Heat Gain from Lighting Calculator

Related Resources: heat transfer

Heat Gain from Lighting Calculator

Heat Transfer Engineering
Thermodynamics

Heat Gain from Lighting Calculator and Equation (estimate)

The energy emitted by lighting and then absorbed by the structure and contents contributes to space cooling load only after a time lag, some still reradiating after the heat sources have been switched off. This may make load lower than instantaneous heat gain, thus affecting the peak load.

Instantaneous rate of heat gain from lights, :

Eq. 1
q_el = 3.412 W F_ul F_sa

where

q_el = heat gain, Btu/h
W = total lighting wattage (W)
F_ul = lighting use factor, 1.0 or decimal fraction <1.0
F_sa = lighting special allowance factor, ratio of lighting fixtures' power consumption (including lamps and ballasts) to nominal power consumption of the lamps
3.412 = conversion factor (Btu/hr/W)

The total light wattage is obtained from the ratings of all lamps installed, both for general illumination and for display use. Ballasts are not included, but are addressed by a separate factor. Wattages of magnetic ballasts are significant; the energy consumption of high-efficiency electronic ballasts might be insignificant compared to that of the lamps.

The lighting use factor is the ratio of wattage in use, for the conditions under which the load estimate is being made, to total installed wattage. For commercial applications such as stores, the use factor is generally 1.0.

The special allowance factor is the ratio of the lighting fixtures’ power consumption, including lamps and ballast, to the nominal power consumption of the lamps. For incandescent lights, this factor is 1. For fluorescent lights, it accounts for power consumed by the ballast as well as the ballast’s effect on lamp power consumption. The special allowance factor can be less than 1 for electronic ballasts that lower electricity consumption below the lamp’s rated power consumption. Use manufacturers’ values for system (lamps + ballast) power, when available.

For high-intensity-discharge lamps (e.g. metal halide, mercury vapor, high- and low-pressure sodium vapor lamps), the actual lighting system power consumption should be available from the manufacturer of the fixture or ballast. Ballasts available for metal halide and high pressure sodium vapor lamps may have special allowance factors from about 1.3 (for low-wattage lamps) down to 1.1 (for high-wattage lamps).

Derived from resources provided by:

ASHRAE Pocket Guide for HVAC, 2013