Modbus is a communications protocol having nothing whatever to do with network media. Thus, your question is confusing to say the least.
Kinda sounds like you are mixing up your connections. In my experience, flow meters and other devices reporting analog values utilize current loops (4-20ma is common) connected in turn to a device that can convert the value provided into a Modbus message. Such a device could service more than one analog input device. Note the communications from the monitored device is entirely isolated from the Modbus network. Each has a different job to do.
Perhaps you are confusing Modbus with, say, RS-485 or RS-232 which are hardware communication standards. Generally, Modbus is used with RS-485. In turn, RS-485 is a differential signaling hardware protocol that typically uses 5 VDC for connectivity. Assuming (a dangerous thing, I know) that you are referring to something like this, it does not matter what primary power source the communication device uses. It will generate the requisite signal required.