Peter Waterhouse, of Stewart River, Australia, asks :-

When making a single cup of tea is it more efficient to use my electric jug (1400W) or my microwave oven (600W)?

Kim Eccleston, an electrical engineer at the University of Canterbury, responded.

Most electric jugs use immersion heaters which are resistive elements that generate heat when an electric current is passed through them. The heat from the element is directly transferred to the water. The electric jug is therefore nearly 100 per cent efficient at using electrical energy to heat the water, except that some heat is wasted in heating the element, connecting wires, and the jug body. A typical electric jug requires a minimum amount of water (enough to well cover the element) for safe operation so using the 1400 W jug may create more hot water than needed for the cup of tea, which is a waste.

The microwave oven works on the principle of interaction of electromagnetic waves with water molecules. At microwave frequencies (eg. 2.45 GHz), this interaction is lossy and microwave energy is converted to heat. The energy conversion process is 100 per cent efficient provided the cup is lossless (eg porcelain). However, the process of converting electrical energy from the mains to microwave energy is not. Typically the device for doing this (a magnetron) has an efficiency of between 60 - 90 per cent. The balance of the energy is dissipated as heat in the magnetron. Being a vacuum tube, the magnetron requires its own heating element, further consuming power from the mains. Magnetrons also require special electronics to drive them and this additional circuitry dissipates power. Additional electrical power is used to operate the oven light, sensors, touch pad, microprocessor, fan to cool the magnetron, turntable etc. The advantage of the microwave oven is that we only need to place one cup of water in the oven if this is all that is required.

But there is another issue and that is safety. It is dangerous to use a microwave oven to boil water as there is the risk of super-heating the water rather than boiling it. When the cup of superheated water is disturbed (eg by picking it up) it could violently erupt spraying boiling water. Superheating is unlikely if microwaving food (containing water). E.g. to safely make coffee using the microwave oven add ingredients (tea bag, instant coffee powder, suger, milk) to the cup of water before placing the cup in the oven.

In conclusion, if just the right amount of water was used, or if you were making tea for a few people, then the electric jug is more efficient. Note that the right amount must cover the element.