I think you're right about home automation. There is also potential for home analytics/intelligence as well. For example, water sub-meters on each water faucet could help conscientious water users conserve.
Couldn't you calculate this simply by monitoring the "run time" of the devices using the gas? If we can tell how long the furnace and/or water heater run, then you can do the math vs. the gas bill?
Now that's a good idea! I could tell whether our central heating boiler is running by its electricity consumption (which will either be on/off) and measure what it actually consumes when it is on.
Kind of obvious once you mention it - as all the best ideas are!
I've thought about it quite a bit, it wasn't off the top of my head; I would also like some sort of usage monitoring, but I'm not eager to tinker with gas lines or the insides of a furnace.
