If you ever touched an old-style incandescent light bulb after it had been on, you know it gets hot enough to burn you. That heat is not what the bulb was for. The bulb's job is to make light, and most of the energy it takes in comes out as thermal energy instead. An LED bulb gives the same light while staying much cooler, because far less of its energy is wasted. Comparing how well devices turn energy into what we actually want is called efficiency, and it is one of the biggest tools we have for saving energy and money.
Energy transformations in devices
The law of conservation of energy says energy cannot be created or destroyed; it can only change from one form to another. An electrical device takes in electrical energy and transforms it. Some of the output is the form you want, the useful energy. The rest is wasted energy (sometimes called "lost" energy), most often thermal energy that warms the surroundings, plus some sound. The energy is not destroyed; it just spreads out in a form that is no longer useful to us. So: input energy = useful output energy + wasted energy.
| Device | Input | Useful output | Main wasted output |
|---|---|---|---|
| Lamp | Electrical | Light | Thermal |
| Electric fan | Electrical | Kinetic (moving air) | Thermal, sound |
| Kettle | Electrical | Thermal (in the water) | Thermal (to the room), sound |
| Speaker | Electrical | Sound | Thermal |
| Charging a phone | Electrical | Chemical (stored in the battery) | Thermal (the charger and phone get warm) |
| Blender | Electrical | Kinetic (spinning blades) | Sound, thermal |
Percent efficiency
Efficiency is the fraction of the input energy that comes out in the useful form. It is usually written as a percentage. A device that turns 20 J of every 100 J into the useful form is 20% efficient. Because energy cannot be created, the useful output can never be more than the input, so a device's efficiency cannot be more than 100% (heat pumps seem to break this rule; see the callout below for why they do not). You can use power instead of energy in the formula too, as long as both are in the same unit.
A Sankey diagram shows where the energy goes. The input arrow enters on the left. Its width stands for the amount of energy. The arrow splits: the useful energy continues straight on, and the wasted energy branches off, usually bending downward. Because widths are to scale, the branches must add up to the width of the input arrow. A glance tells you how efficient the device is: a wide useful arrow means high efficiency.
| Device or system | Approximate efficiency |
|---|---|
| Incandescent light bulb (light out) | about 5% |
| Typical solar panel | about 20% |
| Gasoline car engine | about 20 to 30% |
| LED light bulb (light out) | roughly 30 to 40% |
| Large electric motor | about 90% or more |
| Hydroelectric generating station | about 90% |
Choosing efficient devices
- ENERGY STAR is a symbol on products, such as fridges, washers, TVs and windows, that meet strict efficiency standards; certified models are among the most efficient in their category.
- EnerGuide labels on major appliances in Canada show how many kWh the appliance is expected to use in a year, so you can compare models and estimate running costs before you buy.
- Heat pumps heat a home by moving thermal energy that is already outdoors (even from cold winter air) to the inside, the way a fridge moves heat out of its food compartment. Cold-climate heat pumps can work through most Ontario winters.
- Small habits add up: switching to LED bulbs, air-drying laundry, using cold-water wash cycles, sealing drafts and turning off devices when not in use.