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How much power does my lamp really use?

The Importance of Power Factor in Lamp Ballasts​

Have you ever looked at your lamp and wondered, "The bulb is labeled 150W, but surely the ballast must consume some power too?" If so, you're right. Some people estimate that a ballast consumes around 5-10% of a lamp's wattage, but this isn't always true, especially for inexpensive HID (High Intensity Discharge) lamp ballasts.

The goal is to determine the relationship between the lamp wattage (W) and the total input power (VA, in Watts), also known as the Power Factor. A high-quality ballast will have a power factor above 0.9, meaning more than 90% of the power drawn is used to power the bulb. In contrast, cheaper ballasts often have power factors of 0.5 or lower, meaning only half of the power reaches the bulb. For a 150W lamp, this would mean it would draw 300W of power.

Real-World Example: Measuring Power Consumption of a 150W HPS Lamp​

Let's examine a 150W HPS lamp produced by Globe, which claims to be equivalent to 9 incandescent bulbs and offer 89% energy savings. We'll use a reliable multimeter to measure the voltage on our 120V line and the AC current. After wiring the leads in series with the lamp's black wires, we turn on the lamp and observe the startup current.

During startup, the lamp consumes a whopping 4.4A, or 535W! The power factor during startup is only 0.28, meaning only 28% of the power is utilized by the lamp. However, HID lamps typically draw more power upon startup, which can impact the power factor calculation.

Power Consumption and Power Factor After Warming Up​

After about fifteen minutes, the light reaches maximum intensity, and the current flowing through the circuit decreases to 3.06A, or 372W. The power factor is now 0.4. As an HPS bulb ages, it consumes more power; the same measurement taken when the bulb was nearly new showed a power factor of 0.5.

Since the ballast's power factor doesn't change significantly over time, we can calculate the lamp's wattage midway through its lifespan: 186W.

Answering the Initial Question: How Much Power Does My Lamp Actually Use?​

  • Bulb Wattage: 150W
  • Power used at Startup: 535W
  • Power used at Operating: 372W
The key takeaway is that running a 150W bulb on a low power factor ballast costs nearly the same as operating a 400W bulb with a high power factor ballast. It's also essential to be cautious when calculating electrical circuit installations, as assumptions can lead to incorrect estimations.

Upcoming Segment: Improving Power Factor with a Correction Capacitor​

In the next segment, we'll demonstrate how to add a power factor correction capacitor to enhance the power factor of your low-cost ballast, improving energy efficiency and reducing costs.
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