Наверное  примерно так
from pymodbus.client import ModbusSerialClient as ModbusClient
import json

devices = [
    {"name": "device1", "id": 1},
    {"name": "device2", "id": 2},
    {"name": "device3", "id": 3},
    {"name": "device4", "id": 4},
    {"name": "device5", "id": 5}
]

data = []
errors = []

client = ModbusClient(method="rtu", port="/dev/ttyUSB0", stopbits=1, bytesize=8, parity='N', baudrate=9600)

try:
    connection = client.connect()
    for device in devices:
        try:
            result = client.read_input_registers(0, 10, device["id"])
            voltage = result.registers[0] / 10
            current = (result.registers[1] + (result.registers[2] << 16)) / 1000
            power = (result.registers[3] + (result.registers[4] << 16)) / 10
            energy = (result.registers[5] + (result.registers[6] << 16)) / 10
            frequency = result.registers[7] / 10
            power_factor = result.registers[8] / 100
            data.append({"name": device["name"], "id": device["id"], "voltage": voltage, "current": current, "power": power, "energy": energy, "frequency": frequency, "power_factor": power_factor})
        except:
            errors.append(device["name"])
            data.append({"name": device["name"], "id": device["id"], "voltage": 0, "current": 0, "power": 0, "energy": 0, "frequency": 0, "power_factor": 0})

except:
    print("Failed to connect to Modbus device")

client.close()

with open("data.json", "w") as f:
    json.dump({"data": data, "errors": errors}, f, indent=4)