I would think it would take several years of operation to recapture the capital outlay you take on buying a 70w DE HQI MH setup just to save 26watts, especially at 1st tier baseline cost per kilowatt. You'd be better off chaning out two 75 watt household bulbs for 60 watters, and you'd be ahead a whopping 4 watts, saving you $4-6 per year depending on your cost basis and how long your run them daily.