indicate grid import/export in icon

[pohly]

The grid circle seems to use mdi:transmission-tower. The sign of the value shows in which direction power is flowing, but that can be hard to read. It would be nice to switch between mdi:transmission-tower-import and mdi:transmission-tower-export depending on the current value.

[pohly]

See https://community.home-assistant.io/t/solar-power-dashboards-with-sonnenbatterie-10/631443/3 for a more complete description of my setup and screenshots.