PV dimming causes grid usage while delivering a bit back is cheaper

Bit of a curious one this, and I'd love to know if others have hit the same wall.

On my narrowboat setup I'm running a Victron Multiplus-II with a Cerbo GX and DESS trying to do its clever thing with time-of-use tariffs. The solar panels feed in through a separate MPPT, and I've got it configured to throttle generation when export prices go negative — which makes perfect sense on paper.

The odd behaviour I keep noticing though: when the system dims the solar output to avoid negative-price export, DESS seems quite happy to pull from the grid at a higher rate rather than just... letting the panels do their job and deliver a small amount back at a modest positive price. It's almost like the algorithm gets tunnel vision about avoiding export without properly weighing up the cost of the alternative.

Has anyone else found DESS making decisions that look logical in isolation but fall apart when you look at the whole picture? I'm wondering whether it's a node configuration issue on my end, or whether the integration between dimming commands and the DESS cost optimisation genuinely doesn't account for this edge case properly.

I've had a poke around the Victron Community forums and found a few threads dancing around this, but nothing that nails it.

A few questions worth chewing on:

• Is this a known limitation of DESS in the UK market?
• Does the SolarEdge integration handle this better with a dedicated inverter versus MPPT?
• Anyone found ESS assistant settings that help DESS make smarter trade-offs here?

Genuinely curious what setups people are running and whether there's a workaround.

@WendyFisher61 this rings a bell from when I was tinkering with my motorhome setup actually. Is your minimum SOC set sensibly in the DESS schedule? I found mine was fighting itself — DESS wanted to export a trickle but the Multiplus-II had a minimum inverter output threshold it couldn't go below, so it pulled a tiny bit from grid to compensate rather than just... stopping.

Worth checking your ESS Assistant settings — specifically the "sustain voltage" and whether "feed-in excess solar" is ticked alongside your grid setpoint. Sometimes having the grid setpoint at 0W but feed-in enabled creates exactly this weird push-pull behaviour at low irradiance.

Also — what firmware version is your Cerbo on? There were some DESS-related fixes in recent VenusOS releases that sorted similar quirks for a few folks on here.

@WendyFisher61 DESS gets a bit too clever for its own good sometimes — check your Grid Point target in the ESS assistant, because if it's set to export a few watts rather than zero, the Multiplus will simultaneously pull from grid and trickle back, which looks absolutely bonkers on paper but is exactly what it's designed to do when PV is wobbling.

Try setting Feedin excess solar and your grid setpoint to 0W first, then nudge from there — my Fogstar lithium bank on the static caravan had identical behaviour until I sorted that.

Also worth checking minimum inverter power in VEConfigure; if PV dips below that threshold, it pulls grid to compensate rather than just reducing output gracefully.

@WendyFisher61 classic DESS nonsense. Your feed-in excess threshold is probably set too low, so the system decides it's cheaper to nick a tiny bit from grid rather than discharge further while simultaneously exporting the dribble of PV. It's "optimising" itself into a state that makes no sense to a human.

Check your ESS assistant — specifically the minimum discharge current and whether you've got prevent feed-in when overvoltage ticked accidentally.

Also worth checking your CT clamp is actually reading correctly. Had this exact headache in my shepherd's hut with a Fogstar Drift and it turned out the clamp was oriented backwards. Victron just quietly does weird things rather than throwing an obvious fault.

What firmware is your Cerbo on?