Measuring Energy in Cellular IoT Devices

Module datasheets publish current per operating mode. Those figures are accurate, and they are not the numbers the decision needs.

Transmit current assumes a link condition that is never stated, because uplink power is set by the network through closed-loop power control. NB-IoT and Cat-M1 extend coverage by repeating transmissions, so the energy cost of a message is a property of the message and the link condition together, not of the message alone. For small payloads, connection establishment and the release tail can cost more than the payload itself. And sleep floors, which dominate the budget for a device reporting once an hour, span five orders of magnitude against transmit bursts.

A three-month field study of thirty NB-IoT nodes across more than twelve hundred locations found energy consumption imbalanced by up to 75 to 1 between nodes running identical workloads. The variable was coverage, not code.

Cellular energy metrology measures module current against a base station emulator. The network is controlled; there is no compute workload.

Edge inference metrology measures per-inference energy using shunt isolation and GPIO triggers. The compute is rigorous; there is no network.

Live-network measurement captures both, but the measuring party controls neither the propagation environment nor the operator's power-saving timers. A result from Tuesday afternoon cannot be reproduced on Wednesday morning, or by anyone else anywhere else.

1. 1A conducted RF chain. Coaxial connection through calibrated attenuators, no antenna fitted. This makes coverage a commanded variable rather than an observed condition, which is what makes a result reproducible by a third party.
2. 2A controlled radio access network. PSM timers, eDRX cycle and paging time window, and connected-mode DRX become settings rather than inheritances.
3. 3Dual-path current measurement. No single instrument spans a sub-microamp sleep floor and a transmit burst of hundreds of milliamps. Eight-bit quantisation on the dynamic path cannot represent a microamp floor at all.
4. 4Phase-resolved attribution. Energy decomposed into wake, pre-processing, inference, connection establishment, uplink, downlink, release tail and return to sleep, so a result says where the energy went and not merely how much.

• At what reporting interval local processing beats offloading
• How that boundary moves as coverage degrades
• Whether model precision shifts it by more than measurement uncertainty
• Whether the power-saving configuration changes which strategy wins
• Whether coincident transmit and compute bursts pull the supply below the module's operating margin

The brief states its own limits in a dedicated section. No real propagation. No operator-specific network behaviour. No cross-vendor generalisation from a single module. No substitute for field trials.

A method described without its limits is marketing.