(“Coffeepot” was not the best word for the title. I wanted to use “urn”, but feared people might think of cremations… Anyway…)
I have an urn which we fill with water for special events, and it makes and keeps the water very hot so people can take from it to make their own tea, instant coffee, cocoa, whatever. But the amount of water that actually gets used is only a fraction of the urn’s capacity. So my question is which way is more energy-efficient:
One idea is to fill the urn with only a bit more water than we expect to use. This will use less energy because there’s less water to heat, but might use more enrgy because it will be difficult to maintain the heat.
Or maybe we should fill it all the way. It will take more energy to heat it, but a larger volume of hot water will lose heat to the outside more slowly, so it will be easier to maintain the heat?
How well-insulated is the urn? If it’s poorly-insulated, i.e., if the outside’s pretty durned hot shortly after the water’s heated to the desired temperature, then you should go with less water. Ohhterwise, fill the thing to the brim. Of course, either way, the energy usage’s probably negligible.
Oh, urn, coffeepot, whatever. I’ve just woken up. If it’s a coffeepot, then it’d be better to just fill the thing halfway up. Coffeepots leak heat like nobodys’ business, being open and made of thin glass. I was imagining a samovar or some such thing.
There are two components: bringing the water up to the desired temperature, and then maintaining it.
It should be obvious that it is most efficient to heat only the water you will use. However, this may be difficult to predict.
Once heated, let’s keep it simple by assuming the urn is a cylinder and heat loss is uniform over the entire surface. A half-full urn will lose more heat per liter of water than a full urn (because only the surface of the sides increase, but the top and bottom stay the same; true only when the diameter is significant compared to the height), but a full urn will lose lots more heat absolutely.
But you won’t even notice the difference in the electric bill.