OK, now I got it working finally with the help of rossko57’s post.
My DSL script is now:
var QuantityType<Energy> tot_energy = 20350.27 | kWh
var PV_tot_energy = (FroniusSymoInverter_TotalEnergy.state as QuantityType<Energy>).toBigDecimal + tot_energy.toBigDecimal
Fronoius_PV_total_energy.sendCommand(PV_tot_energy)