Our rule of thumb – it takes one watt of air conditioning to cool three watts of lighting.
For a more precise calculation there are a few things that we need to know about heat
- A BTU, or British Thermal Unit, is the traditional unit for heat or heat removal
- A ton of refrigerant is 12,000 BTUs.
- One watt of energy is equal to 3.412 BTUs
When you turn on a light some of the energy is turned into photons and some of the energy is turned into heat. In the case of LEDs, about 40% of the energy is turned into light and 60% is turned into heat.
Unfortunately…. you need to cool all of the energy, even the energy emitted as light. This is because the emitted photons will end up heating up the surfaces they hit. The heat/light ratio does impact your HVAC design but a good estimate is that you need to cool every watt of lighting.
Let’s assume you need to cool 10,000 watts. You will need about 2.85 tons of refrigerant
- 10,000 watts x 3.142 BTUs per watt = 34,120 BTUs
- 34,120 BTUs / 12,000 BTUs per ton = 2.85 tons
How many watts will 2.85 tons of HVAC consume? This is related to the EER, or Energy Efficiency Ratio, of the HVAC system.
- kW/ton = 12/EER
Most commercial HVAC systems have an EER of 10-13 meaning the systems will consume 0.923 to 1.2 kilowatts per ton of refrigerant. A system with a 12 EER will consume 1 kWh for every ton of cooling.
In our example of 10,000 watts of light, a HVAC system would consume about 2,850 watts of energy to cool the room. This is very close to our rule of thumb of 3,000 watts.