I was recommended this stack to ask this, as it seemed like a somewhat tricky question to find an answer to!
Originally I had thought that it would be around 300W per square meter, as the general guidelines I find for growing lamps is 30W per square foot (which works out to around 300W per square meter). I have no idea why they use square feet as the standard, I can only assume it's an american / gardener thing!
However this paper - arxiv.org/ftp/arxiv/papers/1406/1406.3016.pdf - seems to suggest 307W per a 12'x12' (144 square feet) square, which would imply that only around 30W is needed per square meter.
Hopefully someone can weigh in on this, or point me in the right direction :)