System Efficiency

Total System Efficiency

System Efficiency is of massive importance to the lighting community. LED efficiency is continually improving but more often than not, the end user isn't aware or doesn't know their total system efficiency. Basically, what is the power in and light out? The mistake many companies make is they take the typical LED output value from the manufacturer's data sheet and multiply the number of LEDs in their fixture to calculate their LED light engine lumen per watt figure. This isn't the right way to calclate the right output. Please see below an example of how to get a ballpark figure of the true system efficiency:


LED Output

The LED will be "binned" at a specific lumen figure. Most use the typical value so it might be worth understanding the minimum figure from the bin code data but for this example, let's assume the LED emits 80 lumens. This figure equates to the device being driven at 350mA and at a Tj (junction temperature) of 25C. In reality the Tj in application will be around 70-80C, which will reduce the output by around 15%, and to calculate the lumen per watt figure, you must work out the output at 300mA because this is a Watt, not 350mA. This means our LED produces approx. 58 lumens per watt in application.


Optical

We then need to consider the optical efficiency. Most polymer based optic manufacturers claim around 85% efficiency so with our LED above, we should expect to see 49 lumens per watt output.


Driver

Then there is the driver solution. Due to our driving technique, we can claim an minimum efficiency of >88% because our driver compensates the power depending upon the LED load on each channel. The issue with all other drivers is that if the max voltage isn't consumed by the LED(s), then the balance of power becomes heat. This is why most LED drivers become hot and why it is hard for driver manufacturers to claim a minimum driver efficiency because they will not know for sure what the user's LED configuration will be. But, for the basis of this example, using our driver in this configuration we would see 43 lumens per watt system efficiency.