Yeah, or whatever the actual voltage is at the electrical outlet you plug it into. One assumes it'll be pretty close to 120V, but if it's off by a few volts then it'll affect your calculation.
If you have a Kill A Watt device and it's still in calibration, it'll have a button you can push to get the actual wattage (and other basic stats). If you have one that isn't reading correctly (perhaps because it got a little "burnt up," lol, and you had to repair it) there is an unofficial calibration procedure floating around on the Internet that looked pretty simple.
The... ah... non-square rectangular shape of the light matches the non-square rectangular shape of your grow space, so that should help.
If you have a Kill A Watt device and it's still in calibration, it'll have a button you can push to get the actual wattage (and other basic stats). If you have one that isn't reading correctly (perhaps because it got a little "burnt up," lol, and you had to repair it) there is an unofficial calibration procedure floating around on the Internet that looked pretty simple.
The... ah... non-square rectangular shape of the light matches the non-square rectangular shape of your grow space, so that should help.