With most lightsources, you can use values outside the [0,1] range. It usually doesn't make much sense for color but it's common to see intensities in the hundreds or thousands.

As for physically realistic units, it might be possible to come up with a conversion factor but you would have to take exposure into account as well. Normally in computer graphics people don't mess with exposure and instead adjust all the lights so the computations make the output fit in the proper range. So it's more of a "visual feedback" method than trying to work out good values with a real-world model. You also have to keep in mind that not all computations done in rendering are realistic. For instsance, many lightsources can be made to produce light in impossible ways and artists are often known to want lightsources which substract light!