Surplus charging

With the cFos Charging Manager and a solar system, you can make sure that your car is (almost (*)) only charged when solar surplus power is available. Surplus = generation minus household consumption.

Note: In order for charging rules to be observed, load management must be active, i.e. the mode must be set to "load distribution", not "observe".

You can set up a "Solar" type charging rule for this. Select "Solar surplus" as the mode. As "Start current limit", select the surplus current that the solar system is to generate from which this rule is to apply.

Screenshot cFos Charging Manager Documentation - Surplus charging

The rule: Solar, start current limit: 6.5 A(6500 mA), "solar surplus", this activates surplus charging with the available surplus from a feed-in of 6.5 A.

In addition, you can specify an undercut time, i.e. the number of seconds that the current limit may be undercut after activating the surplus rule before charging is stopped. In this way, charging is not interrupted in the event of short-term power drops, e.g. due to passing clouds.

You can read the solar surplus by setting up the virtual meter "Surplus (VM)" in the Charging Manager. Furthermore, you can set up a virtual meter "Grid Demand (VM)" that shows how much electricity is currently being drawn from the grid.

Tip: If your PV system no longer generates the minimum power required for charging in winter or in the transitional period, you can also specify a value below 6000mA as the start current limit. In this case, charging is carried out with solar surplus and partial use of the grid.

Tip: To ensure that your car is charged the next morning, you can specify a time-based rule in addition to the surplus rule: Time, Start: 21:00, End 6:00, Current 6000. I.e. if at night the car still needs electricity to be fully charged, you can either charge from the grid or from a storage.

The surplus electricity is the electricity that would be fed into the grid. To determine this, the cFos Charging Manager must be able to measure it. The following options are available for this:

Illustration of measurement concepts
  1. Either: Set up with a "mains supply" meter. You install a (bidirectional) meter at the transfer point of the house connection. If this meter shows negative values, you feed in and this electricity is available for surplus charging. Suitable meters are, for example, Modbus meters or internal grid reference meters of your solar system (e.g. SMA Homemanager 2.0, Kostal Power Meter, E3/DC grid reference meter, etc.). In order for the Charging Manager to be able to calculate the grid reference independently of the charging of the electric cars, a meter with the role "consumption electric car" must be configured for each EVSE in this configuration.
  2. Or: You measure your household consumption with a suitable meter without EVSE(s) and without the generation power. Simple S0 meters are sufficient for this. The Charging Manager then subtracts the household consumption from the generation power and makes the remaining power available for charging.

The generation power can be measured with an extra meter. Alternatively, you may be able to read the values from your solar system directly. Please refer to our list of currently supported devices.

(*) Due to measurement and calculation inaccuracies, a slight grid draw or feed-in may occur in the border area.

User report on surplus loading with Shelly 3EM (PDF)
User report on surplus charging with Solaredge

Balance surplus charging

If you know that the solar system can generate 4.2 kW of power or less, excess charging must be configured with a workaround. In this case, "balanced charging" is applied.

Note: Electric cars need at least 1.4 kW (i.e. 6 A) per phase to be able to charge. For three-phase charging, this results in 3*1.4 kW = 4.2 kW.

Below 4.2 kW solar power, you have to redistribute the power for charging from the three phases to one phase so that at least 1.4 kW is available on this phase. For example, if you feed 500 W of solar power on all phases, you can draw 1500 W on a single phase. Since the two-direction meters of the energy suppliers work on a balance sheet basis, mathematically there is neither grid draw nor feed-in.

Below 4.2 kW, you must therefore switch off two fuses with which the supply lines to the EVSE are protected (only not the one with which the cFos Power Brain Controller is protected). Caution: However, you must not switch individual phases on or off during the charging process. This can destroy the car's charging equipment! If you are not using a meter at the EVSE that can resolve individual phases, you should, if possible, inform the Charging Manager that the car is now charging in single phase by setting the configuration parameter "Phases" accordingly. With a meter that can resolve individual phases, you can leave the phase setting at "determine".

If you want to charge the car again later independently of the solar surplus, switch on the switched-off fuses before the charging process and deactivate the solar surplus charging rule in the Charging Manager. Then you can charge with the normal power.

Setting the "Phases" parameter

The cFos Charging Manager needs to know on which phases the car is charging (e.g. 1-phase or 3-phase), in order to

  • to be able to calculate the utilisation of the individual phases for load management, and
  • to decide when the minimum charging current per used phase is reached and charging is activated in case of solar excess charging.
If you have a meter attached to your wallbox, it is best to set it so that the phases used correspond to those of the electrical installation. With a wallbox that has a phase-resolving meter or to which such a meter is attached, you can set the phases to "Determine".

If the wallbox does not have a (possibly attached) meter with phase-by-phase resolution (e.g. cFos Power Brain with S0 meter, Tesla Wall Connector, etc.) and you always charge the same car, then you should set the phases to the way the car actually uses them. If you charge alternating cars, set the phases of the wallbox to "Determine". With "Determine", the cFos Charging Manager tries to determine the phases actually used. It usually knows whether the respective meter or the wallbox offers phase-by-phase resolution of the currents. E.g.: an S0 meter only provides pulses per kWh of total power and no values for the individual phases. Such a meter therefore measures the total power and allocates it to the set phases. From the wallbox's point of view, 1-phase charging cars always use phase L1 (even if this is installed phase-rotated with respect to the house grid). two-phase charging cars always charge on phases L1 and L2.

The "Phases" parameter set for the wallbox is also used to predict the phases actually used when a car is newly plugged in. When "Determine" is selected, the Charging Manager first assumes 1-phase charging and switches to 2-phase or 3-phase charging if necessary. After unplugging, the prediction starts again at 1-phase.

In the case of solar surplus charging of 3-phase charging cars, this has the consequence that the cFos Charging Manager starts charging too early with the forecast "1-phase" when the solar power increases. It then detects 3-phase use after a few seconds and pauses charging if the solar surplus is too low until enough power is available for surplus charging. For this to work, in the first 2 minutes of charging, the undercut time (which you can set in the charging rules) is limited to exactly 15 seconds. If you want to avoid this brief switching on, you can set the phases in the wallbox settings to the phases that are actually used. If the cars change, it is advisable to create RFIDs in the Charging Manager with the function "Overwrite phases" and use them to tell the Charging Manager which phases the car is charging with when the charging cable is plugged in.

Power reserve

The control in a power storage always tries to minimize the grid consumption and the grid injection. You can tell this to the cFos Charging Manager by setting up a meter with the role "solar storage". In this case, a discharging storage is considered a generator (the meter shows negative power values), which means that this energy is available to charge the car. However, a charging storage unit (the meter shows positive power values) is not considered a consumer because the storage unit stops charging immediately when the charging power is used for charging the electric car. The cFos Charging Manager therefore ignores the consumption of meters with the role "solar storage". You can choose between "Storage All" and "Storage Home" for the role of the storage meter. With "Storage Home", the discharge of the storage is not used for surplus charging, which avoids deep charging cycles of the storage. Here are some more options for using the meter.

If your electricity storage system does not have a bidirectional meter, you can install an external meter. Mostly, bidirectional Modbus meters are recommended, but it may also work with several S0 meters (or other unidirectional meters) depending on the arrangement of the meters in the installation, which has to be checked in each individual case.