When I bought them 2 years ago power in MA was $0.46 per kWh, this included transmission costs and all the other fees. 15W cost me $4.80 a month, so $57.6 a year and $230 over 4 years. At the time 14TB mechanical disks were about $300 so it was about a $270 ‘premium’ for solid state over mechanical so I exaggerated the ROI, but to me the 2x price premium was worth it for silence and no latency on retrieving my data. So in summary the ROI for me was more like 8 years, ignoring the many advantages of SSD.