Solar to hot water tank question

[Barrie]

By 11am my batteries are all topped up and my system just dumps solar energy for the rest of the day. Is there a way to redirect this power to my 110 volt hot water tank without using the inverter? I'm thinking of like a trickle charge to keep the water warm while at anchor for a few days.
My MPPT charge controller has a DC load outlet/option that is not being used. See picture. Might need a bit of rewiring and a switch or two but is this plausible? Thanks
Barrie

 

[READY]

I doubt you can make much impact by using DC that would be output from your solar for the water heater. I also suspect the DC load rating on that output is nowhere near what you would need to even hold the water at a steady hot temp. It is usually meant to turn on a light or similar. You could of course add more battery capacity and use that to run the water heater off AC but then you need to have your inverter sized to handle that load.

 

[rslifkin]

Using the existing AC element with DC power will work, but a typical element won't produce enough heat to actually be useful. A 1200w / 120v heating element will only be good for 12 watts when fed with 12v DC.

 

[mvweebles]

Why don't you want to use the inverter?

Maybe put your water heater on a timer so it only goes on from 12pm to 3pm?

Peter

 

[hollywood8118]

someplace on the forum there was previous mention about a second 12V element in the heater to use the dumped excess energy of a PV system.. I believe it was 2-3 years back.
Hollywood

 

[Iggy]

For 1200W AC to power the hot water you will need over 100A of DC power. But it depends on the size of your back.

If it was me.... First, I would start the genset. 2nd if posable heat off the main engine.

 

[Senojev]

Hot water from Solar

Using DC to power a water heater isn't a viable option. However, if you have a decent size bank and the right inverter, you can power a water heater successfully. Our water heater uses 140 amps and takes about 40 minutes to get to temperature so it uses about 95 Ahr. Our solar panels get us to 100% around 11:00 on most days in Maine, so we have ample time to make up that 95 Ahr. I have an 880 Ahr bank and 1600 watts of solar and have no trouble getting to 100% by the end of the day. I do try to run the heater early on a sunny day and avoid using it if there is significant cloud cover.

 

[Barrie]

Thanks fellas : no surprise with your answers. I was just looking for a way to take advantage of the afternoon sun without having to think about the inverter or generator which i have. Maybe some bright spark-plug out there might come up with some magic. Technology keeps changing.
Happy New Year
Barrie

 

[socalrider]

Thanks fellas : no surprise with your answers. I was just looking for a way to take advantage of the afternoon sun without having to think about the inverter or generator which i have. Maybe some bright spark-plug out there might come up with some magic. Technology keeps changing.
Happy New Year
Barrie

You can indeed do this - I posted a thread on how I'm doing it with my single-bank, no generator, 12V only boat a couple of years back.

The question is whether it's worth it - simplest thing for you to do would be to just use the setup you have & somehow program the inverter to heat water for xx minutes if, say, your batteries were at 100% before 1pm. I've not done this so don't know how tricky it'd be. You'd draw down your batteries, but they'd be charged up during the rest of the day (I'm assuming your bank has sufficient capacity).

You can run 12VDC through a 110VAC heating element (it's just a resistor) BUT you've got two challenges - you need a switching system to ensure that you never ever cross your 12V and 110VAC systems, and you need a dedicated 12V thermostat for safety so you don't overheat the water.

If you have a dual element water heater this could be done fairly easily. If not, it seems like a long walk for a short drink.

 

[boathealer]

I did this.

I changed the water heater to a smaller tank and a lower-wattage element such that running off the inverter was no issue.

By going from a 20 gallon, 240V heater to a smaller 11 gallon, 110V unit (Isotemp), and moving the water heater to an inverter circuit, the recovery time was quite similar - even with a smaller 750W heating element vs. the 1500W in the big tank. 20 gallons was WAAAY too big for just the two of us anyway. We never ran out of hot water with the 11 gallon size.

The only drawback was that I had to turn the water heater breaker on and off as the battery level and solar supply dictated (unless plugged in to shore power, of course).

While in the Les Cheneaux islands this past year, we were anchored for 6 days and never ran the generator. We turned on the breaker to the house bank for the water heater once a day, after about 45min to an hour at -60A (at 12V, via the inverter), mostly excess solar for us, the water was hot, and lasted 24+ hours (very warm) with the Isotemp.

Best of luck!

 

[ejds]

someplace on the forum there was previous mention about a second 12V element in the heater to use the dumped excess energy of a PV system.. I believe it was 2-3 years back.
Hollywood

I did the same on my boat, replaced the original element with a 12 V low wattage element. Maybe 150 Watt. Found a 120V element that fit the anode threaded hole. I can switch to use either 12V or 120V. It took some wiring and a relay to use the thermostat for both 12V OR 120V
It is only a 3 gallon heater and to be honest I don't know how it works in reality since I haven't used to boat for some time.

Egbert

 

[Marco Flamingo]

I was just looking for a way to take advantage of the afternoon sun without having to think about the inverter or generator which i have. Maybe some bright spark-plug out there might come up with some magic. Technology keeps changing.

Maybe you could go low technology with a tried-and-true solar backpacking shower. I've seen several that claim their bags are food safe, so could be used for cooking and save some battery/fuel. There are some solar pool heaters that could be more permanent. Or, this fancy black painted aluminum tube for $500. For our idea of boating, we are happy with the $12 option. No electrical hookup or plumbing. The 5 gallon size heats up quick (a couple hours) with only 2 gallons in it.

 

[Capt Ray]

I do know that a solar energy expert back up in Maine whom I used to hang out with, dumps his excess solar energy into his [electric] hot water tank... in his home. (Sorry, no details.)

For US power boaters, the heat exchanger from the engine makes more hot water than we can use.

Buying more battery storage in the same bank OR a different bank w/ DC-DC charger, may be the only way that comes to my mind to save all that sunlight for a rainy day.

I'm actually waiting for someone to figure out how to back-power their generator to make diesel fuel from the sun.

 

[SteveK]

By 11am my batteries are all topped up and my system just dumps solar energy for the rest of the day. ................
Barrie

Barrie.
I have seen this comment on the forum a few times now. My first thoughts are either you have too many solar panels to battery ratio or have not used many Ah in the previous 24hr.
Which is it? There is an estimated 6 hours of useful sunlight in Ottawa during summer of the ~12hrs of daylight. 5am to 11am is 6 hours.
How many Amps were depleted, how many hours was the sun/solar charging at what average amps per hour would be some useful information.

 

[ronobrien]

You would need to be sure the inverter would handle the wattage.

 

[Barrie]

Thanks all for your comments. It seems this is a subject of interest for many based on the number of views.
As a retired salesman, I know engineers take a dim view of us when we say "sure it will work - just plug it in" !
But I have found some Youtube guys that have made exactly what I was looking for in the way of a solar dump to heat water.
Check out

and also https://windandsolar.com/heat-with-dc-water-heating-elements/ skip to 6.02 in the video.
It appears this technology is not quit plug n' play for us boaters but it looks promising. The companies making electric boats must have figured it out. I will keep working on it.
Cheers
Barrie

 

[Mako]

Yes I recall we had a recent thread, 2022, about this subject. I have found 24VDC heater elements of 900 watts and 48VDC of about 1500 watts. I have not researched 12VDC elements but they sound rather worthless.

I think the ideal solution would be to install a domestic solar water heater. American domestic units are generally all dual-element, so keep one as 110V/240VAC, the second as 24/48VDC and then the loop for engine heating.