For example, my home uses an ancient furnace and steam based heating system where the furnace boils water and the steam then flows to the radiators in the rooms of the home.
Energy efficiency is a concept that describes how much energy is used, vs. how much does useful work that we want. W
useful / W
used is one way of writing it. You must be careful, because different sources of energy behave in very different ways.
The video is about electrical resistance heaters. They work by connecting a resistor across the line, so that the power drawn from the utility (V * I) is dissipated in the resistor (V
2 / R). In practice some voltage is dropped by the supply wiring and the power cord, so they are not quite 100% power efficient, but close enough. Any appliance that runs from mains electricity will have those losses.
When using combustion for heating, the system is absolutely
not 100% efficient, because heat energy is contained in the exhaust gases. If those gases are vented straight up a chimney, they carry away roughly 20-30% of the heat energy released by combustion, leaving 70-80% to actually heat the building. More recently, condensing heat recovery devices have become more common, which reduce the heat losses to between 3-10%. If you see a building with "smoke" pouring out at ground level, that is what is happening. The "smoke" is in fact water droplets or mist that have condensed from the hot exhaust as it exchanges most of its heat with the cold intake air.
However, this efficiency comparison can be misleading, because hydrocarbon fuels and electrical power are not the same. The electrical power has usually to be generated at a remote plant, transmitted over a long distance, and converted several times, which represents a built-in loss of energy that cannot be measured at the point of use. Fuels require energy to refine and transport, which is also not measurable at the point of use. If the electrical power is generated from fossil fuels, it is lower in all-inputs efficiency than even 70%-efficient combustion furnaces.
It is also possible to achieve greater than 100% energy efficiency by using heat pumps, since part of the useful work is being extracted from the environment.