Nope, it's not worth it. And this data does not prove it is.
I need to know everything about the conditions prior to installing a Nest. In particular, I need to know about the thermostat and, if programmable, how it was programmed. I also need to know about living patterns. Did anything change? Was the programmed temperature higher in the summer and lower in the winter with Nest or was it set to about the same conditions?
In terms of energy efficiency. How is the house insulated? Can you add insulation to the walls or attic? Are the windows double-insulated? Do you use a whole house fan in the summer to cool the house at night? Do you have a multiple zone system so you don't have to heat or cool the entire house when you spend all day in, let's say, an office or the kitchen? Is it a two story home? Do you push hot air down in the winter in order to equalize the temperature? Do you have attic ventilators?
I see Nest as more of a neat gizmo with lots of marketing behind it than a real solution to a real problem. To be sure, the real solution requires a lot more work than bolting a piece of slick looking electronics to a wall. The payoff from a real solution would almost certainly be far greater and it doesn't have to cost a lot of money. For example, I use a $200 fan to cool my home at night (taking advantage of my local conditions). We have probably used the air conditioner a grand total of four weeks in the last three years. The savings in three years are already in the thousands and the investment was $200.
Like most engineers/geeks/hackers, you've completely missed the point of the Nest. The point is: people like to adjust their thermostat. They're too hot? They want to cool it down. Too cold? Turn it up. The Nest allows one to perform those actions while retaining some level of efficiency, and additionally attempts remaining possible optimizations (e.g. turning the A/C down while you're not at home).
The Nest is not about ideal efficiency -- it's about a system that works with the way people already behave.
If you're comparing against an ideal savings regime, I imagine it's not worth it. However, in my case, I was always putting off figuring out how to program my thermostats, deciding it was too much trouble to adjust them, etc. Against that, I expect the Nest to pay off fairly quickly.
But given that the average contract developer is charging themselves out at $100/hour investing the time to "program a thermostat" will rarely result in a ROI.
This has got to be one of the most impressive uses of "My time is worth too much to do X" I've ever seen.
It took me like 10 minutes to program my schedule into my cheap honeywell programmable thermostat. A leisurely 10 minutes. I can't speak to my ROI on the endeavor, but since the thermostat was ~$25 I can only assume I'm rocketing toward breaking even.
my hat is off to you if you are able to sell all the 16 hours that you're awake every day to someone for 100$ each without dropping in productivity and maintaining mental well beeing.
I need to know everything about the conditions prior to installing a Nest. In particular, I need to know about the thermostat and, if programmable, how it was programmed. I also need to know about living patterns. Did anything change? Was the programmed temperature higher in the summer and lower in the winter with Nest or was it set to about the same conditions?
In terms of energy efficiency. How is the house insulated? Can you add insulation to the walls or attic? Are the windows double-insulated? Do you use a whole house fan in the summer to cool the house at night? Do you have a multiple zone system so you don't have to heat or cool the entire house when you spend all day in, let's say, an office or the kitchen? Is it a two story home? Do you push hot air down in the winter in order to equalize the temperature? Do you have attic ventilators?
I see Nest as more of a neat gizmo with lots of marketing behind it than a real solution to a real problem. To be sure, the real solution requires a lot more work than bolting a piece of slick looking electronics to a wall. The payoff from a real solution would almost certainly be far greater and it doesn't have to cost a lot of money. For example, I use a $200 fan to cool my home at night (taking advantage of my local conditions). We have probably used the air conditioner a grand total of four weeks in the last three years. The savings in three years are already in the thousands and the investment was $200.