A popular use for mqtt brokers seems to be coupling them with a CurrentCost (or similar) energy monitor to then log energy data and produce pretty (and useful!) graphs.
Google recently opened up their PowerMeter API which looks to provide very nice graphing of energy data. They are working with utility companies directly with in home monitors, but it's also possible to use it as an individual.
Toby Evans got to the bottom of registering a device (see his explanatory blog post) which just leaves getting data to Google.
If you're already logging energy data to an MQTT broker, it's as simple as adding another subscriber to send the data to Google. You could use the mosquitto_sub client and a script I wrote for posting to google, google_powermeter_update_mqtt.pl like so:
mosquitto_sub -t sensors/cc128/ch1 | google_powermeter_update_mqtt.pl
This assumes that the data appearing on the sensors/cc128/ch1 topic is in the
format <unix timestamp> <power reading in Watts>.
If you're not logging your energy data to a broker, you should probably consider doing so :) There is another script google_powermeter_update.pl which may be more suitable and can be used as:
google_powermeter_update.pl <unix timestamp> <power in Watts>
Both of the scripts need your user details adding and should be easy to modify to match your own particular need. They also assume you're using a single cumulative variable with your device and will need modifying if you're using more than one variable or aren't using cumulative variables.
For reference, I use the script cc128.pl to read data from my CurrentCost CC128 (Envi).
Update:
Google has a limit of 6 API requests per hour, so the above description will only work for a short while (the 6 request limit doesn't appear to be a hard limit when you first exceed it, but becomes increasingly stricter). I'm now logging my CC128 data to a mysql database and sending batch updates every 15 minutes with google_powermeter_update_mysql.pl.