How can I combine surplus-only smart charging with my home battery system?

The combination of a battery system with Smappee's smart charging system can enhance your building's self-sufficiency and optimize your usage of locally produced green energy. However, it is important to take some limitations into account. Battery systems usually include software that decides when to charge/discharge the batteries, often based on trying to keep the grid import/export close to zero. However, Smappee's smart charging (especially the 'Surplus only' functionality) also tries to keep the grid import/export close to zero. It is important that these two control systems do not conflict.


Battery systems and solar panels both operate on Direct Current (DC) which must first be converted to Alternating Current (AC) via an inverter. In general, there are two ways a battery can be connected:

  • Hybrid system: the battery and solar panels share the same inverter. This topology is usually used when a battery is being installed at the same time as solar panels.
  • Stand-alone system: the battery has its own inverter, separate from the solar inverter. This topology is usually used when a battery is being retrofitted to an existing solar installation.

The behaviour of our smart charging system depends largely on the type of topology you have.


Hybrid inverter

In a hybrid system, there is only 1 inverter which is shared by the battery and the solar panels. We can only measure AC currents, so we only know the current after the inverter, i.e. the sum of the battery output and solar output. It is impossible for our measurement system to differentiate between current coming from the battery and current coming from the solar panels.




If the Smappee system knows that your inverter is a hybrid (solar + battery) system, we can optimize our surplus-only smart charging. During installation of your energy monitoring system, the installer will be asked whether there is a hybrid inverter. However, it is also possible to change this afterwards:


Setting this flag will ensure that when the 'surplus only' charging mode is used, the EV will only charge once the home battery is full, and will stop charging when the solar panels no longer produce enough electricity. When other charging modes (e.g. Standard Charging) are used, the battery will provide (part of) the power.


How does this work? Usually the 'surplus only' functionality tries to keep the grid power close to zero, but in this case we will allow a small amount (~200W) of export. If the power is coming from the battery, then the battery's control system will see that there is export and it will reduce its output. This will continue until there is no power coming from the battery and the EV will only be charging with surplus power coming from the solar panels and not from the battery. It is important to note that this is only active when using surplus-only smart charging and the 'minimum percentage of surplus' is set to 100%


Note: if you have a hybrid inverter, the measurement will always be the sum of the battery output and solar panel output. This may make it look like the solar panels are producing energy at night, while in fact this energy is coming from the battery. Setting the 'hybrid solar inverter' flag (shown above) will not change this.



  • The Hybrid Battery function will only work with the Smart Charging - Surplus only function with the percentage slider set to 100%
  • The Hybrid Battery function does not function correctly with Smart Charging - Schedules or Standard Charging

Stand-alone (retrofit)

If the battery has its own inverter, separate from the solar inverter, we can measure the battery's output separately. This is highly recommended. To do this, attach CT(s) to the battery inverter and use the Smappee app to configure it as submeter of type 'storage'. By doing this, you will be able to see the power going in/out of the battery at all times, and the calculated consumption will be corrected such that charging the battery will not be seen as consumption.


If you wish to prioritize charging of the EV (i.e. while using 'surplus only' smart charging, surplus energy should first go to the EV instead of the home battery) we suggest to change the physical connections to achieve a topology like the one shown below:


In this case, any surplus energy from the solar panels will be seen as export by Smappee and can be used for smart charging. Only if it is not used by the car charger, it will go into the battery. In any case, if you set the charging station to use 'normal charging', power will be drawn from the battery until it is empty.