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How Much Power Does My Lamp Really Use?

Smokin Moose

Fallen Cannabis Warrior
Have you ever looked at your lamp and thought, "Hmm, the bulb says 150W but surely this ballast thingy must use some power too" Well if you did, youre absolutely right. Some people say that the rule of thumb is about 5-10% of the lamp's wattage. While this may be true for some ballasts, it certainly isn't the case for all ballasts, particularly cheap HID (High Intensity Discharge) lamp ballasts.

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.


New Member
wheres the next instalment try to mention where to get these correction capasitors i assume that these will make my light brighter and more efficiant?

Harry Red

Nug of the Month: Nov & Dec 2008, Mar 2009 - Plant of the Month: Oct 2008, Feb & May 2009
Here's a neat little online calculator. Just plugin your wattage, light schedule and your cost per kwh (from power bill), click submit and you get the results. :smokin2:

Super-Grow Electricity Costs Calculator




Member of the Month: May 2009, Oct 2010, Sept 2017
I see this thread is ancient but was there ever a second installment posted anywhere?

IIRC, it came from the old Overgrow. I do not recall ever seeing a second installment. I do recall seeing images that went along with (inline) the text in post #1. I suppose those images might be helpful if anyone is having trouble.

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