How to use the WattNode Modbus registers for a PV or wind power application.

For many power and energy monitoring applications, two Modbus commands are sufficient to acquire basic readings from the WattNode®.

The first command reads registers 1201-1221 which include:

  • PowerA to PowerC: the instantaneous power readings.
  • VoltA to VoltC: the line-to-neutral voltage readings.

The second command reads registers 1301-1322 and includes the following:

  • The EnergyA to EnergyC new energy registers (can be positive or negative depending on whether more energy went in than out through the corresponding CT).
  • The EnergyPosA to EnergyPosC registers, which only include the positive energy.
  • The EnergyNegA to EnergyNegC registers, which only include the negative energy.

The WNB-Pulse-Option-PV.pdf application note shows the different wiring possibilities for PV systems using Modbus as well as Pulse WattNodes. The figures in the app note show a WNB-3Y-208-P Pulse WattNode but for PV applications, you can also think of it as a WNC-3Y-208-MB (i.e. a Modbus model).

Figure 1 on page 3 of the application note shows the wiring arrangement for a single-phase PV inverter system.

Figure 2 on page 4 shows the wiring arrangement for a two-phase PV inverter system.

For a two-phase PV inverter system as in Fig. 2, the EnergyPosA and EnergyPosB registers will tell you how much energy was consumed from the grid. The EnergyNegA and EnergyNegB registers will tell you how much energy was fed back into the grid from the PV system. The net energies reported via the EnergyA and EnergyB registers are just the corresponding differences. If the sum of EnergyA and EnergyB is negative, then the utility received more energy than it supplied and they owe you a credit. If the sum is positive, then the building consumed more energy than the PV system produced and you owe them for that much.

To know how much total energy the PV system produced (some of which the building consumed and the rest fed back into the grid) you would read the EnergyPosC register. For a single phase PV inverter, the only difference compared to the above register usage is that the EnergyA register will always be positive and the EnergyNegA register will always be zero.

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