The majority of office and industrial spaces today would be lit by Fluorescent lighting tubes. Common Wisdom has it that, replacing a standard fluorescent tube with a pin-compatible LED light is an easy way to save money. On the face of it, it makes sense, after all the LED light has a lower power rating when compared with a fluorescent.

But that’s not the whole story.

Understanding Energy

Lamp are rated in Watts. This is the instantaneous power usage of the lamp from second to second. The total energy consumed by a lamp is measured in kilowatt-hours, a sum of the power use of a system over a whole hour.

A kilowatt-hour is the equivalent total energy use of a single 1 kilowatt electric fire running for one hour, ten 100 Watt lightbulbs left running for a single hour, or a single 100 Watt bulb running for ten hours.

A kilowatt-hour of energy, is one Unit of electricity.

Watt’s a Lumen.

The Lumen is a standard metric for light intensity – or Lux – emitted by a light source. Lumens per Watt is a good measure of the energy efficiency of a light source. The more Lumens per Watt of energy, the better the quality of the lamp.

Also important is how well a lamp maintains its lumen output over time.

Shining a Light on things.

SEAI guidelines suggest a minimum Lux of 500 Lumens per square metre for office buildings where writing, typing or reading is required. We will assume a small office of 10m² which will, for our purpose require us to fit 5,000 Lumens of lighting.

We will assume that the business currently utilises a pair efficient T5 Flourescent tubes, each giving 2640 Lumens to light this office. They have measured the power consumption of the lighting tubes through Energy Supervisor, and received a total average power consumption of 56 Watts – equivalent to 28 Watts each.

The lights are reaching the end of their service life, and the company had to decide whether to replace them with another pair of T5 Flourescent tubes, or make the change to LED lighting.

To achieve the same level of illumination, they would need to install a pair of 22 watt, 2000 Lumen LED lamps along with a single 10 Watt, 900 Lumen LED Lamp.

To help them make the decision, they will compare the actual data recorded by Energy Supervisor, with the manufacturer’s stated ratings for the LED lights.

The lifetime cost of each option over 40,000 hours is summarised in the table below:

While the LED lamps do use less power than upgraded Fluorescents, they supply less illumination per watt of energy, meaning that they are less energy efficient overall. To meet our required level of illumination, we need to install more of them – or more energy hungry lights. This actually increases our energy cost when compared to the Fluorescent lamps.

Fluorescent lamps do degrade with age. After 20,000 hours, the Fluorescent will be nearing the end of their service lives. The additional cost of these new lamps eats up any energy savings achieved.

One thing we have not accounted for however, is the cost of installing the third LED lamp to meet our minimum illumination requirement. This requires at minimum, an hour’s work from an electrician, along with cables being run and a new lamp holder being mounted. The cost of this hour’s work, more than eats up any savings generated over the life of the system.

 The Conclusion.

Admittedly, this is an artificial example, but it’s the simplest example of what is a common business question: Is it worth making the capital investment now to change an existing process to be more efficient over the long term?

The actual performance of a real system may be different from the manufacturer’s rating– either due to natural wear-and-tear, production quality differences or environmental changes. There are other variables involved – a high quality fluorescent light matched to a poor-quality ballast will always give poor energy performance, for example. But by monitoring the running process, you can see how the real system is actually performing and make decisions based on real measurements, rather than estimates based on manufacturer’s ratings.

This could easily be applicable for example, to a whole office building which might have potentially hundreds of similar lights, in which case small savings per-lamp can rapidly stack up.

What you cannot measure, you cannot improve.

What we at Energy Supervisor allow you to do is test assertions and analyses like these, measuring the performance of a real system, enabling you proactively improve it.

In this case, it appears the upgrade just isn’t worth making. While there is a small potential saving to be had, the cost of implementing the change outweighs the benefits for the time being.