Internal Heat Gain Calculator
Everything that uses electricity in a room ends up as heat, and so does every person in it. Add them up to see how much cooling those internal gains need.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Total internal heat gain
- 7,518BTU/h
- Total internal heat gain
- 2,203W
People, lighting and equipment add about 7,518 BTU/h (2,203 W) of heat.
- Internal gains help cooling and offset some heating. Sizing heating equipment normally ignores them, because the room may be empty on the coldest night.
- Cooling needed for these gains
- 0.63tons
Breakdown
- People
- 2,400BTU/h
- Lighting
- 1,706BTU/h
- Equipment
- 3,412BTU/h
- Other
- 0BTU/h
Show the working
- People = count × allowance per person4 × 600 = 2,400 BTU/h
- Lighting = watts × 3.412500 × 3.412 = 1,706 BTU/h
- Equipment = watts × share running × 3.4121,000 × 1 × 3.412 = 3,412 BTU/h
- Total internal gain2,400 + 1,706 + 3,412 + 0 = 7,518 BTU/h
How internal gains are counted
Electric lights and equipment turn the power they use into heat in the room: each watt adds 3.412 BTU/h. Use the power they actually draw — a plug-in meter or the equipment’s typical consumption — rather than the maximum on the nameplate, and reduce it by the share that is running at the busiest hour.
People give off heat too. The default allowance of 600 BTU/h per person is what ENERGY STAR adds for each extra occupant when sizing a room air conditioner. People doing physical work give off more; enter the figure your design reference uses.
Cooling versus heating
Internal gains add to the cooling load, so they matter for sizing air conditioners, especially in offices, kitchens and equipment rooms. For heating, designers usually ignore them: the room may be empty and dark on the coldest night, and the heater must still keep up.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- People in the room
- 4
- Heat allowance per person
- 600 BTU/h
- Lighting power
- 500 W
- Equipment power
- 1,000 W
- Share of equipment running at the busiest hour
- 100 %
- Other heat gains (optional)
- 0 BTU/h
Result
- Total internal heat gain
- 7,518 BTU/h
- Total internal heat gain
- 2,203 W
- Cooling needed for these gains
- 0.63 tons
People, lighting and equipment add about 7,518 BTU/h (2,203 W) of heat.
Assumptions
- All electrical power used in the room ends up as heat in the room: 1 W = 3.412 BTU/h.
- Each person adds the allowance you enter (600 BTU/h by default, the ENERGY STAR per-occupant addition).
Limitations
- People give off both sensible heat and moisture; this tool gives one total figure per person and does not split them.
- Equipment that vents heat outdoors (for example a clothes dryer or a hooded range) adds less than its rating.
Frequently asked questions
How many BTU/h does a computer add?
Its actual power draw × 3.412. A computer drawing 150 W adds about 512 BTU/h. The power supply’s rating is a maximum, not the typical draw.
Do LED lights add heat?
Yes — all the electricity they use becomes heat in the room, just less of it than older lamps for the same light. Enter their actual watts.
Should I count a range hood or dryer?
Equipment that exhausts its heat outdoors adds much less to the room than its rating. Count only the part that stays in the room, or leave it out.
Sources
- NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
- Room Air Conditioners — sizing chart — ENERGY STAR (U.S. EPA / DOE). Floor-area sizing chart (5,000–34,000 BTU/h) and the shade, sun, occupancy and kitchen adjustments.
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