RF energy meter

Disclosed is an apparatus having radio-frequency (RF) energy harvester 1 comprising: an antenna 21; a rectifier 22; an energy storage module 23 such as a capacitor or battery, and; a light output 24, together with and optically coupleable to RF energy meter 2. The rectifier 22 generates DC voltage from an RF signal (e.g. in the range 2.4-2.5 GHz) harvested by the antenna 21. The output 24 produces light pulses, each pulse corresponding to a harvested energy amount (in an embodiment ~95µJ), for example via an LED, using the energy stored. An output given at module 13 is indicative of an amount of energy harvested and is based upon a count of light pulses detected at 11, possibly by a photodiode. This output may be an average of counted pulses per unit time. The embodiment may determine the amount of background energy available for harvesting at different locations and/or times.