Definition of Hype
exaggeration, embellishment; marketing message that exaggerates its description of the facts and embellishes the truth; ballyhoo; dishonest scheme
Definition of MTBF
MTBF, or Mean Time Between (or Before) Failure is a basic measure of reliability for repairable items.
Think Critically
What does MTBF really tell you about a product?
It doesn’t tell you how long it will last.
Can you compare the MTBF of one product to another?
No you can’t.
“Data from the European Power Supply Manufacturers Association, an independent trade body, found that MTBF figures for the same product could vary by 10:1, depending on the methodology used in calculating them. This is further evidence that an over-emphasis on MTBF as a measure of real-world reliability is just marketing smoke and mirrors.” (2)
Here is a nice explanation that I located that gives a basic example of how to calculate MTBF and illustrates the problems with MTBF calculations.
“If we have 50 units that all run for 100 hours and right at the end of 100 hours one of the units fails. We can calculate the MTBF as follows. First determine the total hours all the units operated. That’s easy, 50 units times 100 hours is 5,000 hours. Then divide the total operating hours by the number of failures. In this simple example, that is one, for a resulting MTBF = 5,000/1 = 5,000 hours.
Note: if we had 100 units run for 50 hours and had one failure at the end of 50 hours, the result is the same. Or, if one unit runs for 5,000 hours before failing. Or, 5,000 units each running for one hour, then one fails.” (1)
The above image is hype straight for an inverter manufacturer. Their comparison is completely misleading.
“Most people would assume that they should buy a product that has an MTBF of 600 years vs. one with 300 years. The tacit assumption is that one is going to last twice as long as the other. However, the truth is that there is nothing about these numbers to say how long either product will survive before “wearing out.” It could be that the product with the MTBF of 300 years could actually live 40 years before the wear-out mechanisms it. By contrast, the product with 600 years MTBF might have wear-out mechanisms that limit its life expectancy to 15 years or less. If you know this, you would clearly choose the 40-year life expectancy product over the 15-year life expectancy product.” (2)
MTBF doesn’t belong on marketing data and in press releases.
Statements like the above inverter example are simply hype and pushing the use of this information to make a buying decision is dishonest. Don’t fall for the Hype!
Is there a better way to compare inverters and to better define an inverters life?
Yes there are different reliability metrics that can better describe the actual reliability of inverters. One that I would like to personally see used “is “Bx” Bearing Life of “Lx” (described below) . Bx “defines the life point in time (hours, days or years) or cycles when no more that x% of the units in a population will have failed. This reliability metric was developed in the ball and roller bearing industry, where a B10 life is a frequently used metric and requirement. B10 defines a life point (such as 10 years or 100,000 miles or 1 million cycles) when 10% of a population will fail by. In other words, the reliability is 90% at a specific point in life time that is appropriate to the type of equipment it is being applied to. Any value other that 10 can be used 5, 2, 1, 0.5 and even 0.1 are regularly used “Bx” failure risk values.” (1)
“Use of the “Bx” metric spread to other industries where it is still widely used. Sometimes instead of using the “Bx” (i.e. Bearing Life) nomenclature it is listed as “Lx” to denote “MINIMUM TIME TO FAILURE LIFE” for any type of system equipment or component.” An example would be the automotive industry might seek “minimum of a 97% Reliability over 10 yrs of N. American Environmental Exposure and 100,000 Miles of Vehicle Usage that had to be demonstrated with a statistical confidence of at least 50%.” (1) “In other words a L3 (3.0% failure maximum) at 10 years and 100K miles.” (1)
Sources:
(1) http://nomtbf.com/perils/
(2) http://www.renewableenergyworld.com/rea/news/article/2009/09/mtbf-and-reliability-a-misunderstood-relationship-in-solar-pv
