BTUs are added to an indoor garden from light fixtures, fans, and CO2 burners. If your equipment is using electricity, it’s adding BTUs. Short for “British Thermal Unit”, a BTU is a measure of the quantity of heat contributed by a device to its environment. BTU comparison also aids in demonstrating the effect both LED and HPS lights have on the temperature of an indoor garden, and how you can use this relationship to your plant’s advantage.
To illustrate the heat distributed by an HPS lamp we can compare it to the sun in our solar system. Scientists estimate the amount of heat available from the sun striking the surface of the earth is approximately 240 BTUs per square foot, every hour. One 1,000 watt HPS lamp adds roughly 3,400 BTUs to your garden every hour. Granted, the HPS doesn’t radiate this heat from 150 million kilometers away, but it is a significant source of heat. Less than 20% of the light emitted from an HPS is made up of wavelengths plants use efficiently for photosynthesis.
Next, let’s look at the BTUs generated by a low heat LED plant light. If the LED has a cooling fan or metal heat sink you can assume it isn’t low heat. One hundred and ninety 5mm HBLEDs in operation create only 0.024 BTUs of heat; in fact the 40w power adapter emits more, at 0.625 BTUs. Your own body heat would provide over six hundred times more BTUs than some LEDs. Ninety-nine percent of the wavelengths emitted in a quality LED plant light are efficiently used for photosynthesis.
The ideal indoor garden is a greenhouse or area with access to plenty of natural lighting. The sun is free and gives plants both heat and quality wavelengths. Plants need radiant heat that can be generated by HPS lights, but the amount of heat emitted by HPS lights can require additional ventilation and cooling expenses. If natural lighting is not available or an area needs additional heat, the best option is to use LEDs and HPS lights together.
More than 80% of the light emitted from an HID light source is emitted as heat. Watt for watt the HPS will use about the same amount of electricity as a space heater. By using the HPS as a heat source you gain additional productivity (light) for your plants that a heater doesn’t provide. Turn the HPS on only when temperature adjustments are needed and use the LEDs to provide quality light for the entire photoperiod. This dramatically reduces energy costs and eliminates excess heat. Using a combination of LED and HPS lights in this way can decrease your energy use by as much as 80% and gives you greater control over the BTUs in your garden.
Angela Lundmark is the CEO of LED Grow Master Global and editor of the LED Gardener.