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Heat Gain from Lights

Heat contributed by lights may have major impact on air-condition systems.

Heat gain from lights in modern office or production areas may be of significant amount. The heat emitted from lights to the room depends on

  • light level in the room
  • type of lights and their construction
  • location of the light equipment

The light level in a room depends primarily on type of activity. For typical office work the level may be in the range 500 - 1000 lux.

  • The lux is the SI unit of illuminance and luminous emittance - measuring luminous flux per unit area

Required Electric Power for Lights

Unless special arrangements as local cooling or air outlets through the lighting equipment are used, the electric power to the lights are converted to heat transferred to the room. Required electric power to achieve recommended light level can be estimated

P = b / (ηe ηr ls)                                                       (1)


P = installed electric power (W/m2 floor area)

b = recommended light level (lux, lumen/m2, lm/m2)

ηe = light equipment efficiency

ηr = room lighting efficiency

ls = emitted light from the source (lumen/W, lm/W)

  • The lumen is the SI derived unit of luminous flux - a measure of the total visible light emitted by a source

Emitted Light From Source - ls

The purpose of a lamp is to convert electrical power (watts) into light (lumens). Lamps do this with varying efficiencies and the light emitted from a source depends on the type of source.

Typical efficiency of lamp types are indicated below

Lights - Emitted Light from various Lamp Types
Lamp TypeEmitted Light from The Source
GLS Light Bulb1) 10 - 15 1000
Low Voltage Halogen 20 2000 - 5000
Mercury Vapor 40 - 60 22000
Fluorescent 50 - 90 more than 7000
Metal Halide 70 - 90 more than 12000
White LED 80+
High Pressure Sodium 90 - 125 25000
Low Pressure Sodium 120 - 200 20000
  • A typical incandescent GLS light bulb emits approximately 10 lumen/Watt
  • A typical fluorescent tube emits up to approximately 60 lumen/Watt

1) A GLS Light bulb is also known as a traditional shape standard light bulb. GLS bulbs are used in table lamps and ceiling pendants.

Light Equipment Efficiency - ηe

The light equipment efficiency expresses how much of the light is really emitted from the light source to the room.

A bare fluorescent tube emits 100% to the room. A shielded tube emit less - 50% to 80% is common.

Room Lighting Efficiency - ηr

The room lighting efficiency express how much of the light is absorbed by the room before entering the activity area.

Light Equipment Efficiency and Room Lighting Efficiency influences on each other. Common values of the product ηe ηr are in the range 0.3 - 0.6.

Example - Heat Load from Lights

1000 lux is the recommended light level in a office where detailed drawing work is performed. The room and lighting equipment efficiency can be set to 0.5.

By using standard GLS bulbs - the electric power for the lighting can be calculated to

P = (1000 lumen/m2) / (0.5 (10 lumen/W))

    = 200 W/m2

By using standard fluorescent tubes - the electric power for the lighting can be calculated to

P = (1000 lumen/m2) / (0.5 (60 lumen/W))

    = 33.3 W/m2

Due to the high energy consumption and the major impact on air condition heat loads standard GLS light bulbs are not an alternative in high illuminance areas.

The table below indicates typically installed electrical power for different light levels:

Lights - Illumination vs. Installed Effects
Installed effect (W)Illumination - Light Level (lux)
Incandescent GLS bulb lamp 40 80 120 160 200
Fluorescent tubes 6.7 13.3 20 26.7 33.3

Note! Datasheets from manufacturers should always be consulted before detailed engineering. The numbers above do for rough preliminary calculations.

Related Topics

  • Air Conditioning Systems

    Design of Air Conditioning systems - heating, cooling and dehumidification of indoor air for thermal comfort.

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