While the article, you, and i'm sure more to come keep mentioning the need to "place senors" the reality is any Utility company worth it's salt already has this data logged as part of normal operations through SCADA/DCS systems.
This systems monitor (and log) so many different variables that it forces the companies to store everything in databases for reference & analysis. When it comes to power generation nearly all power generation is done by a "utility" company all of which are heavy government regulated. In a lot of areas it is actually the government which determines bill rates and adjustments to generation capacities (or at least responsible for the play book the operators work by).
It would be far easier and less far fetched to believe that the NSA would have access to theses logs/DBs for what ever use they wanted. Especially with most major power generation sites being covered under FERC regulations and several of the regulation requirements for Reliability requires operators to track and monitor this exact data that the NSA would need.
And trust me when i say that these sites log everything and keep it incase of an Audit. The consequences for failing to be able to provide the data in case of an Audit or Incident Investigation is worse (for the company) than just about any incident would be. They log it, they keep it, even if they will will never look at it again, because the government might come asking for it (and they will give it when asked with no questions as they are required to by regulations).
Honestly going this route i'd say compared to the wiretapping network the NSA has put together, this would be trivial for them to do (not cheap or quick, just not all that difficult).