Most of the online data are confusing or out-dated. Some are misleading.
Here is a simplified math answer:
Garden plants generally need around 400 μmol m−2s−1 light to be healthy, roughly equals to 23,000 lm/m2.
Nowadays (year 2020) most of the good LEDs give ~100lm per watt, so we need ~230 watts LED / m2 (or 21 watts per square foot) for indoor plants grow in dark rooms.
Note1: This is the actual power that LED dies draws, not the “marketing power rate” showing on the package. Some marketed “1000w LED grow light” only takes 150w actual power. Be aware!
Note2: Since the power converter and chipboard drop efficiency, you may want the grow light to actually draws ~300 watts from the wall socket, for a square meter of indoor plants.
Note3: Some plants need only 200 μmol light and some sun lovers need 600 μmol, find it out and calculate it accordingly.