Space-by-space allowances from ANSI/ASHRAE/IES 90.1-2022 Tables 9.5.2.1-1 and 9.5.2.1-2 (numbered Table 9.6.1 in earlier editions). Compliance is judged on the building’s total installed interior lighting power against the sum of the space allowances, so one space may exceed its value if others make up the difference, and lighting controls are required separately. Confirm against the edition your jurisdiction has adopted.
Inputs, results, and method — as an Excel workbook or plain CSV.
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This lighting heat gain calculator finds the sensible heat that interior lighting adds to a space and how much of it is cooling load at the hour of interest. Enter the room area, lighting type, installed lighting power density, the ballast or driver allowance factor, the use factor, and the space fraction; the tool returns connected power, total heat gain, the room cooling load, its radiant and convective portions, the share that goes to a return plenum, and the equivalent tons.
Lighting is a core internal gain in a cooling load calculation, and its radiant share appears as a delayed load. The page also compares the installed lighting power density with the ASHRAE 90.1-2022 space-by-space allowance for a selected space type — a quick screen, not a full code compliance submittal.
This is the lighting heat gain of ASHRAE Fundamentals Ch. 18: installed watts times the use factor Ful and the special allowance factor Fsa (ballast or driver losses when the LPD is in lamp watts; 1.0 when it is already luminaire input watts), at 3.412 BTU/hr per watt. The space fraction Fspace is the share that reaches the room; the rest heats a return-air plenum and is a load on the system, not the room. The room gain splits into an immediate convective part and a radiant part that surfaces absorb and release later; the radiative fractions offered (LED ≈0.37, fluorescent ≈0.54, incandescent ≈0.87) are typical values — both fractions depend on the luminaire and venting (Fundamentals Ch. 18 Table 3). The delay factor is the share of the radiant gain released by the hour of interest; in the Radiant Time Series method it is the sum of the radiant time factors over the hours the lights have been on, and it is 1.0 for lights on long enough to reach steady state, so the default 1.0 gives cooling load = heat gain. The LPD check compares LPD × Fsa (luminaire input watts per ft²) with the ASHRAE 90.1-2022 space-by-space allowance.
Frequently Asked Questions
How do I convert lighting watts to cooling load?
First find total power: area times lighting power density times ballast factor times the fraction of fixtures on. Then convert watts to BTU/hr by multiplying by 3.412. All electrical energy used by lighting ends up as heat in the building, in the room itself or, for recessed fixtures vented to a return plenum, partly in the plenum, so the total heat gain equals that power in BTU/hr before any radiant delay is applied.
What is the radiant and convective split for lighting?
Lighting heat is part convective, which warms the air immediately, and part radiant, which is absorbed by surfaces and released later. Typical radiative fractions are roughly 0.37 for LED, 0.54 for fluorescent, and up to about 0.87 for incandescent, but the real split depends on the luminaire and how the ceiling is vented. The convective part is an immediate cooling load; the radiant part is delayed, which is what the delay factor represents.
What does the radiant delay factor do?
Radiant heat from lights is absorbed by room surfaces and released to the air over the following hours, so not all of it is a cooling load the moment the lights come on. The delay factor is the share of the radiant gain that has reached the load by the hour you are checking; the convective portion is always immediate. Once lights have been on long enough to reach steady state the factor is 1.0 and the cooling load equals the heat gain, which is the default here. A lower value applies only when lights switch on a few hours before the peak, most of all in heavy construction; in the Radiant Time Series method it is the sum of the radiant time factors over the hours the lights have been on.
How does the ASHRAE 90.1 LPD check work?
It compares your installed lighting power density, the input LPD times the allowance factor, with the ASHRAE 90.1-2022 space-by-space allowance for the selected space type and reports whether it is within that allowance. Compliance under 90.1 is judged on the total installed interior lighting power against the sum of all the space allowances, and controls are required separately, so treat this as a screen and confirm against the energy code adopted for your project.
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Peak loads (RTS or CLTD/CLF) and ASHRAE 62.1-2019 ventilation
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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.