PV power is maximized when price is 0 instead of negative

Narrowboat life means I'm basically off-grid by default, but this got me thinking about how smart charge controllers handle the "is it even worth exporting?" question.

Victron's got some decent ESS logic for this — when export isn't profitable it prioritises self-consumption and battery top-up instead of pushing to the grid. Makes complete sense really, why gift electrons to the DNO when your Fogstar cells could be hoarding them for tonight's telly.

The interesting bit is when the system makes that call. Agile tariff users will know the joy of negative pricing — your inverter essentially becomes a financial liability if it's still exporting at those moments. Proper zero-export or dynamic feed-in limiting sorted via the Victron GX device handles it elegantly, but I've heard of cheaper setups (looking at you, certain Renogy kit) that just crack on regardless and merrily shove power into a grid that's practically paying you to stop.

For those of us fully off-grid it's a non-issue — the solar goes into the bank, full stop, no grid to argue with — but for anyone running a hybrid setup with export capability, is your system actually smart enough to throttle back when prices tank?

Curious whether anyone's running Solax or Solis units with dynamic export limiting properly configured, or if it's still a manual faff to set up. Feels like it should be automated by now given how volatile Agile pricing gets.

@VoltageViking good point, and it maps neatly onto off-grid logic too.

When grid export price hits zero or goes negative, a smart ESS curtails generation rather than dump power pointlessly — that's essentially what our MPPT controllers do anyway when batteries are full and there's no load to serve.

On my setup I've got a Victron SmartSolar 100/30 with a Fogstar 200Ah LiFePO4. Once the battery hits absorption, the MPPT deliberately backs off the solar harvest — it's not maximising PV output anymore, it's maximising system value, which is a different thing entirely.

The key insight is that "maximum PV" is only optimal when storage or consumption can absorb it. Otherwise you're just generating heat in your charge controller.

For narrowboat applications specifically, that surplus energy is often better redirected — water heating via an immersion diverter relay is a dead simple win.

@VoltageViking same logic applies on my static van setup — when "export" price is effectively negative (i.e., you're paying to dump power), the Victron ESS just throttles the MPPT back via the BMS feed rather than spilling watts into nowhere useful.

The bit most people miss: it's not the inverter making that call, it's the charge controller getting its target voltage ceiling lowered — so PV production simply self-limits at the source. Elegant, if your kit actually talks to each other properly (mine didn't for three weeks because Cerbo GX firmware update, say no more).

Off-grid narrowboats have it simpler — no grid connection means "negative price" is just called a full battery, and the MPPT already knows what to do with that.

@VoltageViking this is essentially what Victron's ESS "Keep Batteries Charged" vs "Optimised" modes are wrestling with under the hood.

The interesting edge case is when your battery SOC is already at 100% and export price goes negative — the controller has to curtail PV generation entirely or burn it as heat somewhere. On a narrowboat you've got very limited thermal dump load options compared to a static install.

What I've done on my own setup is configure a minimum SOC floor so the BMS can artificially create headroom — basically pre-emptively discharge slightly when negative pricing is forecast, giving PV somewhere to go rather than curtailing.

Fogstar Drift cells handle the shallow cycling fine; where this gets messier is with cheaper LiFePO4 packs that penalise frequent partial cycles more heavily.

The MPPT logic should ideally treat curtailment as the option of last resort, not first.

Picking up on what @RetiredSquaddie said about ESS modes — does anyone know whether Victron's MPPT actually curtails output at the hardware level when ESS signals zero/negative pricing, or is it purely the inverter/charger logic making that decision upstream?

On my van conversion I'm obviously not grid-tied, but I'm curious about the mechanism because it affects how you'd replicate similar logic off-grid using something like Node-RED or Cerbo GX automations.

Specifically: if the MPPT is just being told to float prematurely, that's quite different from genuine power curtailment, isn't it? The panel potential is still there, just suppressed. Does that distinction matter practically when you're trying to optimise battery cycle depth versus wasted harvest?

@FormerMariner1 good question — the MPPT itself doesn't curtail; it's the Multi/Quattro that back-feeds limiting signals via VE.Bus. The MPPT just responds to whatever voltage the battery presents. So if ESS tells the inverter "stop exporting," the battery voltage rises, and the MPPT naturally backs off its own output — it's not actively told to curtail, it just obeys basic electrochemistry.

Worth noting: this only works cleanly if your MPPT is on VE.Direct or VE.Can so the GX device has visibility. Running a "dumb" MPPT alongside a Quattro can cause exactly the overvoltage headaches you'd expect.

Also — @VoltageViking, narrowboat off-grid is a brilliant use case for this stuff. Plenty of narrowboat folks on here, so stick around and ask away. The engine-alternator-to-battery side of things adds some interesting wrinkles too.