IDM — 47 intervals in every packet
The only message on the band that carries history rather than a single reading. If any meter at your address speaks IDM, that is the one to work on.
IDM — the Interval Data Message — is the most informative thing an RTL-SDR will hear on 902–928 MHz, and it is structurally different from everything else in the ERT family.
Every other message type carries one cumulative number. IDM carries differential consumption across 47 recent intervals of five minutes each — roughly four hours of five-minute-resolution history, in a single packet.
Properties
| Property | Value |
|---|---|
| Band | 902–928 MHz |
| Utilities | Electric |
| Intervals per packet | 47 |
| Interval length | 5 min |
| History per packet | ≈ 4 hours |
| Payload style | Differential — per-interval consumption |
| Encryption | None |
| Decoders | rtlamr |
rtlamr -msgtype=idm -filterid=YOUR_METER_ID
Why the differential payload changes everything
This is the practical consequence and it is easy to underrate. With SCM, a packet you fail to decode is a sample you have lost permanently — the meter never resends it and nothing later fills it in.
With IDM, every packet contains the last four hours. Miss one, miss twenty in a row, lose an afternoon to a power cut: the next successful decode backfills all of it. You can run a marginal receive path and still end up with a complete series, which is a completely different engineering problem.
The second consequence is resolution. Five minutes is fine enough to see individual appliances in the trace rather than just a daily total:
- A well pump or sump pump cycling on and off
- An electric water heater's recovery after a shower
- An oven's thermostat duty cycle
- An EV starting and stopping a charge session
- Air conditioning compressor cycles, and how they lengthen with outdoor temperature
None of that is visible in a cumulative reading, and none of it is visible in a gas or water meter broadcast at any poll rate.
The catch
Only electric meters send IDM, and many electric meters do not send anything you can decode at all. The modern alternative is a proprietary two-way mesh, often encrypted, which produces energy in a spectrum sweep and nothing in a decoder.
So IDM is simultaneously the best outcome available and the least reliable one to plan on. Check before investing effort: look up your meter's FCC ID, or read the electric meter guide for what the faceplate tells you.
A clamp-on CT sensor at your panel gives sub-second resolution, per-circuit detail, and no dependence on the utility whatsoever — better data than IDM on every axis except agreement with your bill. That comparison is laid out in full on the electric meter page. It is not a consolation prize.
Next
Where this leads.