Solar to heat water?

[wgerstmyer]

Hi electrical whizzes,
I have an abundance of solar amps that quickly bring my AGMs up to level each day. I’d like to add a water heater as a load to my controller so that any extra solar output can be used to heat up even a small amount of water. I see there are 12v heating elements out there, or would I be better off running a 120v heater off my inverter?
I’m sure I can buy a small insulated container for a heating element, but how to determine if it’s worth it, and what size element/container?
Even a small amount of hot or warm water would go a long way toward a contribution to a sunshower bag or comfort for washing dishes.

[Pete7]

How much solar? and do you have a large inverter?

Pete

[Ded reckoner]

We have 850W nameplate capacity solar and a 500A*hr house bank. We are a 30A boat and have a Victron Multiplus 12/3000/120 charger/inverter. We have a Whale S700E water heater. The water heater is only heated electrically (i.e. there is no circulation of hot water from the engine) using 120V (either through dockside power, or our house bank and the inverter, or our diesel generator). The hot water heater has a 1250W element. Using 120V operation, the current draw is almost 10.5A. 12V operation will either use very high current or else the heating will take a long time. Our experience in New England in the spring the year before last was we would heat the water for about 30 min. Further south now, we only heat it for 4min, preferring cooler water for showers.

[Jammer]

If you already have a large inverter, you can just use a 120v electric tea kettle. Typically they're 1500 watts. If you look around you can find lower wattage ones if you need that to work with your inverter. Here's a 300 watt one for example:

https://www.amazon.com/Electric-Port.../dp/B09SZFQ1G9

12 volt tea kettles also exist:

https://www.amazon.com/Electric-Kett.../dp/B08ZWHSPF2


If you're thinking more in terms of a plumbed-in water heater, you can get any marine water heater that will fit your space and then change the element to match your requirements. 12v and 24v elements are available in 150w, 300w, 600w, and probably other sizes too. 120v elements are usually 1200 or 1500 watts. If you want a lower wattage element, run a 240 volt element on 120 volts and you'll get a quarter the nameplate wattage.

[wgerstmyer]

….12 volt tea kettles also exist:

https://www.amazon.com/Electric-Kett.../dp/B08ZWHSPF2 …..

1liter in 50 minutes! For $30! Brilliant! Perfect for our needs. Thanks Jammer, and the other replies as well.

[Cheechako]

I'd go right for the 12V heater elements. KISS.

[dksail]

We heat our water tank with excess solar. We just use a 2000w inverter for the 1250w load. We have 400 amp hrs of lithium so it is an easy task. We usually just run it for 25 minutes. It is a 12 gallon tank so we get about 1 degree per minute. Works great, but you have to watch it close or it will eat up all the batteries in a hurry. We have 750w of solar of which we get about 500 w because of shading.

[valhalla360]

Originally Posted by Pete7

How much solar? and do you have a large inverter?

Pete

^This.

If you only have 200w-hr to burn, it's likely more hassle than benefit.

If you have 3kw-hr to burn, it might be worthwhile.

[senormechanico]

600 watts solar, 2k sine inverter, 200 ah LiPo4 bank. Hot water tank insulation rewrapped with Aerogel. Life is good.

[bergius]

This is the piece of kit we’re considering: https://www.bobilvans.co.uk/product-...bil-air-hybrid

It allows heating water with three sources:

• Heat exchanger connected to our forced air diesel heater
• 230V shore power
• 12V DC when there is excess power coming from solar/hydro

[Rapanui]

We have been running our 1200W 240V water heater from our inverter for 5+ years now. 3 x 120Ahr AGM 12v batteries and 480W (now 720W) solar in Croatia. We use it in timed bursts of 10 minutes, usually in the morning for showers when the sun is up and the solar charging. The batteries are typically at 85-90% SOC as measured by Smartgauge at the start. The inverter draws 100A @ 12v. Any longer than 10 mins and the battery voltage dips so that the inverter trips even though the batteries are still around 80% SOC. A single 10 minute burst will allow 2 showers.
The limiting factor is the eventual voltage drop when you pull high currents, even from AGMs. It is obviously kinder to the batteries if you have lots of solar pushing in lots of amps to reduce the net draw from the batteries.

[NYSail]

I bought a new water heater….. an Isotemp and got the 750 element so that running off my 3000 watt inverter was an option. However have to say at anchorage in summer months we have a 10 gallon solar shower bag that gets overly hot and fine for showers….

Greg

[chris mac]

I swapped the heating element out on our hot water tank. I changed it to a 12 volt, 300 watt element. I bought it on Amazon for less than $20 and it took an hour to do.
I turn it on when we are in absorbtion phase. We can typically run it for 2-4 hours a day off excess solar. We have 8, 6volt batteries. That's 840ah. And we have a little over 1300 watts of solar.
We use a victron mppt controller, which has a virtual load feature. I plan on setting that up this year so it will automatically turn on

[noelex 77]

A traditional marine calorifier does not work well for cruising boats spending time at anchor if you wish to spend more than a couple of nights in one spot.

A much better solution is a simple insulated container that feeds the shower directly. This means hot water from many different sources can be used. The water can heated using electricity, propane, diesel, directly from sun or from a mixture of these methods. Cold water is added to reach the ideal shower temperature and this is pumped directly to the shower rose. Simple.

It provides a very pleasant shower that is a perfect temperature without the need to fiddle with tap settings, or having the risk of scalding. As just the desired amount of water required is heated, it is very energy efficient, enabling the electricity from the solar panels to be used even if you have a modest solar array.