I think people in this thread make a fair point about watts to square foot. This principle of ~30W/sq ft gives a decent idea of what flowering cannabis plants need in terms of light. What this "watt/sq ft"-principle doesn't take into account, though, is how the light spreads. COBs, for instance, usually have very high light intensity (hot spot) right underneath the lamp but the intensity quickly diminishes away from the center spot. Comparing a 100W COB which has a poor spread of light to a 100W QB or strip which have a greater spread of light will give you poor data to base your light configuration on.
I suggest you consult PPFD maps (and make sure they are credible, i.e. not Mars', Vipars', Spider's -- see Albo Pepper's video on Youtube for more info) and based on the PPFD you find a lamp that suits your needs. There's absolutely no need to buy a massive grow light if the goal is to maintain a mother plant and grow clones. The mother plant will do fine with around 300-400 PPFD while the clones barely need 100. PPFD is, simply put, photon density or light intensity.
We've written an article on what to look for when trying to decide what lamp is good for a certain grow space. We mentioned the watt/sq ft-principle but again, looking at PPFD maps is far more accurate.
How to find the right LED grow light
I'd also like to mention that our Q7 (320W) lamp performs very well in 2x4' and 3x4' spaces and our smaller Q2 (36W) is great for clones and mothers.
You can read more about them on
LEDTonic LED Grow Lights or PM me if you have any questions.