Solar water heating

mountainman · August 2018

I have an rv with a 10 gal lp and ac hwh. Using midday sun with 400 watts dirrectly connected, can someone tell me how long it would to take to raise the water temperature 50 degrees from 70f to 120? And what ohm or wattage element would I need?

jonr · August 2018

You need 4170 BTU = 1222 Wh, so about 3 hours. What voltage (Vmp) are the panels? How do you intend to make sure that you don't overheat the water?

mountainman · August 2018

Thanks for replying @jonr. Panels are 18.9 vmp and 5.29 imp. I had planned to use a breaker to disconnect from the array after the time was up. On second thought with clouds and such I guess the time would vary. So a thermostat would be needed.

Estragon · August 2018

You might want something like:
https://www.solar-electric.com/diversion-load-water-heating-element-12-24-volts.html

The thermostat or whatever breaking the circuit should be DC rated.

mike95490 · August 2018

Few thermostats are rated for DC useage

MrM1 · August 2018

DC stats

http://mwands.com/store/water-heater-thermostat

jonr · August 2018

Around 1.4 ohms for the element resistance, depending on low light/sun hours tradeoffs. You must solve the thermostat problem .

Consider this (search ebay for Solar-Water-Heater-EZ-DIY-Save-No-Pipe-Changes-Hot-Water-System-PV-MPPT).

mountainman · August 2018

Thanks for all the useful info. With 400 watts on one array and 100 on another. Most days in summer if i adjust the panels easterly I'm getting to float by 11 or 12. So from roughly 12 to 3 my thoughts were to switch the 400 watt array from the batteries to the hot water heater. I'm not sure of what type of switching I'll use or the effect on the controller. The 100 watt array should run my small day loads and keep me in float. A 120f dc thermostat will be used.

mountainman · August 2018

@jonr would a 24 volt 400 watt element be a good choice? If i used 48 volts at 1600 watts would it last longer or heat slower etc? By my calculator either has 1.44 ohms

jonr · August 2018

Maybe the larger one would last longer.

mountainman · August 2018

For full midday sun shouldn't panels be parrelell? What am I doing wrong? How do you calculate ohms. This is how I'm doing it 18.9x18.9÷400 =0.893025x 1.4=1.250235 ohms.

jonr · August 2018

You can use R = VxV/P. But use a very conservative value for P (watts) to account for cloudy days.

mountainman · August 2018

On cloudy days I had planned to use lp. Because I probably wouldn't get to float early enough to get 3 hrs for hwh. Sun goes behind mountain in summer around 3:30.

jonr · August 2018

Then I agree, 1.25 ohms might be better than 1.4.

MrM1 · August 2018

Perhaps another way of looking at it, or at least you might get some ideas, is from this site:

http://waterheatertimer.org/Convert-AC-water-heater-to-DC-water-heater.html

mountainman · August 2018

I checked the 120 volt hwh element that came in the rv. With my ohm meter set on 200 I get a reading of 35.3. So 120 x120 ÷35.3=407.93 watts. Am i doing this the right way?

mike95490 · August 2018

mountainman said:

I checked the 120 volt hwh element that came in the rv. With my ohm meter set on 200 I get a reading of 35.3. So 120 x120 ÷35.3=407.93 watts. Am i doing this the right way?

Many materials will change resistance (usually increase) when they get hot. Tungsten elements in light bulbs and toasters are like that, low resistance when cold, higher when hot.

Solar water heating

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