A customer has a question and I hope we can get some opinions on it, thanks

If I buy an led light for growing I have read 50 Watts is optimal. However this is what confuses me. A light might say 150 Watts but also only uses 15 Watts. So, do I say I have 15 or 150 Watts to use for growing even though I know only 15 is being consumed PR another example is for a 4x4 or 16 square foot tent to grow optimal would be around 400 Watts so do I need 400 Watts being used or say 3000 Watts light but only consumes 450 ?