600-watt HPS light with a good bulb = 95000 lumens. My 400-watt light generates 55000 lumens. So, T5s don't save power. My T5 light (eight-bulb, 4-foot) takes 432 watts for 40000 lumens, so less light than a 400-watt HPS light, and it takes more power. T5s do generate a lot less heat though, that's true. Not nearly as good with canopy penetration though, or light/energy efficiency.

Each T5 4' bulb generates 5000 lumens, so you'd need 19 bulbs to equal the 95000 lumens of a 600-watt HPS light. Each 4' bulb takes 54 watts of power though, so that'd be 1026 watts of T5s to equal the light from 600 watts of HPS. So, T5s are a a bit better than half as efficient, power-wise.

I use mine solely for veg, for this reason. You *can* flower with them (with warm bulbs), but don't expect the same results that a 400+ watt HPS light would give.