Generally, when selecting LED luminaires, the attention is drawn to energy consumption. Efficacy, therefore, the power consumption, is the most important selection criterion. But do you pay attention to the lifetime of the luminaire? What happens if the better efficacy luminaire has shorter lifetime? This means that you may have to renew the luminaires much quicker. This adds expenses and eats off the savings from the electricity bill.

## Lifetime of LED Light Source

Compared to traditional light sources, the lifetime of an LED light source is long. When a traditional incandescent bulb or a fluorescent tube runs out of its lifetime, it can’t be used anymore. It either won’t produce light or starts to flicker. LED only loses some of its brightness and is, at least in theory, eternal.

This is the reason why the lifetime of LED light sources is measured in a different fashion compared to the traditional. The lifetime of a traditional light source means literally lifetime. LED’s lifetime tells that at what point the amount of light drops below the desired value.

The terms for LED Lifetime:

- L70, L80, LXX = How much of the original lumens are still available. For example L70 means that the light source still produces 70% of the original lumens. So if the light source produced 1000lm at the beginning, this has dropped to 700lm.
- B50, B60, BXX = How many light sources are below the given lumen value. So e.g. B50 means that 50% of the light sources don’t produce the desired amount of lumens anymore.

Typically the lifetime is given as a combination of these two. For example L70B50: 60.000h means that after 60.000 operational hours 50% of the light sources still produces at least 70% of original lumens.

The lifetime can also be given using only the L-value. For example L70: 60.000h. Then the manufacturer doesn’t actually promise how many of the luminaires are still over 70% after 60.000h.

## Taking Lifetime into Account When Selecting Luminaires

Let’s assume that you are lighting a space that has 1000 luminaires. For example a shop. You have narrowed your luminaire choices down to two: Option A and Option B. Currently the space has fluorescent lighting and more specifically 58W T8 luminaires, which produce roughly 4000 lumens each. Here are your options:

- Option A
- Luminous Flux :4000 lumens
- Efficacy 150 lm/W
- Power: 26.7 W
- Lifetime L70B50: 50 000 hours
- Price 120€/Luminaire

- Option B
- Luminous Flux :4000 lumens
- Efficacy 130 lm/W
- Power: 30.8 W
- Lifetime L70B50: 90 000 hours
- Price 120€/luminaire

It’s easy to choose option A. The power consumption is lower as the power is around 4 watts smaller. For example in your 1000 luminaire space this means 4kW and a significant savings in your electricity bill.

When you know the daily operational hours, you can calculate the annual electricity consumption and compare that to the old solution. For the sake of an example, let’s assume that your shop is open for 14 hours a day. When you add the time for cleaning etc., your daily operational hours are 16. At least in this example.

This way you get the following math:

- Annual operational hours: 16h*365= 5840 h/year
- Price of electricity: 0.1€/kWh
- Traditional (58W Fluorescent)
- Annual electricity consumption: 1000*58W*5840h=338720000Wh=
__338720kWh__ - Electricity bill: 338720*0.1=
__33 872.00€__

- Annual electricity consumption: 1000*58W*5840h=338720000Wh=
- Option A:
- Annual electricity consumption: 1000*26.7W*5840h=155928000Wh=
__155928kWh__ - Electricity bill: 155928*0.1=
__15 592.80€__

- Annual electricity consumption: 1000*26.7W*5840h=155928000Wh=
- Option B
- Annual electricity consumption: 1000*30.8W*5840h=179872000Wh=
__179872kWh__ - Electricity bill: 179872*0.1=
__17 987.20€__

- Annual electricity consumption: 1000*30.8W*5840h=179872000Wh=

Both LED options drop the electricity bill down to half of the old solution, saving you a lot of money. Option A saves a little bit more, thanks to the better efficacy and lower power consumption.

In short term, option A would be the better solution. As it saves more per year. Option A saves annually around 2.4k€ more than Option B.

However, it is very rare that investment this size is made with a one, two or even five years scope. That’s why we should do the math for a longer period.

When we take the lifetime into account, the situation changes a little.

Let’s first calculate the lifetime in your application. When the daily operational hours are 16, the annual operational time is 5840h, as calculated above.

With this, we can transform the lifetime into a more understandable form:

- Option A:
- 50 000h/5840h=8.6 years

- Option B:
- 90 000h/5840h=15.4 years

So you would have to change the option A after 8 years while the option B can still light your space for 7 more years (15 in total).

When we look at the total savings caused by the luminaires, the numbers look like this:

Lifetime (a) | Option A | Option B |

1 | 17 582.29 € | 14 447.26 € |

3 | 52 746.88 € | 43 341.78 € |

5 | 87 911.47 € | 72 236.31 € |

10 | 55 822.93 € | 144 472.62 € |

15 | 143 734.40 € | 216 708.92 € |

20 | 111 645.87 € | 168 945.23 € |

As you can see from the calculations and the chart above, the option A is better in the short term. But when you look for a long term savings, the option B’s longer lifetime kicks in before the investment reaches 10 years. You will have to replace the option A almost twice as often as option B.

So when you look at the investment in a long-term, the lifetime becomes very important.

When choosing luminaires, you should focus your attencion to both: efficacy and lifetime. The more expensive the luminaire and the investment is, the more important the lifetime is.