Does a Light Bulb Make a Room Hotter?


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Whether you’re trying to heat up or cool down a particular room, you might be interested in the impact that the light bulbs in that room have on the temperature.

So, does a light bulb make a room hotter? Yes, a light bulb makes a room hotter, albeit barely. While certain light bulbs certainly produce heat (sometimes upwards of 90 percent of the energy is “wasted” as heat), the temperature of a room is not going to rise in any significant way if you have a few light bulbs on. 

This article is going to discuss exactly how much heat different kinds of light bulbs give off, right down to the degree. Whether you’re trying to optimize the temperature of a room, or you’re merely interested in how light bulbs impact the temperature of a room, the rest of this article will be of interest to you. 

Pro-tip: Fluorescent bulbs produce the least heat of all light bulbs (75% less than incandescent bulbs) and use 75% less electricity. If you want to maximize your room’s cooling and save money on electricity usage, check out the best fluorescent light bulbs on Amazon.com now.

Can light bulbs heat up a room?

Light bulbs can heat up a room if the room is quite small. 

Your typical incandescent light bulbs emit light through the heating of a small metal filament enclosed in a glass bulb filled with inert gas. The filament is heated using electrical current until it glows. The temperature of the filament when fully heated is approximately 4000°F!

Incandescent bulbs emit 90 to 98 percent of their energy as heat. The amount of heat emitted depends on several factors, including bulb shape, bulb materials, and surrounding airflow. 

For a regular 100-watt bulb, the filament temperature is approximately 4600°F, and the outside temperature of the glass bulb varies between 150 to more than 250°F.

Where incandescent bulbs and air conditioning are used, incandescent light bulbs can increase the load on the air conditioning system due to their inefficiency. The heat from the lights may help reduce the amount of heating required for a building. However, there are many more efficient ways to heat a building.  

There are certain applications in which an incandescent light bulb can be used to warm the area (or room). Here are a few:

  • Incubators
  • Brooding boxes for poultry
  • Heat lights for reptile tanks
  • Infrared heating for industrial processes
  • Lava lamps, 
  • Easy Bake oven toys. 

Also, for industrial processes such as paint curing or space heating, a quartz tube heat lamp might be used.

How much heat does a light bulb give off?

One of the basic laws of physics is the law of conservation of energy. The law of conservation of energy states that energy can’t be created nor destroyed. Rather, it can only be transformed or transferred from one form to another.

Here’s a real-life explanation that should make understanding this principle a bit easier:

Does a Light Bulb Make a Room Hotter?

When a 100-watt lamp is turned on, 100-watts of electricity is transformed into 100-watts of light and heat.

The same is true for a 50-watt light bulb; 50-watts of electrical energy becomes 50-watts of light and heat. (A watt is a unit of power).

Some lamps are more efficient at producing light than others. This efficiency determines what percentage of that 100-watts is transformed into visible light and what percentage is transformed into heat (or “wasted”) or other energy types (for example, infrared light). 

Here are some quick numbers for reference:

The primary factors for determining how much heat will be produced are what type of bulb is being used and the wattage. 

A secondary factor is the shape of the bulb, meaning the direction or directions that the light and heat are projected at. If the light and heat are concentrated in one area, that area will feel warmer than light and heat that is diffused over a larger area.  

The Science of Heating a Room with a Light Bulb

Let’s say you really want to heat a room with an incandescent light bulb. Using science, you can determine how many bulbs you would need to heat a room with a light bulb. Let’s get into it. 

Specific heat is the amount of heat required to change the temperature of a substance by 1 degree. The units are joule per kelvin and kilogram – kJ/(K*kg)

The specific heat for air is approximately 1.00 kJ/K*kg (In this example, it has been rounded down for simplicity). The density of air at sea level is 1.2 kilograms per cubic meter.

A 420 square foot room with 10-foot ceilings has 145 kilograms of air. This means that it takes 145 kJ to heat the room 1 degree Celsius. With a 40-watt light bulb, it would take approximately 1 hour to heat the room 1 degree. Therefore, a 40-watt light bulb would heat a room 1 degree every hour (with perfect insulation). 

Here is the math breakdown:

  • Volume of the room = 21-foot x 20-foot x 10-foot = 4250 cubic feet = 120.3 cubic meters
  • Volume of air = 120.3 cubic meters x 1.2 kg/m3 = 144.36 kilograms of air
  • Kilojoules = 40-watts x (3600 seconds/1000) = 144 kJ

So, the science says that you can heat a room with a light bulb. The cost to power that light bulb may be more than it is worth, though.

Do led lights make a room hotter?

With all the hype surrounding their efficiency, you might consider if an LED light will make a room hotter. 

However, the answer is no. LED light bulbs will not cause an increase in room temperature because of their high efficiency. Because they are highly efficient, much less energy is transformed into heat. 

LED lamps or light bulbs produce light using light-emitting diodes (LEDs). A light-emitting diode is a semiconductor light source that emits light when a current runs through it.

Does a Light Bulb Make a Room Hotter?

Typically, LED bulbs are cool to the touch. For example, in plant growing applications, plants can be placed close to the bulbs without the risk of overheating or scorching the leaves. This means that less space is required for cultivation as compared to hot-running lights.

LED bulbs must be kept cool, as high temperatures can cause failure or reduced light output. Typical LED lamps have heat sinks and cooling fins to allow for heat dissipation, which means they don’t emit much of their heat to the outside area. 

The hottest surface of an LED light bulb (the heat sink) that is exposed to surface is approximately half the temperature of an equivalent incandescent bulb and approximately 20 percent cooler than fluorescent bulbs. In testing of a fully lit LED bulb, the heat sink temperature was around 60°C to 100°C (140°F – 212°F).

Do fluorescent lights make a room hotter?

You might also be wondering if a fluorescent light bulb would make a room hotter. However, the answer is also no. Typical fluorescent lights will not cause a noticeable increase in room temperature.

Fluorescent light bulbs produce visible light using fluorescence. Electric current (electrical energy) passes through and excites mercury vapor producing short wave ultraviolet light. The ultraviolet light causes a phosphor coating inside the light tube to glow.

The fluorescence method means that only 30 percent of the energy is used to create heat. Fluorescent light bulbs are far more energy-efficient and far cooler than regular light bulbs such as incandescent.

Does a Light Bulb Make a Room Hotter?

Fluorescent bulbs give off about one-fifth of the heat of an equivalent incandescent bulb. When used in cool temperature applications such as road or rail signaling, the fluorescent bulbs do not generate enough heat to melt snow or ice, which may result in reduced visibility.

Hopefully, you found this help article helpful. To recap, lightbulbs of all different kinds give off a small amount of heat, but the amount of heat given off is not enough to affect the temperature of a room in any significant way. 

Steve Rajeckas

Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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