Stop in for a cup of coffee

Here's the post about the H4 bulb tests.
Interesting headlight bulb test results
He tested at 13.5 Volts and then converts to some of the other common reference voltages.
He mentions that
US standard is for wattage to be stated at 12.8 Volts.
UN/ECE standard for wattage for same type of bulb is at 13.2 Volts.
So based on that, we clearly can't compare 'wattage' of an 9003 or HB2/H4 given in US Standard to an H4 in ECE standard without doing some math.
Fun 'eh?

Got the following from Stern's website:
Headlamp bulb light output is severely compromised with decreased voltage. The drop in light output is not linear, it is exponential with the power 3.4. For example, let's consider a 9006 low beam bulb rated 1000 lumens at 12.8 Volts and plug in different voltages:

10.5V : 510 lumens
11.0V : 597 lumens
11.5V : 695 lumens
12.0V : 803 lumens
12.5V : 923 lumens
12.8V : 1000 lumens ←Rated output voltage
13.0V : 1054 lumens
13.5V : 1198 lumens
14.0V : 1356 lumens ←Rated life voltage
14.5V : 1528 lumens
from here: Daniel Stern Lighting Consultancy and Supply