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
Materials List - These are the materials I used. You may substitute anything that works
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