Will's Thoughtstream

I have long been the very satisfied owner of a Zojirushi electric water kettle, officially known as the Zojirushi CD-LCC30 Micom 3.0-Liter Electric Dispensing Pot. As I originally wrote in my Amazon.com product review:

I purchased the Zojirushi 3 liter dispensing pot, and it runs 24 hours a day, 7 days a week in my house. We drink tea constantly, and it is wonderful not to have to wait to boil water. The Zojirushi allows you to set the desired water temperature based on your needs (i.e. 175 for green/white teas, 195 for other teas, 205 for cooking). The unit is tall enough to fit even our tallest to-go cup under the dispensing spout.

Since we've had the kettle, I wondered about the energy efficiency of it, since the outside of the unit can be fairly warm, suggesting that it isn't very well insulated. I also happen to have a Brand electric meter, an expensive and highly accurate meter used for measuring electric consumption. So I plugged the Zojirushi kettle into the meter, and let it run.

Measured over the course of three days, at about 70-72 degrees F ambient temperature, and with a water temperature setting of 195, the kettle used 0.9kwh/day! That's quite a bit just to have hot water. An average house might use about 30kwh/day, so 0.9kwh represents 3% of the entire energy budget. A very efficient house might use 15kwh/day, in which case 0.9kwh represents 6% of the energy budget.

I have previously read the typical home water heater wastes about 30% of energy consumed keeping the water hot. This ratio appears to be much higher for the Zojirushi. As measured by the Brand meter, initially heating the 3 liters of water takes ~0.25kwh. Maintaining that temperature over the next 24 hours then takes about 0.65kwh. (These numbers are somewhat approximate, since I didn't rigorously control for usage of the water kettle during testing, and assume that the entire 3 liter capacity is used every day, which is close to our actual usage.) This suggests that almost 70% of the energy consumed is wasted keeping the water hot, as opposed to the 30% of the energy used that is necessary for actually heating the water that we will consume.

I happen to have some foil insulation bubble wrap laying around. This inexpensive insulation product is often used to insulate crawlspaces. It has a pretty low R value, but is compact and flexible. I cut a piece to cover the water kettle, leaving room to dispense water, see the water level indicator, and creating a flap to allow access to the lid for adding water. Ideally the insulation would have been tight to the body of kettle, with as little room for airflow as possible to give the best possible performance. But in practice, using rubber bands and paper clips, there were still some big gaps.

I put the insulation on and checked back a couple of hours later. I noticed right away that the outside of the foil insulation was cool. And the space in between the foil insulation and the plastic housing of the kettle was quite hot: so indeed the insulation was working.

After three days, I checked the power consumption and found that with the extra insulation, the consumption was down to 0.75kwh/day. This is a 17% decrease in total power consumption compared to the unaltered kettle. And if you exclude the energy required for initial heating of the water, this is a 23% reduction in the energy that is wasted in maintaining the temperature. So a few cents invested in foil insulation wrap fastened with two rubbers saved 17% in total power consumption, and 23% in wasted power consumption. I could easily imagine the same product manufactured with more insulation (properly applied and in strategic locations like the top) could achieve power savings of 50%.

So we've decided to put our Zojirushi away for now, and have gone back to boiling water on the stove. It's not as convenient, but it is an easy way to save energy. Another alternative would be to buy a large capacity insulated thermos, boil water once per day, and store it in the thermos.

I'd really like to see Zojirushi develop a highly energy efficient version of their electric water kettle that has extra insulation.