Opportunity loads - DC load or relay activation

Been thinking about this quite a bit since the 3.80 firmware dropped — the Opportunity Loads feature is genuinely useful for making the most of excess solar rather than just watching it bleed off into nothing.

On my boat setup I've been using a Victron Cerbo GX relay to trigger a DC immersion element when the batteries hit a certain SOC threshold, which works well enough, but it's fairly blunt. What I really want is something more dynamic that responds to actual surplus power rather than just battery state.

Has anyone here experimented with using the Ekrano GX's spare relay outputs specifically for DC loads under Opportunity Loads logic? I'm wondering whether:

• It's cleaner to activate a relay-switched DC load directly
• Or whether it's better to trigger an AC load via the inverter and let VRM handle the intelligence

For the shepherd's hut I've been planning, I'd love to divert surplus into a small DC water heater or even a battery-fed fan heater — something genuinely useful rather than just curtailing.

The Fogstar 48V cells I've got spec'd for the hut build should handle the charge/discharge cycling fine, but I don't want to be hammering them unnecessarily if the logic isn't dialled in properly.

Anyone running Opportunity Loads on a Cerbo or Ekrano in a real off-grid (not ESS) configuration? Curious whether the feature behaves differently without a grid reference point. The Victron documentation is a bit thin on that scenario.

Would be good to hear how others have implemented this practically rather than just in theory.

Really glad this is getting more attention — I spent a good while puzzling over the same choice on my boat setup.

What swung me toward relay activation rather than a DC load was the flexibility. Using the relay to trigger a contactor meant I could control a proper 240V immersion heater through the inverter — effectively getting hot water "for free" on sunny afternoons without any complicated wiring into the DC side.

The DC load option is cleaner if you're running something like a dedicated Victron MPPT-controlled pump or a smaller resistive load directly. Less latency in the switching too.

One thing worth flagging: check your hysteresis settings carefully. I had mine too tight initially and the relay was chattering every few minutes, which isn't great for whatever you've got connected downstream.

What's your actual load you're trying to opportunistically power?

The relay activation route is where I landed after a frustrating week of experimenting on the narrowboat. Had a Victron Cerbo GX managing everything and initially wired an immersion element directly as a DC load — seemed obvious at the time.

Problem was the voltage fluctuations playing havoc with the element's longevity. Switched to relay activation triggering a dedicated AC immersion controller instead, and it's been rock solid ever since.

The real advantage @VoltChloe might not have considered: with relay activation you can stack conditions — SOC threshold AND minimum voltage AND time window. That granularity just isn't there with straight DC load control.

On the static caravan I run a similar setup through a Victron MultiPlus, using the relay to fire a Myenergi Eddi for water heating. Tidy, reliable, and the immersion element isn't seeing any dodgy switching behaviour.

Great thread, @VoltChloe — timing is spot on as I've just been through this exact decision on my shed/workshop setup.

One thing worth flagging that I haven't seen mentioned yet: if you're going the relay route like @DerekDixon, pay close attention to the hysteresis settings. I initially had mine triggering the immersion element on and off every few minutes because the threshold was too tight, which is obviously not ideal for the element or the relay contacts over time.

Ended up setting a wider band — roughly 5-10% SOC gap between activation and deactivation — and it's been rock solid since. The DC load option is cleaner in principle but your appliance really does need to be properly rated for whatever DC voltage you're running, which rules out a fair bit of kit practically speaking.

Good points all round, but nobody's mentioned the assistant load current monitoring issue with the DC load option yet.

If your opportunity load draws variable current — a pump, a compressor, anything inductive — the MPPT can misread the net current and make poor decisions about when to curtail. Caught me out on my shepherd's hut setup with a 12V water pump; the system kept terminating the opportunity window prematurely because of inrush spikes.

The relay route sidesteps this entirely since the MPPT isn't trying to monitor what's downstream — it just closes/opens the contact based on your configured voltage thresholds.

Worth being precise about your hysteresis settings too. I run 0.4V on the garden office system and it's night and day versus the default — stops the relay chattering on partly cloudy days when voltage is hovering around threshold.

@Trigger raises a fair point — my tiny house setup nearly went sideways when the assistant load current threw readings so far off it thought my kettle was a LED strip.

Went relay activation in the end; dead simple, no current sensing drama, and my immersion heater doesn't care whether it's philosophically a "DC load" or not.

One thing nobody's flagged: if you're using a Victron Cerbo GX, the relay logic via Node-RED gives you far more nuance than the basic ESS assistant alone — I've got mine triggering at different SOC thresholds depending on time of day, which feels almost too clever for a man living in a box on wheels.