I suspect the Phillips agro light, 50w r20, is a halogen light. (??)
If so, we are talking apples and oranges. Remember that a halogen is an upgraded version of an incandescent. With a halogen, your energy will be utilized creating primarily infrared light (heat), even with an agro light, which tends to have added blue spectrum.
These are ideal for terrestrial plants. No idea how they will work with macroalgae. My concern is that microalgae, the type that grows near the surface, will love it. Cyanobacteria may love it also.
Let us know how it works, but keep in mind that with a halogen (if it is) you are paying higher electricity bills for less lumens and more heat.