We essentially want to know the ratio of the lamp wattage (W) to total input power (VA, in Watts):

This is called the Power Factor.

A good ballast will have a power factor above 0.9 (i.e. more than 90% of the power it draws is used to power the bulb). Cheaper (lower quality) ballasts often have power factors of 0.5 or less. That's right, only half of the power gets to the bulb. So a 150W lamp would be drawing 300W of power.

Let's look at a real world example. Here's a 150W HPS lamp made by Globe. Notice they claim that this 150W HPS is equivalent to 9 incandescent bulbs for a saving of 89% in energy costs. This lamp has been used for about 8 months (about 4-5000hrs).

OK, let's take our trusty multimeter and measure the voltage on our 120V line.

So now we've measured V = 121.6V. We now set our multimeter to measure AC current and plug the leads into the 10A slot on it.

WARNING! Never put the leads in the 10A slot for any purpose other than measuring current! Failure to heed this warning will at very least result in a tripped breaker and a blown fuse inside your multimeter, if you're lucky.

We then wire the leads in series with the black wires of the lamp. Let's turn on the lamp and see what the startup current looks like.

Holy cow! 4.4A! That means that it's using 121.6V x 4.4A = 535W! Let\'s figure out the power factor at startup.

Power Factor = W / VA = 150 / (121.6 x 4.4) = 0.28!

That means that only 28% of the power is being used by the lamp. Let's not forget that it's normal for HID lamps to draw more juice at startup. Therefore, the lamp is probably drawing more than 150W and is skewing our power factor calculation.

So let's let it run for a while....

After a good fifteen minutes has gone by, the light is at maximum intensity. Let's have a look at the current flowing through the circuit now!

As we can see, it has indeed dropped. It's now drawing 3.06A or 121.6 x 3.06 = 372W! Let's figure out the power factor.

Power Factor = W / VA = 150 / (121.6 x 3.06) = 0.4

It's important to mention that as an HPS bulb ages, it draws more power. I performed the same measurement when the bulb was almost new and it was drawing 2.6-2.7A at operating temperature then, which would give something closer to a more realistic power factor of 0.5.

Since the power factor of the ballast doesn't change much over time, we can work backwards to determine how many watts the lamp is drawing part way through its life:

W = PF x V x A = 0.5 x 121.6 x 3.06 = 186W.

To answer our initial question, "How much power does my lamp really use?"

Bulb Wattage -> 150W

Power used at Startup -> 535W

Power used at Operating -> 372W

I think the most important thing to learn here is that it costs almost as much to run a 150W bulb on a low power factor ballast as it does to run a 400W bulb with a high power factor ballast.

It should also be apparent that one should be careful about the assumptions they make when making calculations for the installation of electrical circuits. You may think that you can safely put 7-8 of these 150W lamps on a 15A circuit but guess again!

In the next installment, I'll show you how to add a power factor correction capacitor to increase the power factor of your el cheapo ballast.