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.

Phase use

If you use meters that record the current in phases, the Charging Manager (especially for cars charging in one or two phases) can control the charging power in phases and thus optimise it. In the case of meters that only output a power value related to the number of phases used, the cFos Charging Manager allocates the power equally to the phases, which leads to certain inaccuracies. In this case (and also in the case of sudden heavy consumption), you should set a sufficiently high power reserve. If you know which phases are actually used, you can configure this for the meter or the EVSE using the 'Phases' parameter. Automatic phase detection: If you set the 'Phases' parameter to 'Determine' for a device without phase resolution, the Charging Manager will attempt to determine the phase usage per transation itself. If, for example, a meter is attached to the EVSE, it takes over the phase usage of this meter. If the power is clearly too high in relation to the number of phases, it corrects the phase usage from L1 to L1+L2, or L1 to L1+L2+L3 or L1+L2 to L1+L2+L3. If no further information is available, the cFos Charging Manager initially assumes 1-phase charging. The value determined in this way is used during the charging process and deleted again after the vehicle is unplugged. In the case of excess charging, this can lead to up to two short charging attempts with a three-phase charging car until the Charging Manager has determined the correct number of phases. The goal here is to provide the highest possible functionality even without phase-accurate meters when cars with different phase usage are charged at one EVSE.

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 (the meter shows positive power values) is not considered as a consumer because the storage stops charging immediately when the charging power is used for charging the electric car. So, for meters with the role "solar storage", the cFos Charging Manager ignores its consumption.

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.