FrSKY 40A current sensor

This sensor (cost about A$27) enables current, voltage and consumed mAh telemetry. This is one of the most useful modules for flying powered planes because it means you don't need to keep landing to check the battery level.

It plugs in-line between the battery and ESC and connects to the receiver via the Smart Port.

This video covers connection and setup using OpenTX companion, Taranis Plus transmitter and OpenTX 2.0. On-screen display and spoken voltage and mAh consumption values. Viewing of recorded logs in OpenTX companion.