We’ve explained why wattage is a poor measure of LED performance, but there is still some value to this measurement. Before you use the power consumption to estimate your electricity cost, you’ll first need to understand how these measurements are taken.
Watts are a measure of power. Electricity can be measured in three different metrics: voltage, current, and resistance.
Voltage measures the difference in electrical charge between two contacts. Current measures the rate of flow, and resistance measures the amount of current a circuit can handle. Voltage is equal to the total current multiplied by the resistance.
You can think of this like water flowing through a hose. Resistance is the size of the hose, current is the volume of water flowing through it, and voltage is the pressure of the water. Imagine that you have two hoses, one large and one small. If you were to push one liter through both of these hoses over the course of 60 seconds, they would each flow a little differently. In the small hose, the water would flow quickly and come out at high pressure. In the large hose, the water would be low pressure and have a more gradual flow.
Watts are equal to the current multiplied by the voltage. In this analogy, watts are a measure of both the quantity of water and the pressure. We often think of wattage as a simple measure of the volume of electricity used. But as you can see from this example, it’s measuring both the quantity of electricity and the amount of strain being applied to the circuit. As you learned in our efficiency guide, LED grow lights are complicated little devices. Manufacturers use watts to describe several different functions of their lights, so not all measurements are the same.
If you want to know how much power your lights are going to use, true wattage is the measurement you’ll want to know. True wattage, as the name implies, measures the actual power used by both the electrical circuits and the lights themselves. Although true wattage is a good measure of power draw, it is not a good measure of light output because you have no idea how efficient the system is.
This is the measurement of the maximum potential power that your LEDs can handle. If you have a light with 100 LEDs that are each rated at 3 watts, then your LED wattage would be 300. It’s important to remember that this is a maximum, just like the top speed on your car. If you’ve read our efficiency guide, you know that LEDs take power and turn it into both heat and light. When you are running LEDs at their maximum potential power, they generate more heat and tend to burn out quickly. For this reason, most LED grow lights supply less than the maximum power to these lights, ensuring they are more efficient.
The perfect example of this is the MarsHydro Reflector LED grow light. The first generation of this product used 3 watt LEDs and ran them at full power. Once consumers reported problems with lights dying after only a few years, they replaced all the LEDs with 5-watt versions. For the highest end model, this means that the LED wattage rating went from 576 watts to 960 watts. But because they are running these LEDs at less than their capacity, the true wattage rating remained the same.
This is probably the most misunderstood power rating in the industry. When LED grow lights first hit the market, they were not well received. Remember, LEDs usually produce more light per watt than other types of lighting. At the time, this fact wasn’t very well known. Consumers would see a 250 watt LED light for the same price as a 500-watt HPS light, and assume that the HPS was twice as powerful. For this reason, manufacturers advertising equivalent wattage. When you’re told that an LED light has an equivalent wattage of 500, they’re saying that you would need to purchase a 500 W HPS light to produce the same quantity of light.
Using Power Ratings
Each of these ratings can tell you a little bit about the grow light you are about to purchase. The most important rating is true wattage. If you purchase lights that draw, cumulatively, 1000 watts of power, you can quickly calculate what your electrical cost is going to be.
LED wattage can, in some cases, help you estimate longevity. For example, let’s say that you are choosing between two cheap LED grow lights. One requires 330 watts of power and the LEDs are rated at 300w. The other requires the same 330 watts of power, but the LEDs are rated at 500 watts. It’s clear that the first one is operating their LEDs at their maximum. This means that you’ll likely generate more heat, and the lights will not last as long. If you’re purchasing a high-end grow light, you don’t want to worry about this value quite as much. With high-quality components and careful engineering, LEDs can run at or near their peak performance without degradation.
For the most part, equivalent wattage can be ignored. In most cases, this is simply a marketing trick that allows manufacturers too but big, impressive numbers on their packaging. However, there are two ways you can use this value. There is a little correlation between light output and equivalent power draw, so this can be a quick way to narrow down your choices of LED grow lights. If you’re upgrading your HPS growing system, then this value can help you understand which lights will be a suitable replacement. Remember, this rating is always a rough estimate. Every manufacturer calculates it differently. At the end of the day, you’ll want to make sure you are using the correct metrics to measure the output of your lights.