HPS is great and all, but perhaps for a small gorw like yours, you should consider induction lighting? This is the lowest wattage I could find:
http://www.hydroponicshut.com/50-watt-plasma-induction-grow-lamp-hydroponics-hut.html (you would need at least two to replace the HPS) The advantages are dual spectrum (more light gets absorbed by the plant), much longer lifespan than with HIDs, runs cooler, does not need a ballast and can be screwed into a regular E27 socket... you can bring this light closer to your plants than HPS, which is handy if you are growing in a small space. It would also make a great supplemental light.