Motivation

Shared utility metering is the norm in Malawian multi-tenant housing ("compound housing"), where a single ESCOM prepaid meter serves between two and ten households. Cost apportionment is typically negotiated — per room, per appliance count, or per head — and the resulting disputes are a persistent source of tenant-landlord friction. Certified submeters exist but their per-point cost (US$120–180 installed) exceeds what the rental economics of compound housing can absorb.

The question this work addresses: how accurate and how durable can a submetering point be at a materials cost an order of magnitude lower, and is that accuracy sufficient to displace heuristic apportionment in practice?

System design

Each metering point comprises a split-core current transformer (SCT-013-030) on the tenant's feed, an ESP32 module sampling at 2 kHz, and a 25 F supercapacitor bank providing write-through persistence across outages. Points report over 433 MHz LoRa to a compound-level hub, which handles prepaid credit accounting and tenant display.

Three design decisions proved consequential:

  1. Measurement-side simplicity. True RMS computation on-device over 10-cycle windows, with per-point two-load calibration at install time (kettle + known resistive load). No voltage measurement per point — compound voltage is measured once at the hub and broadcast, trading per-point accuracy for cost.
  2. Outage-first persistence. Credit balances and cumulative energy live in a double-buffered flash journal committed on every 0.01 kWh increment and on brownout interrupt. No observed balance loss across 214 logged outages.
  3. Social legibility. The tenant display shows the same units (kWh and Kwacha) in the same format as ESCOM's own prepaid interface. Early prototypes showing percentages were consistently distrusted.

Results

Against a class-1 reference meter rotated across points monthly:

  • Median absolute energy error: 2.2%; worst point-month: 3.1%.
  • Error was dominated by the shared-voltage assumption during localized voltage sag (long feeder runs within the compound).
  • Calibration drift over six months was below 0.5% for 10 of 12 points; the two outliers correlated with CT cores disturbed during unrelated wiring work.
  • Billing disputes reported by the landlord fell from a baseline of 3.2/month to 0.4/month by study end.

Limitations and the certification gap

This system measures for information, not for legally binding billing — Malawi, like most jurisdictions, requires type-approved meters for revenue metering. In practice, all parties in the study treated the readings as authoritative because the alternative was not a certified meter but a heuristic. This regulatory gap — where the de facto competitor to an uncertified meter is no meter — deserves more attention in appropriate-technology literature than it receives.

Tamper behaviour was mild and creative: one household relocated a freezer to a neighbour's circuit on a rotating schedule. CT placement audits, not electronics, are the countermeasure.

Artifacts

Preprint PDFplaceholder — 18 pages

Hardware design files and firmware are described in the associated project write-up on this site.