We are implementing the openenergymonitor transmitter, and were wondering whether there is a convention. Cutting through all the Active Power, Reactive Power, Apparent Power, we should get a figure of plus or minus watts. If the user turns the CT clamp round, this will be reversed.
Is there a convention that Import is positive, and export negative, or the other way round?
We can of course write software to cope with either - and then be consistent - but I was wondering if there is a convention?
I guess that if import meters have been “positive” for more than a century, then “export” would be a negative value?
The Mk2 PV Router continuously monitors the energy state of the premises by means of its “energy bucket”. When energy is generated on site by any means, the numerical value of the “level” in the (virtual) energy bucket increases. Conversely, when energy is consumed by any on-site appliance, the level in the energy bucket decreases. As Robert has noted, the “polarity” of this logic which I have used throughout the Mk2 project is contrary to the normal convention.
The purpose of the Mk2 PV Router is to control one or more dump-loads so that surplus energy can be freely used on site rather than leaking away to the grid. While surplus power is being diverted in this way, small bursts of energy will move to and fro between the premises and the grid in equal amounts. When the system is set up connectly, many types of meter will go into their sleep mode because there will be no net flow of energy at the grid connection point.
For Mk2 sketches that support datalogging, an inversion has been included in the software. The recorded values for average power at relevant locations are thereby aligned with the Open Energy Monitor’s convention that export is negative and import is positive.