Megawatts are indicative of compute load, not storage load. I can definitely believe that Google is doing more compute than NSA, but that sounds more like a difference of need, not of ability.
I think the query pipeline for NSA (relative to the scales of Google's query pipeline) looks like absense-of-query-pipeline. Hence NSA using less compute and thus (the reasoning goes) less power consumption.
I can assure you that it's not mapping each query down to a single-sector disk read off an inverted index.