I was surprised. Are you surprised? I mean, burning the fuel is itself highly efficient. The pot sits right atop the flame. Dirt simple. Some heat escapes around the side, some goes to heating the pot itself, but come-on—16%?!
In another test on another night, I boiled 500 mℓ of water starting at 16.5°C on a smaller burner (1670 W, or 5,700 Btu/hr). I used the same pot, but this time with its lid. I put the burner on full power, but its smaller diameter meant that no flames visibly licked around the bottom of the pot. Want to guess as to the efficiency in this mode? I deem this mode to be about as efficient as I am likely to get on the stove-top using standard practices. The answer: I used 640 kJ of gas for something that should have taken 175 kJ for an efficiency of 27%. An enclosed kettle on the same burner delivered the same result.
Okay, so a 68% improvement over the full-throttle approach is a significant gain (although it took almost twice as long). But still, I was surprised that I top out at less than 30% on a gas stove burner.
Incidentally, from the rate of cooling after the burner was turned off, I gather that the water remaining in the open pot lost heat at a rate of about 300 W through convection, radiation, and possibly some conduction to the grate. These mechanisms are roughly proportional toΔT between the water and the environment, so on average come to about 150 W during the linear ramp-up of temperature from ambient to boiling. This would have cost 63 kJ in the slow case (about 10% of the total), or about 35 kJ in the full-blast case (3% of the total). But the lidded case would actually lose heat less quickly than these numbers indicate, as the loss rate is based on an open pot after the boiling test was complete. In either case, loss from the pot to the environment appears to play a relatively minor role.
How About Microwaves?
Okay, the stove top delivered disappointing efficiency results. But right above the stove, I have a microwave. Naively, I expected the microwave to be something like 80% efficient at delivering energy to the water. The thinking goes like this: the interior of the microwave oven is a good reflector for microwaves, so they rattle around until they find an absorber; namely, the food/water. Of course there will be some loss in generation at the magnetron tube (I imagine it gets hot). And the walls may not be perfect reflectors, so that after twenty bounces, there may not be much energy left. But still, 80%—right?
This time, I put 500 mℓ of 18°C water into a plastic measuring cup (63 g) and heated on full power for 90 seconds, measuring the temperature afterwards. The water reached 50°C, demanding 67 kJ of energy. Meanwhile, both a Kill-A-Watt and TED (The Energy Detective) told me that I used 157 kJ of utility energy. That’s 43%, folks. Yes, I’m disappointed too. A later test at 120 seconds produced 60°C water (this time from 22°C) for a computed 40% efficiency.
Considering that electricity often comes from fossil fuels at 30–40% efficiency, the microwave (as tested by me) is only 15% efficient at transforming fossil fuel heat into heated water. That’s worse than the stove top—especially the slow-and-steady version.
What about electric kettles, where a heating element is immersed into the water, directly heating the thing you care about? To boot, these kettles often have insulated sides, keeping the heat where you want it.
I borrowed a kettle from work, and heated 500 mℓ of water from 18°C. I was able to suspend a thermometer in the water in a way that did not compromise the lid or spout. After 152 s, the water was making plenty of happy bubbling noise and the thermometer read 100°C—although apparently it was not as well-calibrated as I would have expected, since it sailed up to 105°C after 173 s. The water was obviously in a vigorous boil by then, and I shut the kettle off at 210 s (I should have let it turn off on its own, but lost patience with its inefficient lethargy).
Both the Kill-A-Watt and TED agreed that the kettle consumed about 1440 W. So while I needed 0.5×4184×82 = 171.5 kJ to raise the water to boiling from 18°C, I spent 219 kJ by the time the (erroneous) thermometer read 100°C; 249 kJ by the time a full, roiling boil was obvious; and 302 kJ by the time I shut the device off. Using the middle number, I calculate 69% efficiency. I get 78% if I use the shortest time, when I thought I was at boil. But by the time I shut it off, we were down to 57%, and falling. Who knows how far it would have sunk before shutting itself off (kicking myself that I don’t know).
If the shutoff is sufficiently sensitive, it seems electric kettles are capable of something like 70% efficiency or better. It would seem to beat the pants off the other methods—except for two caveats.
The first is the one raised for the microwave. If your electricity comes from fossil fuels, 70% efficiency becomes 25% in turning fossil fuel energy into hot water. The stove top is competitive.
The second caveat is that kettles suffer a sometimes rather large inefficiency due to lax filling practices. A kettle is considered to be a reservoir. Few measure the amount of water they put in. As long as it’s enough, game on. So the tendency is to overfill. All the water gets heated. Only some is used. This practice is probably even more pervasive in communally-shared kettles, and could easily cut a deep hole into the net efficiency. By contrast, microwave practices typically heat exactly as much as is consumed. A kettle with 50% over-fill turning off on its own sweet time can easily sink to the levels achieved by the microwave.