There’s an advert that’s well-featured on the television at the moment for a plug-in wall-mounted heater.  As part of the advert, the product is described as ‘efficient’.  Now, I’m not at all saying that these heaters aren’t a good purchase, but a bit of physics tells me that this statement about efficiency doesn’t really mean very much – all heaters that work by converting elecricity to heat are going to be 100% efficient.

Energy comes in various forms. There’s electrical energy, movement (kinetic) energy, sound energy, gravitational potential energy (the energy stored in a hydro lake), light and heat, just to name some. Energy can convert between forms, but is always conserved – you can’t create energy out of nothing (at least, not for long periods of time, but I’ll leave quantum electrodynamics aside hre) and you can’t destroy it.   In which case, you might ask the question, "If it’s never destroyed or used up, why do we have to pay for it?"

Unfortunately, with energy, it is easier to convert it in some directions rather than others. Formally, this is encapsulated with the second law of thermodynamics – that entropy (‘disorder’) increases with time. Take friction for example. A rolling car on a flat road will eventually come to rest because of friction in the moving parts of the car, between the tyres and the road, etc. (and due to air resistance). Here, the movement (kinetic) energy of the car has been converted to heat energy.  That process happens naturally on its own. What doesn’t happen naturally is that a car sitting stationary on a flat road starts moving, because heat in its parts turns itself into kinetic energy.  Energy does not flow naturally that way.

Essentially, the second law of thermodynamics tells us that producing heat is really straightforward. Take just about any physical entity containing some form of energy, and it will eventually generate heat. It’s not difficult.  So, our plug in wall-heater, is designed to take energy from the power station (transmitted through the electricity grid) and turn it into heat. This it can do perfectly; every bit of energy that gets to the heater through the electricity supply gets turned into heat.   (Contrast that with a light bulb, where some gets turned into light, and some to heat, or with your TV, where some goes to light, some to sound, and some to heat). So the heater is 100% efficient at converting electrical energy to heat energy.

Just where that heat goes next is determined by your house design, particularly how much insulation is in the roof – hopefully it hangs around inside your house for a while, before leaving (still as heat).

And just to end, you can get more than 100% ‘efficiency’ though in electrical heating – namely the heat pump. In this case heat energy is transferred from the air outside (yes, cold air contains heat) and is pumped inside. This flow doesn’t happen naturally (second law of thermodynamics), and there is a cost you pay. You need to input more energy to get it to happen – that’s the electricity supply to your heat pump – but you get that energy back as heat as well, so you don’t lose out.

So a claim of 100% efficiency for an electrical heater doesn’t mean a great deal – don’t take much heed of it when deciding what plug-in electrical heater to buy!