This topic covers the reported power in the DC system tile as shown on a Venus device (such as CCGX, Cerbo, Ekrano or Raspberry Pi etc.) or on VRM dashboard. The symptoms are typically that the DC power does not match with a calculation using the power shown and the battery, solar and ac tiles and/or that the DC System Power is erratic or jumping about.
To see the DC System tile, it needs “Has DC System” enabled in the System Setup Menu of the Venus device.
If you have a system which does not measure the DC System Power using a SmartShunt or BMV 712 (see later) then this is entirely normal.
In this case the DC load is a calculated value, not a measured value. The Venus device knows the power flowing into or out of the battery accurately from the battery monitor and the assumption is the shunt is wired up correctly, so all DC loads and chargers pass through the shunt. The Venus device also the power from the MPPT reasonably accurately and also knows the DC power into or out of the inverter/charger. The DC power used by the inverter/charger is not a measured value, but an estimate based on the AC power and the inverter/charger efficiency so has some error in it. There may be other loads or chargers in the system with their own reported value (alternator, wind turbine, DC to DC charger etc.). The Venus device then calculates the DC System Power as being what is not accounted for from the data it does have and calls this the DC System Power. What makes this more complex is the different devices may be polled or report their DC power values at different sampling intervals, which can account for the erratic nature of the DC System Power. This also means that the DC load is where all inaccuracies in the other equipment measurements ends up.
One way to get an accurate and more stable reading for the DC System Power is to install a SmartShunt or BMV 712 measuring all DC loads that are not measured. This shunt is then set up as a DC Energy Meter and it MUST be given the energy meter type “DC system”.