The first month after a charger goes in, the electricity bill answers a question you didn't ask. It went up — by about what you'd expect, or by more — and nothing in the house can tell you how much of that was the car. The charger's own app, if it has one, reports what the charger thinks it delivered; your supplier reports the whole house; and between the two there is no figure for what the car took, at the price you paid for it at that hour.
That figure is one clamp away. The complication is that a charger is usually on a garage board, and the garage is often not where the hub is.
What you need:
- an RBgrid hub on your home Wi-Fi, and a tariff entered — without one you get energy but no cost;
- the charger's circuit reachable by an rbAmp module, at the board it is fed from;
- for a detached garage the hub's I²C cable can't reach, a remote node: a second ESP32 with its own rbAmp modules, reporting by radio. On v1.1.7, up to 6 nodes per hub, 4 inputs each (Remote nodes).
Safety. For any work inside an electrical panel, call a qualified electrician.
1. 🔑 Meter the sub-panel or the charger — not both
Read this before you buy anything, because a detached garage makes both look sensible and the pair is the one combination that spoils a figure.
- Your house total is safe either way. System totals are taken at the grid boundary — the supply point only. Load points are never summed into it, so adding the charger as its own point does not inflate the house total or double-count a single watt-hour of it.
- The "unmetered" figure is what breaks. It is the input minus the sum of all your line points, and node points are lines like any other. Meter the garage sub-panel and the charger inside it, and the charger's energy is subtracted twice: the remainder reads too low and can go negative.
So pick one. Which one depends on the question you actually have:
| You want to know | Meter |
|---|---|
| what the car took | the charger's circuit |
| what the whole garage costs — charger, lighting, freezer, workshop | the sub-panel |
| both, properly | the sub-panel and the charger, and accept that "unmetered" is now meaningless — or use two of your twelve history points and meter the garage's other circuits individually instead |
Most people want the first line. The car is the thing that changed.
⚠️ There is no hierarchy of points. Every point in RBgrid is flat and independent: the hub does not know your charger sits behind a panel you also measure, and nothing warns you. The arithmetic above is yours to get right.
The same arrangement describes a second home. A sub-panel with its own loads, often in a separate building, is as good a description of an annexe, a garden flat or an ADU as it is of a garage — and metering that sub-panel tells you what the unit used, per 15-minute period and per tariff zone, instead of leaving it to be guessed at. Two things it isn't. It isn't a utility meter: your supplier bills the single meter at the boundary, and nothing here changes that, or who is liable for it. And it isn't a basis for charging anyone — what should pass between you and whoever lives there is a question about your tenancy and your local law, not about a measurement. What you get is the figure.
2. What to buy for a node
🔑 A node records energy only if one of its rbAmp modules measures voltage. With current-only modules it reports live current and closes no energy records — so you would watch the car charging and never learn the kilowatt-hours or the cost, which is the entire point here. Buy a single-phase rbAmp set with voltage measurement.
- An original ESP32 — 4 MB flash, no PSRAM. S2, S3 and C3 are not built or tested, and an S3 is what most people have spare.
- No over-the-air update: flashed over USB, so leave it reachable with a laptop.
- On v1.1.7 a node is single-phase only, with no phase grouping. A three-phase charger is not something a node covers today — at the board, the hub meters three phases properly, including per-phase. If your charger is three-phase and in a detached garage, meter it at a board the hub can reach.
3. Will the radio reach the garage?
You find out in ten minutes, before anything is mounted: pair the node on the bench, carry it to where it will live, and read the link while the car is actually charging. A charger at full current is the honest test — and for this install it is easy to arrange, which not every load allows.
The procedure and the three numbers it gives you are in pairing a remote node, with the fallbacks in the order people try them. The one to consider first here: if the charger's circuit starts at a board the hub can already reach, measure it there — a node is for the board you can't reach, not for every charger.
4. Give it a point, and give it the right role
In Management → Points, bind the channel as its own point, name it before anything else sees it, and set its Role to EV charging.
The role is not decoration: the standby-load card takes EV charging and heating lines out of the house's baseline before it estimates what is always on, because a car that charges at 02:00 is not standby. Set it wrong and your standby figure quietly includes the car. The app also shows a charging Status for such a point, derived from the point's own power — the units and points documentation has the detail.
One caveat worth knowing if you send this to Home Assistant: the role doesn't travel. In Home Assistant the charger's point looks like any other point, so whatever you want to do with the role, do it on the RBgrid side.
5. What you get to see
- Live, every 3 seconds — power while the car charges. A charge is close to a plateau: ramp up, a long flat stretch, stop. The live view tells you immediately whether the car is drawing now and at roughly what rate.
- Energy, per 15-minute period, kept as history — so the shape of a charge is visible afterwards, and the energy of a whole session is the sum of its periods.
- Cost per tariff zone. Each 15-minute period is priced at the zone in effect for it. If your charging crosses a zone boundary — most overnight charging does — the cost follows the boundary rather than averaging over it. Monitoring → Economics gives the charger's energy and cost for an hour, today, yesterday, the last 7 days, a month or a year.
⚠️ There is no per-session record. RBgrid does not know a "charge" began or ended, and keeps no session list, no per-session kWh and no start or stop event. What it keeps is energy per 15-minute period, for ever; a session is something you read off those periods, or add up yourself. If you need a session log, your charger's own app is where it lives.
And the figures will not match your charger's app exactly. That is expected rather than wrong: the two measure different things in different places — one reports what it delivered, the other measures what the circuit drew.
What this doesn't do
- RBgrid switches nothing. No relay, no outputs. It will not start or stop a charge, will not move charging into a cheap zone, and will not do load management. People who act on these figures do it in Home Assistant, against their own charger.
- It makes no claim about savings. You get what the car took and what it cost at your rates, per period and per zone.
- No per-session record, no start/stop events — see §5.
- It can't take a shared circuit apart afterwards. One clamp on the sub-panel gives you the garage, not the car; the separation happens at the clamp.
- On v1.1.7, history covers up to 12 measurement points, and a node's points come out of that same pool rather than adding to it (which loads are worth a history slot).
- No alerts. Nothing watches the pattern or tells you a charge finished.
Next
- Pairing a remote node, and reading the link — the ten-minute procedure
- Remote nodes — requirements, flashing, pairing, field by field
- Your supplier's day and night prices, applied to every circuit — if the cost column is empty, this is why
- What the heat pump and the EV cost, per phase and per zone — the same two loads on a three-phase supply, metered at the board
- Questions → Support