The industry standard for High Intensity Discharge or HID lights is 50 watts per square foot. Or about 500 watts per square meter.
Roughly, 40-45 watts per square foot for T5 high output or CFL fluorescent lights, and this is mostly as you can bring the light a lot closer to the plants without burning them, and the intensity of light geometrically increases with proximity due to the inverse square law of radiation.
But watt for watt is is hard to get the intensity that HID lights provide.
LEDs and certain other lights can maybe do as little as 30-35 watts per square foot, maybe. A lot of this depends on how good the light is and how it is used. LEDs can often also be brought closer to the plant’s canopy than you might be able to do with HID.
Keep in mind, although it is rare with most types of lights, as you will often burn them with heat before you make the light too intense, some lights that do not put out much heat, like with certain high output high performance LEDs, or maybe a 1000 watt HID with a very efficient enclosed cooled hood, these can sometimes create a condition known as photo inhibition, also know as light burn or light bleaching.
In general, as @Donaldj has said, 40-60 watts per square foot might not be a bad guide to go by.