"the difference between 2012-01-01 and 2013-01-01 is just way beyond what even the biggest Nest hyper claims" Doesn't the temperature chart explain most of the savings? A colder winter and hotter summer for the first year (pre-Nest). The simple total of net degrees by month disguises the fact that the second year is much milder. What you want to look at is KWH from months (days if possible) with nearly identical avg temperatures, come up with composite "savings" for different month types and project that on a year.