My tank is 24''x12''x12'' which equals 57 litres or 15 US gallons. Does that mean I need light of between 30 and 45 watts and if so what does 6500K mean

This is a picture of my set up so far, I,ve got some bog wood soaking in a tub out in the shed, and alist of plants from the previos posts that I hope to get tomorrow. This tank has been doing a fishless cycle for about 4 weeks now, so shouldn,t be long before I can get some fish in it.
Brian.