Where do evcharger Session/Cost / Session/SavedCost and the "Guest EV" vehicle name come from?

Been scratching my head over something similar on my boat setup, and I reckon this deserves a proper thread.

Running a Cerbo GX with VenusOS (recently updated to the latest stable), and I've been poking around with EV charging integration — partly out of curiosity, partly because I'm planning a shore-power hybrid setup for the marina berth. When I pulled up the MQTT data and started digging through the dbus paths, I noticed Session/Cost, Session/SavedCost, and this mysterious "Guest EV" vehicle label appearing in the console.

Now, I've got no wallbox physically connected at the moment — I was just experimenting with how Victron handles EV sessions conceptually. But those values were there, populated with zeros, as if the system was anticipating something.

My questions for the collective brain here:

• Where exactly does VenusOS pull the Session/Cost and Session/SavedCost figures from? Is it calculating against your configured energy tariff, or does it need an external integration (like an openWB or similar OCPP-compliant charger) to feed those in?
• The "Guest EV" name — is that purely a VenusOS default placeholder, or does it get overwritten when a proper charger handshake occurs?

I'm wondering if Victron built this out anticipating OCPP integration becoming more mainstream in off-grid/hybrid installs. Fogstar and others pushing into the EV battery repurposing space makes me think this whole ecosystem is going to get messier before it gets cleaner.

Anyone actually got a working wallbox feeding live session data into their Cerbo? Would love to know how the cost fields behave in practice.

@OldSailor86 the Session/Cost and Session/SavedCost values are pulled from your EVCS device (usually via Modbus or the internal VenusOS driver), calculated against whatever tariff rate you've set in the GX device settings — and "Guest EV" is literally just the default vehicle name Venus assigns when no custom name has been pushed via MQTT or the portal. Change it under Settings → EV Charging Station on your Cerbo, or shove a custom name down the MQTT topic W/<VRM_ID>/evcharger/<instance>/Ac/Name if you're feeling adventurous. Mine just says "Floating Money Pit" which is arguably more accurate anyway.

@OldSailor86 right, so the "Guest EV" vehicle name — that's your EVCS firmware's default label when no RFID card or app session is authenticated. Basically your charger sees an unknown vehicle rocking up and just calls it "Guest EV" like a suspicious pub landlord writing "Anonymous" in the visitors book.

If

@OldSailor86 worth noting that on the Victron side, those Session/Cost values only populate correctly if your EVCS has its tariff rate configured — if it's left at zero you'll just see null or zeroes in the dbus path regardless of what VenusOS is doing. Check /Settings/CGwacs/AcPowerSetPoint isn't interfering if you're running ESS alongside it. On my cabin build I had a similar ghost-value situation where the DBUS object simply wasn't being written by the charger firmware at all — confirmed it via dbus-spy on the Cerbo directly. The "Guest EV" label specifically is hardcoded fallback behaviour in most OCPP-lite implementations when no user token is presented, so you won't shift it without either flashing the EVCS firmware or pushing a custom vehicle profile via the manufacturer's app if supported.

@CamperGeek is onto something important there — worth being precise though: it's not just that the EVCS has tariff config, it needs to have the correct tariff actively set on the charger itself, not just in the VRM portal. Two different things.

On my boat I had this exact confusion with a DEFA charger — the portal showed £0.00 for Session/Cost for ages because I'd set the rate in VRM but never in the charger's own interface.

Also worth noting: Session/SavedCost specifically compares against your configured "petrol equivalent" rate, so if that baseline figure isn't set in the EVCS firmware, that value will stay null regardless of anything you do on the Victron side.