How does ballast factor affect energy savings calculations in lighting?

We’ve written separately on the blog about energy savings calculations and ballast factors with ballast-dependent lighting, but how do these two topics come together? This can be a really tricky topic in lighting if you’re new to ballast factors. 

Here’s how it often plays out:

You’re considering going with linear LED lamps in your parking deck. You currently have two 32W T8 bulbs in each vapor tight, and you are curious how much you would save.

You: “How much will I save by replacing my T8 bulbs with linear LED?”

Lighting Rep: “Well, your existing fixtures use about 59 watts each. If we look at a 12 watt LED bulb, your wattage per fixture would go to about 29 watts. That’s about 30 watts saved per fixture!”

You: (To yourself) "Did this person flunk basic math? A fixture with two 32 watt bulbs should use 64 watts, right? That makes no sense."

Lighting Rep: “What do you think? Should we get some samples for you to look at?”

You: “I’ll think about it and get back to you.”

That response would be totally understandable if the rep didn't explain the impact that a ballast has on the fixture’s energy usage.

If you’re looking for an accurate picture of how much you could save by swapping either fluorescent lighting or HID lighting to a plug-and-play LED, one of the most important factors to consider is the ballast you’re using. This will give you an accurate picture of your current energy usage as well as realistic expectations for savings estimates.

Let’s go through a concrete example.

Looking for more background on this topic? Here is a primer on how ballast factor affects energy usage.

Example: Ballast factor with linear fluorescent-to-LED retrofits

Consider the parking garage scenario we started with. If you opened up your average fluorescent light fixture, you would probably find an electronic ballast with a normal ballast factor. For this example, let’s use a typical ballast factor of 0.88.

Now, there are two ways to calculate the energy usage of our fixture. The most accurate way to get energy usage is to look up the specific light bulb and ballast combination in a ballast catalog and find the “Input Wattage” or “Input Power” rating. The second way that many people use to estimate energy usage is an approximation using the ballast factor. In this approach, you multiply the light bulb wattage by the ballast factor. 

Here’s how the energy usage breaks down for our example fluorescent fixtures:

Here’s how the energy usage breaks down for these fluorescent fixtures:

32 W bulb x 2 bulbs x 0.88 (normal ballast factor) 

=

56.32 W estimated total fixture wattage

(Ballast catalog range: 55W to 59W)

Which number should you use for your calculations? Our teams tend to use the 59W rating from the ballast catalogs for existing systems because it’s based on a more common, middle-of-the-road ballast.

So, what about the LED you’re installing? Unfortunately, there’s no approximation here. You need to consult the manufacturer’s specifications to be sure about the energy usage for a plug-and-play linear LED.

If we take the Philips InstantFit product for an example, the table on page three of the spec sheet tells us that a 12 watt LED tube paired with a 0.88 ballast factor uses 14.5 watts. So, our new fixture wattage with the LED tubes would total 29 watts.

Wait, what? More energy used than the rating on the tube?

Yep. That’s one of the reasons that it’s so important to get the right data before making a purchase decision on a retrofit. Let’s do a side-by-side comparison of the two ways we can look at this calculation: 

As you can see, getting the numbers right at the beginning can make a huge difference in your final financial analysis and payback estimates.

Choosing the right LED with ballast factors in mind

At this point, you may be wondering whether or not it’s worth bypassing the ballast if you are upgrading to LED to save a little more energy. And that would be a valid question.

While there is likely a small amount of additional energy savings using a direct-wire, or ballast-bypass, LED tube, there still remains a safety risk with line voltage going straight to the sockets. Additionally, you should consider whether or not the additional labor in having to re-wire the fixture negates the incremental wattage savings you would see.

Click here to read our comprehensive review: 'Plug-and-play vs. ballast-bypass and other linear LED solutions.'

If the idea of running an LED lamp with a ballast is something you’re against, your best options may be a retrofit kit that includes a driver installed in place of the ballast or a new LED fixture. There are even some innovative “retrofit kits” that are quick and easy to install and upgrade the look of the entire fixture.

Here’s a guide for how we break down the decision of going with a replacement LED lamp versus a complete fixture: 'LED lamp replacement vs. LED fixture: Which is best for you?'

As you consider your next LED retrofit of ballast dependent lighting, make sure you gather all the facts on the real energy savings to expect. Once you have the wattages sorted out, you can use our energy savings calculator or e-book on payback and return on investment to get to the bottom-line financial impact of your decision.