How many lights/watts can I put on a 15 Amp service?
The rule of thumb is to only load a breaker up to 80% of its capacity. On a 15-amp service with 14/2 gauge wire one should only load it up to 12 Amps MAX. Since most lights will draw 1 amp per 100 watts a 15-amp breaker can handle one 1K light each. This brings up another point in home growing. That it is always good practice to have the fans and the light that they cool on the same breaker. That way if the breaker is somehow tripped (Murphy's Law) then both the light and its cooling system are down. Instead of the cooling system for the light going down and the light still blazes away because it was on another breaker.
Note: above figures are estimated based on 110V supply.
Nietzsche recommends purchasing a CAT III meter before doing any electrical work. It will allow for safe testing of the circuit before commencing work. A clamp meter is also handy for testing how many amps the circuit is drawing without having to break the circuit to do so, great to see if you get a peak when your lights first come on or just how many amps a given device is using.