Equivalent wattages between LED and HPS?

[BigB]

Is there a guide somewhere that compares the usable lumens per watt of LED to MH or HPS? I am used to growing under halide and sodium lights, but would like to experiment with triband and quadband LED. Is there a table somewhere that shows how a given wattage in LED compares to HPS or MH? What wattage HPS or MH would be required to get a comparable amount of light usable by the plant from a triband or quand band LED (ie x LED = y HPS)?