Your inverter must be approved by PEA, and the installation also has to be inspected and signed off with the required drawings and paperwork, as Carly mentioned. In the inverter app you should then be able to set the priorities, house load first, battery charging second, and after that export any surplus to PEA. The rules also seem to have changed regarding system size. PEA now allows systems larger than 5 kW for your own consumption, including batteries, while export is limited to 5 kW AC per meter. So a larger inverter should not automatically prevent selling anymore, provided the system is approved and the export is limited correctly. We paid 12,000 baht for the registration and final paperwork with PEA, all handled by the installer.