Water Heater switching while on Solar - Victron System

[Escape Plan]

Victron System, Multiplus Compact 12V/2000VA/80A, MPPT 100/50 Solar, Cerbo GX with touchscreen controller. I believe I can use those Cerbo GX relay outputs to switch the water heater on and off based on battery state.

I looked on another thread that discussed doing this, so my question is more about would you be inclined to use a 300W 12VDC heating element or what is currently in my water heater (AC powered, have to see what the wattage is)? I guess the downside is that the AC element requires the inverter to be on, but it's on anyway since it powers outlets on boat. My research shows the DC element is not quite as aggressive in heating the water as AC.

I'm not opposed to running the genset each day, but you know how sailors can be (the wind is free, shouldn't everything be?)...since we have solar and wind turbine, I thought I could maybe snatch some heating energy from them. We have 760 W of solar, only 400 AH battery capacity on house bank. Thanks for your comments and suggestions.

If you've done this already, I'd really love a quick diagram/schematic of what you've done!

Enjoy the day

[chris mac]

I just switched out my element. It was 110v, 1500 watt. Which is a 125 amp draw at 12 v. That's a pretty heavy draw against golf cart batteries(which is what I have)
I installed a 12v 300 watt element. That is only a 25 amp draw. Which means the solar panels are able to replace the power as it's being used and isnt hurting the batteries.
The downside of course is needing to run the heater for 5 times as long to get the same temp increase.
For our 11 gallon tank, 3 hours seems to get it hot enough that we need to mix cold in for a shower. Here in the Bahamas, I'm able to run it 3 hours per day and still have full batteries at sunset.
I have 640 watts of solar, and 840 AH battery bank.
I am looking at increasing solar and getting the victron monitor as well.

[Escape Plan]

Thanks Chris - is the water heater on full time, or switched?

[chris mac]

Originally Posted by SaltyDogsLLC

Thanks Chris - is the water heater on full time, or switched?

I ran it to the breaker panel. So I manually turn it on and off.

[JustMurph]

If there is a 12V option, I'd go for that in your case. I have a 240V 750W water heater and use my Cerbo GX to control it so that it runs on excess solar (1.7kW system).

Happy to offer some advice on configuring the Cerbo GX to do that.

[redog]

Just want to share some information on what is working for me.
At home 3x 270w panels in series with a 4500w 240v element, increase of 10 deg per day in 400 ltr system. Controlled by digital thermostat. That's DC to AC element direct. Backup element in the top of the tank set to 60 deg and connected to the grid. This setup maintains temperature even during a string of rainy days, solar panels still work better on rainy days than thermal heating panels do. My backup element hasn't used one bit of power during 5 continuous rainy days.

In the caravan 300w 240v element run off 1 K 12/240 inverter same digital thermostat. Mostly have turned on when tug is running so alternator handles the load and have hot water when we pull up. Caravans 320w solar handles this pretty well also including other loads while we are anchored up, although I limit the time I have it turned on. I see an increase in temperature of about 2 to 3 deg per hour in an 11ltr tank. No problems getting to 45deg which is what our temperance valve is set to.

Haven't bothered on the boat as it's a stink boat and don't have them white sheets and poles, so we have to run the engine anytime we move. Heat exchanger has HW system up to 80deg within 15 minutes. Then the Genset cranks the 240v element anytime we are anchored. Over 1k of solar on the roof but haven't found the need to go down that road.

Hope this helps someone.
Cheers Dave

[CartMan]

Originally Posted by JustMurph

If there is a 12V option, I'd go for that in your case. I have a 240V 750W water heater and use my Cerbo GX to control it so that it runs on excess solar (1.7kW system).

Happy to offer some advice on configuring the Cerbo GX to do that.

Hi JustMurph,

I would like to do exactly what you describe here using a Cerbo GX. Any advice you can offer would be much appreciated.

Many thanks

[JustMurph]

Hi CartMan.

So I do this using the generator start/stop control in the Cerbo GX with an inverted output (wire to the normally closed contact of the relay on the cerbo). This way when the batteries are full, rather than turning a generator off, you're turning the hot water on. When the batteries get low, it turns it off. You can also use the "quiet hours" function so that it only turns the hotter on during specific hours of the day, namely, while the sun is up.

So, in my case I've set it so that it allows the hot water to run between 8am and 5pm, when the batteries are over 90% (off at <88%) full and the current going into the battery is over 28A (750W) (off at <0A). This seems to be a good compromise. On nice sunny summer day, the batteries are at 90% from an overnight low of 85% by say 9am, the hot water is made by the time they hit 95% and they're 100% not long after. If I turn on a big load, or the clouds come out and power starts being pulled from the battery, the HW will turn off until power into the battery climbs over 750W again.

For control of the hot water system, I bought a 240V AC relay with a 24V DC coil. This is switched by the Cerbo.

Let me know if you want any specifics of the setup, or screen caps of the settings.