We shall refer to two types of such forecasts: based on Cree's model (which we call the "worst case scenario" or "pessimistic model"), and in parenthesis we provide figures based on the Philips model for their LUXEON Rebel ES (which we call the "optimistic model"). If all the required operation conditions are fulfilled, we will still be getting about 70% of LED's initial radiation power after 40 (150) thousand hours of operation. These figures translate to 10 (33) years of operation of a light fixture, providing 12 hours of operation daily! After this period the LEDs will continue to lose luminance, reaching about 50% of the initial value after 100 (200) thousand hours!
The probability of a single LED failure on a fixture is quite low, about 1% during the period of 50 thousand hours of operation, and after this period the probability increases to 50% by 200 thousand hours. Several LEDs in a light fixture are usually connected in series, and therefore, if one LED dies, the whole string will be effected. If we look at these figures statistically, is likely that for a fixture with about 200 LEDs this can happen in 10 years. However a LED's death is a probabilistic event and it can happen that a particular light emitting diode may get "fried" during the very first hours of its life. In practice, if the conditions are good, lifespan of modern LEDs is quite long.
In comparison, conventional fluorescent tubes need to be replaced once every four to six months. Based on our worst case scenario, it means that they will have to be replaced at least 20 times during the lifetime of a LED fixture. Providing that the cost of specialized tubes for reef lighting can be quite high, a LED fixture can provide significant savings; e.g., not only monetary, but also of the time that was to be spent for acquisition and replacement of light bulbs.
Let us try to calculate the possible savings from using a LED fixture. A 300W LED fixture can replace a 900W T5 fixture used on a 160 gallon SPS reef tank. In 10 years the LED fixture will save ((900-300)/1000)*12*365*10=26280KWh of electric power. The cost of electricity depends on where you live, how much you use, and possibly when you use it and the rates from the same provider can range from 12c to 50c per kWh [17]. For our estimate we shall use a sample rate of 15c per kWh, which is a reasonable example (you can find out how much you are actually paying for electric power by looking at your bill). Based on the 15c per KWH example, the fixture will save you $3942 in power alone. If we take the average cost of a specialized 80W T5 bulb to be around $25, we shall additionally save $25*10*20=$5000 in bulb replacements. Your total savings in 10 years will be about $8942. This is a "best scenario" estimate and we did not consider many additional expenses - for example, the cost of an aquarium chiller to remove the excessive heat from the tanks, as well as energy costs related to its operation. Besides, there are non-monetary values - such as the comfort of not having to provide maintenance on a light fixture in 10 years! Thus far, direct savings during the operational period are quite a few times higher than the cost of even most expensive LED fixture. In other words, not only you are getting it free, but it will even bring you some profit in its lifetime!
