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