I know that when making tea it uses less energy when heating the water when using a water boiler(cooker), than when using an electrical plate. A water boiler is directly heating the water, while a cooking plate first transmits heat to the pot and then to the water.

Now I need to boil water for cooking broccoli. Shall I first boil water with a water boiler an than pour it into my pot (where it will be cooled down again by the walls of my cold pot and I will have to heat the pot anyway to keep it boiling for a while), or shall I boil immediately in my pot without the water boiler?

Or are there other techniques to use less energy to boil my broccoli? Heating the pot a bit before pouring the water (although this sounds very stupid)? Using heated tap water (from an electrical heater/gas heater or geyser).

If it matters, I'm having an electrical cooking plate (not ceramic or glass).

up vote 14 down vote accepted

Operating one cooking appliance would be more efficient.

Using your two-pot method, to maintain the boiling water once it is transferred to the cooking pot, you'll have to first bring that cooking pot up to temperature. During that time, the system inefficiencies are in effect for two systems, and heat and energy are being lost.

The heat being radiated during the pre-heating of the cooking pot might as well be used to heat the water it will eventually contain. Besides, electrical and heating appliances are not efficient. We're probably talking minutia here, but heating up two completely different systems is more wasteful than sticking to the one.

If you really want to save energy, steam your broccoli. It's healthier, tastier, it needs less cooking time… and you'll have to boil much, much, much less water.

  • 3
    +1 for mentioning steaming, which is really the only way to prepare broccoli. – Earthliŋ Apr 25 '13 at 21:47

Based on personal experience, I agree with Zach's conclusion that the dedicated water boiling kettle is more effcient than the stove.

My 1500W electric kettle boils a liter of water about 2-1/2 times faster than a covered pan on my stove's 2100W element (4 minutes versus 10 minutes)... and more than 3 times faster than the stove's smaller 1600W element).

My pan just covers the entire large heating element if I place it carefully, and has considerable overlap over the smaller element.

I believe the heating element in the kettle has much more efficient heat transfer to the water - while I can't see the element itself,I can see a raised bump at the bottom of the kettle where the heating element is, and I assume that it's mechanically bonded to the metal bottom of the kettle allowing for very efficient heat transfer. Further, the plastic bottom of the kettle doesn't get extremely hot, so the bottom of the element must be insulated to keep most of the heat against the water. And since the bottom of the kettle is not vented, there's not much opportunity for convection to pull heat away from the heating element.

The stove, on the other hand, relies on the weight of the pan to make contact with the element, and the spiral element may not be completely level, so some parts of the element are in better contact than others. Since the bottom of the element is exposed, it is free to radiate some of the heat down into the stove drip pan. Also, there seem to be some convection air currents carrying heat away, since if I hold my hand a few inches over the pan while heating water, it becomes uncomfortably warm in a short period of time.

While I agree that there is some heat lost in heating up both the kettle and the the pan, the specific heat of water is about 8 times greater than stainless steel, so the energy wasted in heating up the 24 ounce kettle is around the same energy needed to heat up 3 ounces of water.

Of course, it all depends on what type of appliances you're using. I have a stove with traditional spiral elements, if I had an inductive cooktop, it's likely that it would be more efficient than the electric kettle since more of the stove's energy would be going into the cooking pan. A glass cooktop with heating elements or infrared elements might also change the equation. As would a gas stove.

A water boiler or hot water tap are the most efficient ways to heat up water since they are insulated, keeping all of the heat in the container and water (and not the air). I would say that your best bet is heating water with the water boiler, pouring it into your pot, and cooking on the stove from there. Some heat would be lost to the container in the water boiler, but I think it would be less than the heat lost to the air from a hot pot on a stove. If you could cook the broccoli in the water boiler, that would be even better.

Using hot water from your tap is probably second best (and maybe even best), only because it won't get the water up to boiling and the water will have to spend more time on the stove.

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