When does a lighting retrofit pay off?

Retrofit quotes typically show a single number: how much the electricity bill will drop. That number may even be true, but on its own it isn't enough to decide on, and it often makes the weaker quote look better.

What counts besides consumption?

Over a luminaire's lifetime, its light output steadily declines. A low-quality LED can lose a third of its output in five years, while a good one still holds 80-90 percent after fifty to sixty thousand hours. If the plan doesn't account for this lumen depreciation, within a few years the measured light level falls below the standard, and the investment practically has to start over.

The other item is maintenance. In a twelve-meter-high hall, replacing a luminaire means a lift, two people and a production shutdown. If that repeats every five years, the cost can be several times the price of the luminaire. In a place like that, the more expensive luminaire with a longer lifetime often pays for itself sooner than the cheap one.

The third factor is controls. Without occupancy sensing and daylight harvesting, the lights stay on even when nobody is in the space. In a warehouse or an office corridor, this alone can mean an extra 20-40 percent in savings for a relatively small added cost.

A typical calculation

In a 2000-square-meter hall, 120 metal halide luminaires of 400 W each run about 4000 hours a year in two shifts. Switching to 150 W LEDs cuts consumption by roughly 60 percent on its own. The real payback, however, also includes the lamp replacements you no longer need, the avoided maintenance downtime, and the fact that LEDs start at full brightness instantly, so there's no warm-up time to burn through.

With all of this factored in, the investment typically pays for itself in two to three years. If the project also receives grant funding, this can drop below one year.

What to ask for

The calculation is different for every project, which is why the survey is based on measurement, not estimates. Ask for the measurement report of the current state, the photometric calculation for the new state, and the payback broken down by year. Three documents, and the decision is no longer a matter of agonizing.