Design and Volt Amps
In the LED industry, specifically in landscape lighting, there is a term used quite often: 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 number is important when designing and planning for an outdoor lighting system. 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—so it’s crucial to take VA into account.
Most retrofit LED lamps used for landscape lighting have a small circuit board inside. This circuit board is called a driver. This driver is what accepts 12 volts AC from a transformer and turns this voltage into the DC voltage required to light the LED. Now this rectified voltage goes through a series of small electronic components which make the LED light up. NOTE: It is more detailed than described, but we won’t get into circuit design.
Power Factor & Calculations
When voltage passes through the driver, the small electronic components consume watts. In the case of small retrofit lamps, it is a small amount of wattage—but certainly a measurable amount and necessary to plan for. All LED lamps have a Power Factor (PF) value, usually 0-1. In most landscape retrofit lamps, the power factor ranges from .5 to .9. This number is important, because it is used to calculate VA. For example, if you took the apparent power (the wattage listed on the lamp), and 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 have a VA chart on their website to help the designer measure the voltage drop (and then size the transformer correctly). 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 plan and make sure you can safely add light fixtures to an existing design. Planning will ensure a hassle-free upgrade down the road.