In the LED industry, specifically landscape lighting, there is a term tossed around called Volt Amps (VA). In layman’s terms, volt amps refers to the amount of energy in an AC circuit that is consumed but doesn't contribute to light output. This is important when designing and planning for an outdoor lighting system because when the VA is calculated, the LED lamp typically draws more energy than listed on the lamp. It is important to size the transformer correctly; therefore taking VA into account is crucial.
Most retrofit LED lamps used for landscape lighting have a small circuit board inside called a driver. This driver is what accepts 12 volts AC from a transformer and turns this voltage into DC voltage which is required to light the LED. Now this rectified voltage goes through a series of small electronic components which make the LED light up. (It is actually much more complicated than described, however we won’t get into electrical circuit design.)
When the voltage passes through the driver, all of the small electronic components consume watts. In the case of small retrofit lamps it is a small amount of wattage, but certainly measurable and necessary to plan for. All LED lamps have a Power Factor (PF) value, usually 0-1. In most landscape retrofit lamps this number can range from .5 to.9. This number is important because it is what is used to calculate VA. For example, if you took the apparent power, (the wattage listed on the lamp), divided it by the PF you will get the VA, or the real power of the LED lamp. Below is a quick example of how the formula works using a 4 watt LED lamp.
4 watts/ PF .85 = VA 4.7
In this example, the transformer will see a load of 4.7 watts. Most manufacturers will have a VA chart listed on their website to help the designer measure the voltage drop and size the transformer correctly. Now, if you don't know the VA and want to be safe, you can use a PF of .65 to.75 and get very close. It is important to always plan for the future and make sure you can safely add light fixtures to an existing design. This will ensure a hassle free upgrade down the road.