What exactly are you asking for? Are you asking for the range of irradiance under which humans can have "acceptable" vision, however you define that? Because if that's the case, it's going to vary by illuminating source. That is why you can't find convenient converters online. Stare at a light source that's putting out 100W in long range infrared and you can't see a thing, but your face will probably get warm if you're close enough. On the other hand, stare at a white LED that's putting out 100W across the visual spectrum and it's going to look pretty darn bright. Unfortunately, the luminous efficacy (how bright a light source of a particular wavelength appears to a human being given a certain amount of emitted flux) is variable based not only on wavelength but also on overall lighting conditions, because the sensors in your eye which are used at low light have different wavelength dependent response than the sensors in your eye that are used in brighter light.

Lux is the unit for radiative flux that is useful for human vision. It's the unit which is intended to reflect how bright a light source actually looks, without regard to how much total radiation is coming out of it. The range of human vision, in terms of being able to develop a useful visual picture based on the incoming light, is roughly 10^-5 to 10^+5 lux. If we make the further assumption that even at very low radiation power the eye is most sensitive to 555 nanometer wavelength light (which is not true) and use the conversion for luminous flux to radiative flux assuming that all of the light is monochromatic 555 nm light, we have 683 lux = 1 W/m^2. These edges aren't exactly precise, so let's just use 10^3 as the conversion factor between lux and watts per square meter, in which case we get a visual range of 10^-8 to 10^2 W/m^2.

Again, there's no single answer to your question. It's underdetermined. But as a general estimate the above is somewhat reasonable.