If you're running pure T5's, the power savings of going to LED's will probably be minimal. If you're compairing to metal halide with supplementation (VHO or T5) you will likely save quite a bit of power. The main cost savings of the LED's is going to be in your bulb replacement. Ideally metal halide and T5's should be replaced yearly - average about $20-25 a bulb for T5 and $50-75 a bulb for MH. That could be anywhere from $50 for a simple 2 bulb T5 setup to $200 or more for multiple metal halides with T5 supplements.LED are said to be a huge power saver but it's nickel and dime as far as im concerned when talking about the power use on your entire house. a 150w LED that provides a cone and uneven lighting vs. my 3foot 4 bulb t5 putting out 156w of power that evenly lights a 3foot long tank and grows SPS on the sand.
IMO led would take YEARSSSSS to pay for themselves if you neglect changing the lamps in fluorescent/halide.
i've owned both LED and t5. I'd say i'm leaning toward LED for my next fixture to save an extra $100 a year in bulb cost.