Hi @leiv - I’m not an expert in Modbus, but a read of the linked documentation suggests the issue lies in the addressing. The Solax is arranged in pages, so I think you need to include the page as part of the address. Shift the page address, 0x04 here, left by 8 bits, followed by the register. For example (decimals that you’ll need for the interfacer configuration, hex first as it’s easier to deal with the shifts 
):
- GridVoltage @ 0 →
0x400(1024) - GridCurrent @ 1 →
0x401(1025) - GridPower @ 2 →
0x402(1026)
This is from a very quick skim of the minimalmodbus module, I think it’s little Endian so sends the lower byte first. The relevant diagram in the Solax documentation is here:
Give that a try and let us know.