EV is "exposure value", which means a quantity of light coming from a source in the scene. You can think of it as being source brightness, or you can think of it as the camera settings to capture that particular source brightness as a midtone on your film. It's also the value returned by a reflected-light meter.

So EV15 is 4096 cd/m^2 or it's ISO100 f/16 1/100s. Likewise, the sun (according to andrew.roos) is 1.6e9 cd/m^2 = EV33.6, which would be ISO100 f/16 1/39,000,000 (a 25.6 nanosecond exposure) to hit Zone V.

I think you're right about the numbering of welding glasses being stops but I don't know. They also have some odd spectral effects (kind of green, lots of UV-blocking) so it's not a simple ND and therefore you can't state a simple filter-factor without knowing the spectrum.