Why Do LEDs Use So Little Electricity?
– They Make Light Directly, Not Through Heat
Swap an old-fashioned incandescent bulb for an LED bulb, and the wattage on the label can drop to about one seventh, for roughly the same brightness. Leave it on for a while and the LED doesn't get nearly as hot as the incandescent bulb. The secret behind this low power use is that the two bulbs make light in fundamentally different ways.
Have you ever left an incandescent bulb on for a while, then touched it by accident and nearly burned your fingers? An incandescent bulb stays very hot the whole time it is lit.
An LED bulb of the same brightness, on the other hand, doesn't get nearly as hot, even after hours on. And yet the wattage printed on the LED is a much smaller number.
Both give the same "brightness." So why do they need such different amounts of electricity?
It heats a filament until it is very hot and uses the light given off by that heat. Only a small part of the electricity ends up as light.
Inside a semiconductor, the energy of electrons is converted into light with almost no detour through heat.
The two bulbs make light in completely different ways, and that is what creates the gap in power use. Let's look at each in turn.
An Incandescent Bulb Glows Because of Heat
Inside an incandescent bulb is a thin filament made of a metal called tungsten. When current flows, the filament's electrical resistance heats it to well over 2000 °C.
Anything gives off light when it gets hot enough. It is the same principle as iron glowing red when heated. An incandescent bulb relies on this "hot things glow" effect (thermal radiation).
But the light from a heated object includes not only visible light but also a large share of invisible infrared (the light we feel as heat). A large share of the electricity an incandescent bulb uses is said to be lost as invisible infrared. The bulb feels hot to the touch because so much infrared and heat pours out of it.
An LED Turns Electron Energy Directly into Light
An LED (light-emitting diode) glows by a completely different principle. Its core is made of a special material called a semiconductor. When current flows, electrons inside the semiconductor drop into particular spots called holes, places where an electron is missing.
When an electron drops in, it gives off part of its energy directly as light. This is called electroluminescence. The big difference from an incandescent bulb is that there is no detour through heat: the electrical energy becomes light energy in a single step, so to speak.
What's more, the colour (wavelength) of the emitted light is largely fixed in advance by the semiconductor material. Being able to aim for just the colour you want is also said to help LEDs be efficient.
An LED gets light straight from the energy of electrons.
Why This Difference Means Less Electricity
In an incandescent bulb, only a small share of the electrical energy becomes light, and most of the rest is lost as heat. To get the same brightness, you have to keep feeding in that much more electricity.
In an LED, a large share of the electrical energy turns directly into light, with almost no detour through heat. An LED does give off a little heat because of resistance, but far less is wasted than in an incandescent bulb. That is why the same brightness can be made with less electricity. How big the difference really is gets checked with numbers in the fold-out below.
From how hot incandescent bulbs get, you might have the impression that "brighter light means more heat." But as we saw above, heat and light don't necessarily come as a pair. In principle, it is possible to make bright light with almost no heat, as an LED does.
Try It Yourself
- If you have an LED bulb (and an incandescent bulb, if you still have one) at home, turn each on for about 5 minutes.
- Bring your palm close to each bulb (without touching it) and compare how much heat you feel.
- If you have no incandescent bulb, look at an LED bulb package in a shop that says "equivalent to a ○ W incandescent bulb," and check how much smaller the actual wattage is.
You should feel the heat clearly on the palm held near the incandescent bulb. This is a direct, bodily sense of the fact that an incandescent bulb gives off much of its electricity as heat.
Summary
An incandescent bulb heats a filament until it is hot and gets light as part of the resulting thermal radiation. Much of the electricity is lost as invisible heat. An LED turns the energy of electrons directly into light inside a semiconductor, with almost no detour through heat. This difference in how the light is made is said to be behind the large gap in power use.
LEDs save energy not because they skimp on light.
They go straight to light, without the detour through heat.
For Those Who Want More – Terms, Numbers and Links to TextbooksFrom middle-school science to active research, each topic is labelled by level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics
- High school+High-school physics, or advanced or sidebar material in textbooks
- UniversityNot taught in high school; university-level content (semiconductor engineering)
- ResearchNot yet settled even at university level; topics researchers are studying right now
Middle schoolTerms: Words Around LEDs and Bulbs
- Filament: the thin wire inside an incandescent bulb that becomes very hot when current flows through it.
- Semiconductor: a material whose ability to conduct electricity changes with conditions. It is used in LEDs and computer parts.
- Lumen (lm): the unit for the brightness of light (luminous flux).
- Watt (W): the unit for electric power consumed (energy used per second).
High schoolChecking with a Formula: Why Does the Material Decide an LED's Colour?
The colour (wavelength) of an LED's light is set by the amount of energy released when an electron drops in inside the semiconductor. Using the relation between the energy of light and its wavelength, let's calculate how much energy light of a given wavelength carries.
Photon energy = Planck constant × speed of light ÷ wavelength
| Photon energy | Unit: J (joule) |
| Planck constant | A fixed value of about 6.63×10⁻³⁴ J·s |
| Speed of light | About 3.00×10⁸ m/s |
| Wavelength | Unit: m. Red LED light is said to be about 630 nm (6.30×10⁻⁷ m) |
This is a well-known relation that gives the energy of a single grain of light (a photon). The shorter the wavelength (the bluer the light), the more energy each grain carries.
The product of the Planck constant and the speed of light is known to be hc ≒ 1.989×10⁻²⁵ J·m. We divide this by the red LED's wavelength.
| Divide hc by the wavelength | (1.989×10⁻²⁵) ÷ (6.30×10⁻⁷) ≒ 3.157×10⁻¹⁹ |
| Photon energy | About 3.16×10⁻¹⁹ J |
This value is too small to get a feel for, so we convert it to a unit called the electron volt (eV), the energy of one electron accelerated through one volt. 1 eV is about 1.602×10⁻¹⁹ J.
| Convert to eV | (3.157×10⁻¹⁹) ÷ (1.602×10⁻¹⁹) ≒ 1.97 |
| Photon energy (in eV) | About 1.97 eV |
The energy an electron loses when it drops in, in the semiconductor used for red LEDs (the band gap), is reported to be about 1.9 to 2.2 eV, which is very close to our result. So we have confirmed that if you specify a wavelength (colour), you can work out the energy needed.
* The emission wavelength and band gap of real LEDs vary with the material and manufacturing method. Typical guide values are used here.
High school+Why Are Incandescent Bulbs Inefficient? (Blackbody Radiation)
The amount and colour distribution of light from a hot object are well explained by a theory called blackbody radiation. As the temperature rises, the amount of light increases and the peak wavelength gets shorter (bluer), but at the temperature of a bulb filament (roughly 2000 to 3000 °C), the peak of the radiation is still in the invisible infrared region. So there is a limit: however cleverly the filament is designed, in principle a lot of the energy comes out as invisible heat.
UniversityDirect-Gap and Indirect-Gap Semiconductors
Many of the semiconductors used in LEDs (such as gallium arsenide and gallium arsenide phosphide) are of the direct-gap type, which can give off light efficiently when an electron drops into a hole. By contrast, indirect-gap semiconductors such as silicon need an extra step before an electron can emit light, so their light-emission efficiency is very low. This is one reason why LEDs use other semiconductor materials even though most everyday computers are made of silicon.
ResearchWhat Is Still Unclear
- It is known that as you increase the current through an LED, its light-emission efficiency oddly starts to fall (efficiency droop). Several explanations have been proposed, such as energy wasted when electrons collide with one another (Auger recombination) and electrons leaking out of the region where light is made, but how much each factor contributes is still being debated.
- Research is also under way on LEDs made from a new class of materials called perovskites. They are said to have the potential to lower manufacturing costs compared with conventional semiconductor LEDs, but securing the durability needed for practical use remains a challenge.
- Research and development continues on "micro-LED" displays, made by arranging huge numbers of very small LEDs. But because they are hard to make, mass production of large screens still faces challenges.
LEDs are already a widespread technology, but research on raising their efficiency further and putting new materials to practical use is still going on.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: electricity and energy | Basic terms: filament, semiconductor, watt |
| High school | Physics: particle nature of light | Calculating photon energy from wavelength |
| High school+ | Physics: thermal radiation | Blackbody radiation and the efficiency limit of incandescent bulbs |
| University | Semiconductor engineering | Direct-gap vs indirect-gap semiconductors |
| Research | Optical engineering (ongoing research) | Efficiency droop, perovskite LEDs, micro-LEDs |
- Comparative explanations of how incandescent bulbs and LEDs emit light, in lighting engineering textbooks.
- Explanations of electroluminescence and the band gap, in semiconductor engineering textbooks.
- Agency for Natural Resources and Energy (資源エネルギー庁), explanatory material on the energy-saving effect of LED lighting (「LED照明の省エネ効果」).
- Schubert, E. F., Light-Emitting Diodes (a standard specialist book on how LEDs emit light and on efficiency droop).
- Explanations of blackbody radiation and Planck's law in physics textbooks.
* Figures such as wavelength, band gap and efficiency are typical guide values and vary with the actual product or material.
* This article is a general-audience science explainer. For details on the performance and energy-saving effects of bulbs and LED products, please check the package labels and information from public bodies such as Japan's Agency for Natural Resources and Energy.