Photography question: What is the difference between 300W and 400W flash?

My friend, you mean shooting people outdoors when the sun is against the light. You want the background to be black. This is very simple, and the power of the flash is not much different.

Let's assume that the measured background shutter data is 1/250 seconds, then use high-speed synchronous flash to set the shutter to 1/2000 seconds, and the camera uses TV files.

This will make the background black due to underexposure.