For an overview of energy conservation with Vera see Intro Energy Conservation.
A few Z-Wave devices are able to tell Vera exactly how much energy they are using. Then Vera can log the devices' real, live energy usage. But, most Z-Wave devices, like light switches, do not measure energy usage. So, on the 'Devices' tab in Vera's 'Setup' page, you can click the '+' button next to each device and tell Vera how much energy each device uses when it's On by putting its energy usage in the "Watts" field. (Lights default to 100.)
Most devices have the number of watts they use printed on them. For example, a 60W light bulb uses 60 watts. If you put '60' in the "Watts" field, then Vera will assume the device is using 60 watts at full brightness, and a percentage of 60 watts when dimmed to a lower level. Note that you may have a Z-Wave switch that controls several lights. The value you put in the "Watts" field should be the sum of all the lights controlled by a switch. So if a switch controls four 60W overhead lights, in the "Watts" field for that switch, put in 240.
For thermostats, the "Watts" field consists of 3 numbers separated by commas. The default is 1500,1500,300. The first number is how many watts are used when the thermostat is in 'heat' mode; the second is how many watts are used when the thermostat is in 'cool' mode; and the third number is for the 'fan only' mode.