Dynamic ESS limits Fronius to 29W when both PV inverters are producing

Right, so I've been down a rabbit hole this evening that I thought was worth sharing because I suspect a few of you might have bumped into something similar.

My setup is single-phase rather than 3-phase, but I've been experimenting with Dynamic ESS on my Victron MultiPlus-II and noticed something peculiar when my Fogstar-paired battery bank is nearly full and prices go negative on Agile.

When both my PV sources are generating — roof panels via a grid-tie inverter and a small Renogy MPPT — the system seemingly throttles one source down to almost nothing. We're talking figures so low they'd barely charge a phone. Pointlessly low.

My theory, and I'd love someone more technically-minded to either confirm or laugh at me, is that Dynamic ESS gets confused about where power is actually coming from and starts issuing bizarre curtailment instructions to avoid grid export during negative pricing. Noble intention, chaotic execution.

The Cerbo GX logs are... not exactly chatty about why it's making these decisions. Classic Victron — brilliant hardware, documentation written by someone who enjoys suffering.

Has anyone else seen weird limiting behaviour when two generation sources are active simultaneously under Dynamic ESS?

Particularly curious whether this is a firmware thing (I'm on 3.75) or a fundamental limitation in how the algorithm handles multiple simultaneous inputs. If it's the latter, that's going to be a problem as more of us add secondary generation sources to our shepherds hut-style setups.

Would genuinely appreciate hearing from anyone running a similar multi-source arrangement. Pull up a virtual chair.

Hey @JubileeClipHero, interesting one! I've seen this crop up before with Dynamic ESS and dual PV inverter setups. The 29W figure is a dead giveaway — that's essentially the system telling the Fronius it wants just enough production to satisfy minimum output requirements whilst the other inverter handles the bulk.

Worth checking your ESS assistant settings in VEConfig — specifically whether the "minimum inverter power" is being calculated across both inverters combined rather than per-unit. On single-phase setups this can get a bit confused when DVCC is involved.

Also, are both Fronius units on the same AC bus, or have you got one on AC-in and one on AC-out? That topology makes a significant difference to how Venus OS interprets available generation. What firmware are you running on the GX device?

Good timing on this thread — ran into something adjacent when I was commissioning the garden office setup last spring.

Worth checking your AC-coupled PV assistant version in VenusOS. There was a known quirk where Dynamic ESS would throttle the Fronius via the frequency-shift mechanism more aggressively than intended when it detected two AC-coupled sources simultaneously — almost like it was treating combined output as potential backfeed risk and being overly cautious.

The 29W figure specifically rings a bell as being suspiciously close to a minimum clamp value in an older firmware revision.

What's your VenusOS version and which Fronius model? The Gen24 range behaves differently to the older Symo under frequency shifting. Also worth checking whether the Fronius is on the same VLAN as your Victron and comms aren't dropping mid-session — intermittent dropouts can trigger the same symptom.

Lived through something painfully similar on the boat last winter. The GX device was essentially throttling the Fronius because Dynamic ESS couldn't reconcile two PV sources feeding into the same AC bus — it kept interpreting the combined output as a potential feed-in overshoot and clamped down hard.

What fixed it for me was setting the Fronius as AC-coupled PV on AC-out properly within the Victron portal and making sure the grid feed-in excess solar was explicitly permitted per inverter rather than globally. The 29W figure is suspiciously specific — almost like it's holding just enough to keep the inverter "alive" without actually exporting.

Worth checking your ESS minimum feed-in power setting too. Sometimes that interacts oddly when you've got two sources being summed. What firmware version is your GX device running? That narrows it down considerably.

Really interesting thread this one. @JubileeClipHero, one thing worth checking is whether your GX device is correctly identifying both inverters as separate AC-coupled sources. I've seen situations where the system lumps them together oddly and the Dynamic ESS algorithm gets confused about available generation, then clamps output to something daft like that 29W figure as a kind of failsafe.

Have a look in the VRM portal under Advanced → Device List and confirm both inverters are showing distinct entries with sensible power readings. Also worth checking your ESS minimum SOC settings haven't crept up — if the battery is hovering near that threshold the system gets very conservative very quickly.

What firmware are you running on the GX? There were some notable fixes around AC-coupled PV handling in recent Venus OS releases that might be relevant here.