Here are the incorrect ways (but most common) in order of how wrong it is.
- 1000 lumen / 10 watts = 100 L/watt (it will say on box "for 5000k led")
- 850 lumen / 10 watts = 85 L/watt
- 1000 lumen / 13 watts = 77 L/watt (it will say on box "for 5000k led")
- 850 lumen / 13 watts = 65 L/watt
(To stay competitive we have also had to published efficiencies based on the above)
The correct way is as follows:
(750 x .9 x .91) /13 = 47L/watt
The best way is to have the light output measured by a integrating sphere to calculate the output lumen.
In practise it is not easy to make a focused dimming light with an efficiency over 60 - 65 L/watt