Handle energyConsumedToDate counters that decrease

[cammoore]

Reported by rbrewer, Sep 15, 2012
When retrieving data from an eGauge meter, it is possible to have an energy odometer value that is positive but decreasing. This violates WattDepot's assumption that odometers increase monotonically, and prevents all energy calculations from succeeding.

To handle this unusual case, the eGauge sensor could have a configuration parameter that causes the received meter data to be modified before storing in WattDepot. The sensor would retrieve the latest SensorData from WattDepot, extract the (positive) energyConsumedToDate value to use as a starting point. Then the odometer value from the meter would be subtracted from the starting point to determine how much energy has been used since the last sample, and that difference would be added to the starting point and stored as energyConsumedToDate in a new SensorData to be sent to the WattDepot server. This will preserve the monotenacity of the energyConsumedToDate counter, but will mean that the energyConsumedToDate will not match the actual odometer value from the meter, which is something that we have tried to preserve in the past. The sensor will need to remember the last energyConsumedToDate value to difference with each new incoming value.

Note that when the Source has no SensorData (first meter data), the starting odometer value can be whatever the meter value is.