Solar water heating

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?
Blue ridge mts. Renogy 400 watts manual tracking . Epever mppt 30. 2 GC 208 ahr. 300 watt psw inverter. 2 kw genny. Iota 45.

Comments

  • jonrjonr Solar Expert Posts: 1,135 ✭✭✭✭
    edited August 8 #2
    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?
  • mountainmanmountainman Registered Users Posts: 144 ✭✭
    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. 
    Blue ridge mts. Renogy 400 watts manual tracking . Epever mppt 30. 2 GC 208 ahr. 300 watt psw inverter. 2 kw genny. Iota 45.
  • EstragonEstragon Registered Users Posts: 3,043 ✭✭✭✭✭
    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.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • mike95490mike95490 Solar Expert Posts: 8,033 ✭✭✭✭
    Few thermostats are rated for DC useage
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • MrM1MrM1 Registered Users Posts: 306 ✭✭✭
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
  • jonrjonr Solar Expert Posts: 1,135 ✭✭✭✭
    edited August 9 #7
    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).
  • mountainmanmountainman Registered Users Posts: 144 ✭✭
    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.
    Blue ridge mts. Renogy 400 watts manual tracking . Epever mppt 30. 2 GC 208 ahr. 300 watt psw inverter. 2 kw genny. Iota 45.
  • mountainmanmountainman Registered Users Posts: 144 ✭✭
    edited August 10 #9
    @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
    Blue ridge mts. Renogy 400 watts manual tracking . Epever mppt 30. 2 GC 208 ahr. 300 watt psw inverter. 2 kw genny. Iota 45.
  • jonrjonr Solar Expert Posts: 1,135 ✭✭✭✭
    Maybe the larger one would last longer. 
  • mountainmanmountainman Registered Users Posts: 144 ✭✭
    edited August 13 #11
    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.
    Blue ridge mts. Renogy 400 watts manual tracking . Epever mppt 30. 2 GC 208 ahr. 300 watt psw inverter. 2 kw genny. Iota 45.
  • jonrjonr Solar Expert Posts: 1,135 ✭✭✭✭
    You can use R = VxV/P.    But use a very conservative value for P (watts) to account for cloudy days.
  • mountainmanmountainman Registered Users Posts: 144 ✭✭
    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.
    Blue ridge mts. Renogy 400 watts manual tracking . Epever mppt 30. 2 GC 208 ahr. 300 watt psw inverter. 2 kw genny. Iota 45.
  • jonrjonr Solar Expert Posts: 1,135 ✭✭✭✭
    Then I agree, 1.25 ohms might be better than 1.4.
  • MrM1MrM1 Registered Users Posts: 306 ✭✭✭
    edited August 11 #15
    Perhaps another way of looking at it,  or at least you might get some ideas, is from this site:


    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2018 / System, Pics and Discussion
  • mountainmanmountainman Registered Users Posts: 144 ✭✭
    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?
    Blue ridge mts. Renogy 400 watts manual tracking . Epever mppt 30. 2 GC 208 ahr. 300 watt psw inverter. 2 kw genny. Iota 45.
  • mike95490mike95490 Solar Expert Posts: 8,033 ✭✭✭✭
    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.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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