This Instructable describes how I built a low power, electric kettle-thermos to heat up water. It is made of reused/repurposed components and is suitable for use with an off-the-grid PV panel. I have a single panel connected to a small inverter that puts out 120V AC.
Electric kettles that you typically see use 1000 or more watts, because people want their water heated ASAP. However, if you’re not in a rush, then you can get by with a lot less power. Mine uses only about 15 Watts, and, on sunny days, while I’m gone at work, it heats up 1.25 liters of water to boiling. Even on days when it has been mostly cloudy, the heater was on enough of the time that the water temperature was 160 F when I got home from work. In the evening, I can use the water to wash dishes, and since the kettle is also a thermos, the water is still hot enough for dishes the following morning. I don't use the water for drinking because I'm not sure that the heating element is completely free of materials that I would want to ingest.
Pros and Cons
I know that electricity is an inefficient way to heat water, and that a solar thermal system could easily to as well or better. However, if one has the available PV power, then efficiency isn't really an issue. Furthermore, it would be quite challenging and complex to use solar thermal to heat water in a thermos. This device is really compact, and the water will stay plenty hot for hours after the Sun goes down. By using the water to wash dishes, I save a little bit of natural gas. I dilute this water with tap water, and I get enough hot water to wash a load of dinner dishes for 2-3 people.
Step 1: Materials and Tools
An old, discarded thermos, 1.25 liters, the kind made for hot liquids
An old 15 watt soldering element that was discarded from my work
A small piece of rigid foam insulation, 1-inch thick
A discarded computer power cord
Small piece of 1/8 inch copper tubing
Shrink Tubing Heat Gun
Step 2: Electric Heating Element
In my case: V=120
R = ~1000 Ohms
Power = V^2/R = ~15 Watts
Say you are using 12V DC:
V = 12
P = 15W
R = V^2/P = 144/15 ~10 ohms
So, you would want to get a 10 ohm resistor that is rated at, say, 25 Watts (for some added safety). With a resistor, you wouldn’t want to immerse it directly in the water. I haven’t thought about this a lot, but you would want to seal it in something that is waterproof, not electrically conductive, and very good at conducting heat.
Step 3: Joining the Heater to the Power Cable
Step 4: The Insulated Top
I also made a second top identical to the first one, but without any holes poked in it. I use this top when I take the thermos inside to use the water.
Step 5: Plug It In!
This device is not intended to be used, from a wall socket and I wouldn't consider it safe for that. With a small PV system, even if you forget and leave it on, it will either run the battery or turn off at night. For mine, I plug the kettle-thermos in outdoors and have it sitting on concrete in my backyard. I wasn't able to use the ground wire on the power cord as a safety feature. As long as the connections on top are intact, there is little chance of getting shocked, I always connect the electric kettle-thermos to a power strip and turn the power off before handling the power cord or any other part. If there were some kind of short circuit in my absence, the inverter would shut itself off due to the voltage dropping. Also, the PV panel is not capable of supplying much current, should there be some type of short. Of course, handle the thermos carefully, as you would with any other container of boiling water.
Plug it in and enjoy "green" hot water at the end of the day!