopwatch tells me that a cup of water boils in 51 seconds. A full kettle takes nearly 4 minutes.

While on the subject of kettles, I sometimes use ours as a back-up geyser. I try to keep the electric elements in my two, 300 litre solar-heated geysers switched off, and most days I don’t need them. But occasionally I have been seen pouring a few kettles full of boiling water into a bath on a winter night to keep my wife from grumbling about my solar fanaticism. This causes her to ask, “wouldn’t it use less electricity just to turn on the element in the tanks for an hour?” My spreadsheet provides the definitive answer. Boiling even five kettles of lukewarm water from the hot tap uses 2/3 of a kilowatt hour. The 4000 watt element on the geyser would use 4 kWh in an hour. There are times when a full kettle is green.