Solar dump load

[blackrose]

Is this still the best way to install a solar dump to a mains immersion heater? I've also watched a few YouTube videos of people doing the same thing with a voltage controlled relay and variable power controller.

 

Solar Dump Arrangement (1) (1) (1).pdf

 

Or would it be easier if I installed a Cerbo?

 

I have 910w of panels, Victron 150/60 MPPT and a 2000w standalone PSW inverter, 1kW mains immersion with fused switch, 560ah of LiFePo4 batteries and a Victron BMV battery monitor.

 

On sunny days I switch on the immersion heater and I could use the immersion timer, but it would be nice to have a proper solar dump load.

 

 

Edited April 10 by blackrose

[IanD]

6 minutes ago, blackrose said:

Is this still the best way to install a solar dump to a mains immersion heater? I've also watched a few YouTube videos of people doing the same thing with a voltage controlled relay and variable power controller.

 

Solar Dump Arrangement (1) (1) (1).pdf 1.34 MB · 0 downloads

 

Or would it be easier if I installed a Cerbo?

 

I have 910w of panels, Victron 150/60 MPPT and a 2000w standalone PSW inverter, 1kW mains immersion with fused switch, 560ah of LiFePo4 batteries and a Victron BMV battery monitor.

 

On sunny days I switch on the immersion heater and I could use the immersion timer, but it would be nice to have a proper solar dump load.

 

 

If you have a Victron Multiplus or Quattro you can connect the immersion to the (switched) AC2 output, and use an assistant to turn this on and off -- a Cerbo isn't needed. However the capabilities of these assistants are pretty crude (e.g. turn on at 53.9V (just below float voltage) and off at 53V (about 95% SoC), a Cerbo running Node Red gives you a lot more flexibility.

[jonathanA]

Wasn't someone using the load output of the Mppt to switch on the immersion (via a relay/contactor)?.   Which would work for Blackrose as he doesn't have a victron inverter.

 

[blackrose]

2 hours ago, IanD said:

 

If you have a Victron Multiplus or Quattro you can connect the immersion to the (switched) AC2 output, and use an assistant to turn this on and off -- a Cerbo isn't needed. However the capabilities of these assistants are pretty crude (e.g. turn on at 53.9V (just below float voltage) and off at 53V (about 95% SoC), a Cerbo running Node Red gives you a lot more flexibility.

 

I don't have a Victron multiplus, just a Victron smart solar MPPT.

2 hours ago, jonathanA said:

Wasn't someone using the load output of the Mppt to switch on the immersion (via a relay/contactor)?.   Which would work for Blackrose as he doesn't have a victron inverter.

 

 

So is that using the relay connections G below?

 

Why are others buying additional relays if they can do it this way with their MPPT?

 

 

Edited April 10 by blackrose

[IanD]

2 hours ago, blackrose said:

 

I don't have a Victron multiplus, just a Victron smart solar MPPT.

 

So is that using the relay connections G below?

 

Why are others buying additional relays if they can do it this way with their MPPT?

 

Because that relay connection is only capable of switching low currents and low voltages, not high-current 230Vac to drive an immersion heater.

 

So you use that relay to control the coil of an external relay which is capable of doing this.

[GUMPY]

You can use the terminals to drive a   Contactor  

[blackrose]

Thanks. So the control equipment is the MPPT and the output is 12v from the MPPT relay terminals going to the contactor?

 

Then N & L (AC) to the contactor from the boat mains/inverter and load to the immersion? 

 

Is that it? I guess I'd need fuses in the 12v (+) and AC (L) cables?

 

Then I assume there are some voltage settings in the Victron MPPT relay to tell the contactor when to switch the immersion on/off?

 

Or have I got this all wrong? 

 

 

 

Edited April 10 by blackrose

[jonathanA]

4 hours ago, blackrose said:

 

I don't have a Victron multiplus, just a Victron smart solar MPPT.

 

So is that using the relay connections G below?

 

Why are others buying additional relays if they can do it this way with their MPPT?

 

 

Are you sure that's a smart solar diagram. I don't recall seeing those connections ? (Not shown in the manual I have. )

 

I was thinking the load output terminals would switch a relay to control the inverter or immersion 

 

 

[blackrose]

6 hours ago, jonathanA said:

Are you sure that's a smart solar diagram. I don't recall seeing those connections ? (Not shown in the manual I have. )

 

 

That's what I've got on the bottom of my Smart Solar MPPT 150/60 Tr.

 

Also in my Victron manual.

 

https://www.victronenergy.com/upload/documents/Manual_SmartSolar_MPPT_150-60_up_to_250-70/29694-MPPT_solar_charger_manual-pdf-en.pdf

 

 

Edited April 11 by blackrose

[Rob-M]

Not all Victron MPPT controllers have the connections.

[Jen-in-Wellies]

10 minutes ago, Rob-M said:

Not all Victron MPPT controllers have the connections.

My smaller Smartsolar one doesn't. Which makes sense, as there is little point in having a relay output if there is less excess summer energy to divert elsewhere. 

[blackrose]

11 hours ago, jonathanA said:

I was thinking the load output terminals would switch a relay to control the inverter or immersion 

 

 

So not the relay output? What are the load output terminals? Are they the same ones used from the MPPT to the batteries? That's what others who are using voltage controlled relays and variable power controllers (as in my OP) seem to be doing. 

[Tony Brooks]

13 minutes ago, blackrose said:

What are the load output terminals?

 

I think they are the terminals that some solar controllers have that are designed to feed a load but ONLY when the solar is producing enough. I think some people have used hem to charge the engine battery while the main output terminal feed the domestic ones.

 

I can't see how using this gives the facility to wit until the batteries are charged before turning the immersion heater on, so I think that whatever is monitoring the LFP's state of charge needs to be the thing that signals to turn the immersion heater on.

 

Note - I may well not have understood what has been suggested.

 

 

 

[GUMPY]

12 hours ago, blackrose said:

Thanks. So the control equipment is the MPPT and the output is 12v from the MPPT relay terminals going to the contactor?

 

Then N & L (AC) to the contactor from the boat mains/inverter and load to the immersion? 

 

Is that it? I guess I'd need fuses in the 12v (+) and AC (L) cables?

 

Then I assume there are some voltage settings in the Victron MPPT relay to tell the contactor when to switch the immersion on/off?

 

Or have I got this all wrong? 

 

 

 

That is basically correct, however.

The relay terminals don't provide any power it is just a switch.

You need to use the relay terminals for the switching as follows.

Connect 12v to the C terminal with an in line fuse.

Connect the NO terminal to the contactor +

Connect the contactor - to 0v 

There is a relay mode in the MPPT that is controlled by when the MPPT goes into float.

 

Unfortunately there is no load output to feed the contactor, 12v +/- can be taken from the battery terminals on the MPPT.

 

 

Edited April 11 by GUMPY

[jonathanA]

12 hours ago, blackrose said:

That's what I've got on the bottom of my Smart Solar MPPT 150/60 Tr.

OK... now that you have actually told us the model.... then yes it has those relay contacts.  the only option that looks useful is for the relay to operate when the charger goes into Float.  but its not clear (to me) when it would decide to come out of float. Similarly looking at what/how the load terminals can be configured might give some options (which was what i was alluding to in my first reply)

 

Think you need to read the manual and see if you can come up with a configuration that would operate as a solar dump - might be worth looking on the victron support forum to see if anyone has done it. 

[blackrose]

1 hour ago, jonathanA said:

OK... now that you have actually told us the model.... then yes it has those relay contacts.  the only option that looks useful is for the relay to operate when the charger goes into Float.  but its not clear (to me) when it would decide to come out of float. Similarly looking at what/how the load terminals can be configured might give some options (which was what i was alluding to in my first reply)

 

Think you need to read the manual and see if you can come up with a configuration that would operate as a solar dump - might be worth looking on the victron support forum to see if anyone has done it. 

 

I'm pretty sure I mentioned the model of MPPT in my original post. As you can see from the manual, both the Tr and MC4 models of MPPT (150/60 and larger) have those relay contacts. The Blue Solar MPPTs in that size range also have them.

 

Anyway, thanks for your advice. I'm not really looking to reinvent the wheel or do something that needs a lot of research, so it sounds like it might be better to install a system similar to the one in my OP as that's proven. There seem to be quite a few YouTube videos showing similar installations but the link I posted above has the most detail.

 

Here's an example of a YouTuber who's installed a solar dump load and it looks like what he's done is similar to the the guide I posted above in the OP.

 

Edited April 11 by blackrose

[wakey_wake]

20 hours ago, blackrose said:

Is this still the best way to install a solar dump to a mains immersion heater?

There is a 'best' way? 😛

To pre-empt what I often get - I have already been overthinking this, and am obviously continuing...

 

The way I did it recently is not the best way. I guessed there was enough solar in the batteries and coming soon for another day's needs, so I used the manual switch to heat the tank. I wouldn't say it is the 'worst' way because nothing caught fire 😁 but when the clouds rolled over the rest of the day, it did leave me watching the battery voltage and trying to eke a cup of coffee out of same batteries before they hit 10.5V. 🤡

In my defence, I knew I had two buckets of electricity in the car and I applied those to look like a 26V PV input overnight. These are my toys.

 

Skimming over a brickboat solution: "solar diverters" exist such as the "myenergi eddi" single phase power diverter. It assumes all power is on the Grid and aims to balance house PV inverters and house demand such that export is close to zero, by modulating power delivered to the 3kW immersion element. In this way you avoid buying import, and prefer to avoid selling export because it does not pay well. I cannot imagine a boat where that would be useful or relevant, but the comparison is relevant.

 

20 hours ago, blackrose said:

I've also watched a few YouTube videos of people doing the same thing with a voltage controlled relay and variable power controller.

Solar Dump Arrangement (1) (1) (1).pdf 1.34 MB · 6 downloads

 

I looked at the circuit diagram and then only skimmed the text (sorry). My observations and reservations with that scheme

1. from a "battery sufficiency" signal, you want input to drive a domestic-coil mains-contact relay plus a phase-chopping power controller.
2. the battery sufficiency signal is unclear in nature. Perhaps if I knew more about specific kit or read the article more closely 😇 it would be clearer, but I can imagine variations. Phrases like "smart relay" will hide a lot of detail!
3. the power controller seems optional.
   If there is no power controller and the PV input (here max 60A "24V") is under the immersion demand (~43A 26V = 1kW @ 90%), it would be easy to begin discharging the batteries by closing the relay. That may be perfectly fine.
4. The contacts of the "auto/manual" switch appear to be wrong?
   I think the immersion should be the flying pole, while "from inverter > existing immersion switch >" should reach the bottom bole.
5. I am suspicious of the durability of inverters in the face of phase-chopping power controllers. More below.
6. any AC power modulation is going to impose quite a 50Hz ripple on the DC system.
   We're happy to let the batteries eat that? They do it for all the other mains loads...
7. if the power modulation signal comes from a DC shunt, is the aim to keep it average=zero? always net positive? Does it even sample fast enough to see that?
8. system inefficiencies aren't critical if you have spare PV for the immersion, but they will still limit utility on marginal days.
   Something like10% loss in the MPPT, 3~8% loss if round-tripping through LFP batteries, 10% loss in the inverter. Total loss 23% (0.9×0.95×0.9).
9. Electrical losses are after the ~78% losses of the PV from sunlight caught, but I think we all assume that the goal is to increase utility of the PV array rather than best water heating by sun capture. However any heating of the cabin in summer can be unwelcome. Who mounts their MPPT on the uxter plate for cooling?
10. You need a control system of some kind. Initially, simple is better so maybe a small box that does one function; but how many "small boxes doing one simple function" are sensible before you chuck it all out and replace the lot with a small shell script™ running on a microcontroller?
    My limited understanding of the Cerbo is that it offers to be that microcontroller, and comes with software that non-programmers can use.

On control signals from the battery,

1. Plenty of sunshine, batteries are (say) half full already, therefore use (say) half to heat the water and the rest to charge - then hope the batteries are full by sunset?
   Advantage: hot water available sooner.
   Disadvantage: batteries may not reach 100%.
2. Batteries now full (as full as you care to https://www.canalworld.net/forums/index.php?/topic/126671-lifepo4-max-charge-voltage/ ), electricity is free, so warm the tank.
   Advantage: simple and batteries take priority.
   Disadvantage: hot water delayed, maybe there is none and you carry unnecessary battery charge to the next day.
3. Did you want it linked to a weather forecast? Does it have an itinerary for the cruise, with tree-shade prediction? Does cruising heat the water anyway?
4. If you plot battery SoC and the gradient implied by the shunt, you might want to modulate power so the batteries reach full-enough by sunset.
   Compare with the SoC and power draws from recent days to estimate remaining PV input. ("Yesterday's weather" is the simplest forecast.)
   That is about as fancy as I would want to get.

On mains power modulation, the ways I'm aware of at kilowatt level

1. Phase chopping, by delayed conduction each half cycle. Triacs do this, and that ('drill speed controller') seems to be what is described.
2. Phase chopping, by early switch-off each half cycle. I think some older systems did this, but I forget how.
3. Hit-and-miss modulation. The heater runs full power and then pauses, either cycle-by-cycle or over seconds to minutes.
   Ordinary thermostats do this minute by minute.
4. Reduced intrinsic power of the heater,
   e.g. If you have a choice of a 3kW element and 1kW element in the same tank then two power levels are easily available.
   e.g. If you put two 1kW heaters in series, they draw 500W together on full mains voltage, so you have three levels {0, 500, 1000, 2000}W.
5. Reduced mains voltage by autotransformer (Variac). These are usually manual voltage control with a big knob on top.
   I would not buy one for this purpose, but since I have one I keep it on the boat. These are my toys.
6. Reduced mains voltage the modern way, with a mains powered inverter of  variable output voltage, probably driven by a semi-proprietary digital input over CANBUS.
   The 'myenergy eddi' mentioned above is such a thing and there are competitors. It is a specialised and strange beast but you could press it into service if those are your toys.

I don't have sufficient understanding of the filters that modern inverters use to remove the high frequency chopper noise, to get from the PWM sine-derived switched DC to a smooth AC. What understanding I do have makes me reluctant to present these filters with abrupt changes in load impedance, as a phase chopper will, because if you manage to make the filter 'ring' then the voltages may kill the H-bridge that feeds 380V DC to the AC network. I suspect  that is how my EDECOA died - no evidence yet.

 

So overall, what does fancy power modulation get you that a minute-by-minute (ie. slow PWM) switching of that relay will not? Probably not much, and I think it comes to simplicity vs battery care.

 

20 hours ago, blackrose said:

Or would it be easier if I installed a Cerbo?

I would start a list of "what else does the Cerbo offer to do for me" and "what else do I want automated, which the Cerbo might be pressed into"?

 

Then I would look at "does the Cerbo give me sufficient flexibility (in practice, or if someone else makes a software recipe) to generate a control signal that suits my needs?"

 

20 hours ago, blackrose said:

On sunny days I switch on the immersion heater and I could use the immersion timer, but it would be nice to have a proper solar dump load.

 

Some folks may benefit from an "excess power" mains fourway, into which drill battery chargers etc. are plugged. Demand would be much lower and they may prefer constant mains for 10 minutes or more at a time.

 

 

Above I'm looking at the whole system efficiency. What is the alternative? I'm not saying it's a good idea, but here it is...

You can get MPPT DC boost controllers which take a PV input and attempt to generate up to 90V DC, with lower output when sun weakens.

My experiment with sticking this directly into a 500W halogen flood lamp shows that it works as expected, and that device could put 140W into a "1kW" immersion element.
By diverting DC power before it hits the normal MPPT system, you have only one 10% power loss. The big drawback is that if the immersion thermostat tries to open, it is likely to get welded shut after not many cycles ( https://www.canalworld.net/forums/index.php?/topic/126659-12v-or-230v-kettle/#comment-3239275 ) and then you risk boiling the cauliflower and testing the PRV in anger. It would need a reliable additional thermostat set to a lower temperature, and then maybe a weekly mains top-up to get over 55°C for legionella. If the MPPTs are not expecting competition, they will fight each other in silly ways.

Turning that 90V DC into a 180V AC squarewave would achieve 560W into a "1kW" element, but it's not going to allow an arc to extinguish in the thermostat because there is no dead time. Also there would be new losses in the large capacitor involved.

 

Would it be simpler to use a cheap 1kVA queasi-sine inverter with a digital enable input just for the immersion heater? It brings new safety risks of having two independent mains phases within arms length and no efficiency gains, but it is another way to use that DC.

 

Ugh, too much complexity for me.

 

 

I would consider merging with another potentially useful device: a peak-demand-modulating heat controller. It probably doesn't exist but it goes like this.

Suppose the cauli is hot and the immersion element is on the thermostat just maintaining it. Sometimes it draws power, sometimes it doesn't. When I turn the kettle on and/or the fridge kicks in, will it trip the inverter? Will it trip the shoreline?

Peak-demand-modulation would watch the AC current transformer to get instantaneous demand, know the relevant limits, and shed load from heating devices as necessary. This would be useful for running fan heater(s) on shoreline and not having to manually switch off before boiling the kettle. Next best is a radio controlled plug ( https://www.ebay.co.uk/sch/i.html?kw=wireless+remote+control+mains+socket / £15 from Aldi) with transmitter near the kettle - guess how I know this?

 

I know some combi inverters can handle this by running as a shoreline-grid-tied inverter, but that doesn't suit me.

 

Most likely there are small 'modules' suitable to connect to a microcontroller, which can transmit the right signal on 418MHz to one of those mains sockets. For me this is where all those messy thoughts above join back up into something useful.

17 hours ago, GUMPY said:

You can use the terminals to drive a   Contactor  

 

Beware, I have seen some "contactors" in that form factor on Amazon with photos of the sides melted out. If you mount them on DIN rail they may not have enough cooling for the power dissipation of the coil. Best to leave a gap?

 

 

4 hours ago, blackrose said:

So not the relay output? What are the load output terminals?

 

"Relay output" sounds like a dry contact (sense #1 "no current"). The terminals are of a switch without any voltage of their own. You bring power from elsewhere.

"Load output terminals" sound like the thing solar MPPTs have. It provides domestic battery voltage when the MPPT decides.

 

As GUMPY says,

4 hours ago, GUMPY said:

The relay terminals don't provide any power it is just a switch.  [...]

Unfortunately there is no load output to feed the contactor, 12v +/- can be taken from the battery terminals on the MPPT.

except I would rather take power for the relay from a domestic circuit with its own fuse/breaker, than the "battery terminals on the MPPT" which may be rather directly to the battery and also harder to get two conductors into the joint without compromising the high current connection.

 

 

16 hours ago, jonathanA said:

I was thinking the load output terminals would switch a relay to control the inverter or immersion 

 

This scheme has the advantage that many "solar MPPT controller" boxes have such a load output, even the cheap and nasty PWM ones. Driving a mains-contact 12V-coil relay from that looks like an easily accessible solution for many people? However the MPPT I had (which died of smoke emission) and the one I have now (which forgets its load voltage setting and defaults to 9.5V on waking) are not to be trusted.

 

4 hours ago, jonathanA said:

but its not clear (to me) when it would decide to come out of float. [...] read the manual and see if you can come up with a configuration that would operate as a solar dump - might be worth looking on the victron support forum to see if anyone has done it. 

 

Yes, this looks like simplest but trusting the configuration is important.

 

What you may not want is "immersion heater starts when the PVs get the battery charged just before sunset, then the tank is nice and hot three hours after sunset but the battery is half empty again".

 

3 hours ago, blackrose said:

[...] I'm not really looking to reinvent the wheel or do something that needs a lot of research, so it sounds like it might be better to install a system similar to the one in my OP as that's proven. There seem to be quite a few YouTube videos showing similar installations but the link I posted above has the most detail.

 

Sorry, didn't see that until after writing my previous 🤣. Hopefully it will be interesting to somebody.

If "simple" is a requirement, an MPPT-driven relay seems like 'the best way' as requested in your OP. What does that power modulation gubbins buy you?

 

Another thing entirely lacking from all these solutions is some indication of actual hot water availability. On a boat especially, do you really want to run the hot tap for 45 seconds just to discover that the 'hot' water is tepid, so you need to put the kettle on to wash up?

[blackrose]

Ok thanks for the ideas. I can't see anything else that's proven to work, so I'll stick with the way that others are doing it.

[wakey_wake]

I forgot the bit "how do I plan to do it?"

 

My current plan

• a Raspberry Pi which is not on all the time, especially in winter
• a Raspberry Pico or other Arduino-alike, which is on at least at frequent intervals, and can wake the Pi
• if the inverter is on or can be woken, and if the battery is in that nearly-charged regime before battery care suggests not holding the cells at voltage over the knee, then offer power to the immersion heater
• I may use a triac rather than a relay
• once the hardware is done, control logic can change easily

My lingering questions are "what does the power modulation part give us?" and "do the batteries and inverter like phase choppers?"

[wakey_wake]

Turns out 12V 300W (25A nom) stainless immersion heating elements exist, and at that voltage you're less concerned about arcing. It's within range of a 30A automotive relay, but with zero margin at 14.4V (432W).

 

https://www.ebay.co.uk/itm/406536267996

 

Seems like the main limitations would be the apparent lack of any thermostat, and physically fitting the thing into an existing tank.

[GUMPY]

11 minutes ago, wakey_wake said:

Seems like the main limitations would be the apparent lack of any thermostat, and physically fitting the thing into an existing tank.

That's where you can take advantage of a Essex Flange😉

[wakey_wake]

18 minutes ago, GUMPY said:

That's where you can take advantage of a Essex Flange😉

Heard the name lots of times, had to see the vid to understand the trick

 

linked from https://essexflanges.com/pages/how-it-works 

 

The thermostat can be stuck on the outside somewhere, but you have to trust it!

[GUMPY]

15 minutes ago, wakey_wake said:

Heard the name lots of times,

It has a melodic roll to it

There is also a Surrey Flange but that's posher than the Essex one

[Tony Brooks]

4 hours ago, wakey_wake said:

Seems like the main limitations would be the apparent lack of any thermostat

 

I wonder if they are designed so they can't get any hotter than say 80C, or you are expected to use a cylinder stat.

[blackrose]

On 10/04/2026 at 18:55, IanD said:

Because that relay connection is only capable of switching low currents and low voltages, not high-current 230Vac to drive an immersion heater.

 

So you use that relay to control the coil of an external relay which is capable of doing this.

 

Can I use the MPPT relay to control one of these DC to AC relays? 

 

https://amzn.eu/d/0bpssqi5

 

Then use an AC power controller such as this to reduce the draw of the 1kW immersion heater? 

 

https://amzn.eu/d/00VWIfuL

 

Edited April 24 by blackrose