VA vs. Watts in Landscape Lighting: How to Size Your Transformer

Understanding Power and Design Considerations

Is VA a single, fixed number? Not quite. VA (volt-ampere), or apparent power, changes with the conditions in the system. It combines real power, measured in watts, with reactive power, measured in VAR. Here's how VA works and how to use it when you size a landscape lighting transformer.

VA depends on its inputs: volts, resistance (ohms), and current. To find current, measure voltage and resistance and divide volts by ohms (A = V ÷ Ω). To find real power, multiply voltage by current (W = V × A). Once you have watts, you need VAR (volt-ampere reactive). Watts and VAR together give you VA, or apparent power.

How is VAR calculated? You need the phase angle measurement first, and then you can calculate VAR. VA isn't a simple sum of watts and VAR. As the power triangle shows, it's the square root of watts squared plus VAR squared: VA = √(W² + VAR²). In practice, there's no easy way to measure VAR in the field the way you can measure watts. Measuring phase angle takes special equipment, and for most installation crews it isn't worth the time. That's why most designers rely on the manufacturer's published VA rating.

So calculating the VA of a luminaire takes voltage, current, and phase angle measurements plus several formulas. And VA isn't fixed. It changes with voltage (think multi-tap transformers, or a luminaire's distance from the transformer), and it changes with resistance (think long wire runs). Every outdoor lighting system has variables that only show up at installation.

Can you correct for changes in resistance and voltage? Yes, through proper planning. You can specify the length and number of runs and the number of luminaires per run. The goal is to deliver the planned voltage to every luminaire. For example, if the target is 12VAC, you specify the wire gauge, run lengths, and number of luminaires per run so that 12VAC reaches each one. Then the manufacturer's VA rating holds up in your calculations. Note: you need to know the voltage a manufacturer used for its VA rating to get an accurate estimate.

How do you use VA when designing an outdoor system? Every LED draws both watts and VAR. Watts are the real power that makes light, and VAR is power that's sent but not used to make light. To size a transformer correctly, you have to account for the power the luminaires consume and for losses in distribution. VA captures what your system is actually drawing, so you can design with safety in mind.

VA is also a useful way to judge how many luminaires a transformer can carry. In the halogen era, the method was simple. With a 600W transformer, you'd deduct 20% for system losses, leaving 480W. Then you'd divide by the lamp wattage (say, a 35W halogen PAR36) to get the number of fixtures that transformer could handle. Halogen lamps had a power factor close to 1, so that 20% buffer was mostly covering resistance in the system, not reactive power.

With LEDs, estimating the luminaire-to-transformer ratio gets more complicated. It's easy to assume VA is fixed, but resistance and voltage both change the VA at the luminaire. Many LEDs run at multiple voltages. Sterling Lighting's drivers, for example, run on AC or DC anywhere from 9.5V to 22V, and many Sterling luminaires have adjustable drivers that change the LED's wattage. Each voltage and setting combination has a different VA. To use VA in system planning, you need to know the input voltage behind a manufacturer's VA rating and build your system to match it. Sterling luminaires' wide input voltage range and adjustable wattage make that much easier in the field.

What do variable VA numbers mean for you? Every installer wants a safe system: no transformer overheating from excessive amperage, no more current through the lines than they're rated for, and no shortened LED life from over- or under-powering a luminaire. VA accounts for both watts and VAR, but it only stays constant at a consistent input voltage and resistance. Theoretical values are great for planning, but plans change in the field. Check the voltage at every luminaire and the current on each line at the transformer on every install.

For more on sizing your wire runs, see our post on calculating voltage drop in landscape lighting. When you're ready to size your next system, browse Sterling's landscape lighting transformers.

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About the author: Patrick Harders, LC, is co-founder and CEO of Sterling Lighting and founder and principal lighting designer of Dayloom. He has more than 25 years of experience designing and installing outdoor lighting, and he's one of only five landscape lighting designers in North America with Lighting Certified (LC) accreditation. His projects have won multiple international lighting awards, including landscape lighting winners in 2018 and 2021, and he leads design of Sterling's luminaires.