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.

1000 Watt HPS with conventional transformer,capacitor and starter ballast=1125 Watts consumption. They are second only to low pressure sodium in efficiency, LPS are not usable for growing as they produce only yellow light.

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Where have you taken these figures from?

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Standard reference for electrical tradesmen, , these get used for security lighting so have easily avaiable specs. Years of building experiance comes in handy.

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I just realized that my last post was a little short. The wattage figures would have come from Seimes/Sylvania and assume that the ballast is integral with the fixture. I f like 99.99% of growers you are remoting the ballast line losses will be adde. If you are using cable smaller than 3/12 these can be signifigant( the cable will be warm, showing loss through heat). I must admit I have not actually put a meter on one to test, and am takting these figures on faith, has anyone done this testing? If so let us knw results, please. Thanx.

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Thanks for the post mogie, always informative.

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Did you do some calculations or it was an eye experience?

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wood pellets chips

Not eyes but sense of touch. Undersized wires get really hot. Any good basic electricity text will explain this. Smaller gauge wire cannot handle the amperage. Overheated wiring is a major fire hazard, please don't burn down your house.

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Hi Mogie,

Interesting tests. You have a little error in your calculations. When you measure the amperage times the voltage the result is VA not watts. VA x power factor equals watts. You are correct that the low power factor ballast make the current higher than you would think and that people should pay attention to the power factor (or just read the full load amps off the nameplate) when they are figureing haw many lights to put on a circuit. But, the power companys' measure watts, not VA and that is what your bill is calculated from so you don't really use any more electricity with a low power factor ballast.

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Actually what they measure is kilowatt hours(1000 Watts x 1 hour). Amperage is the critical measurement for curcuit load, as voltage is fixed.(120 or 240). This whole discussion is getting quite interesting. I wonder if maybe a new thread on basic electrics would be a good idea?

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