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Thread: I want to run my water heater on solar power (Newbie)

  1. #1

    Default I want to run my water heater on solar power (Newbie)

    I was looking to experiment with solar panels. I wanted to try and run my hot water heater from a battery bank and solar panels.

    Now I have tried running through the numbers but I come up with new numbers every time I try to figure out a design.

    My water heater is a 40 gallon tank. It runs 4500watts at 240 volts. I'm guessing that it's on for an average of 4 hours a day.

    According to a map I looked at, I should have 4-4.5 hours of sunlight where I live

    My latest figures are;

    I need a 4000w panel setup, and a 750Ah 48v battery system. This will leave roughly half the battery charge, with no padding for a cloudy day

    This seems excessive to me. Can someone please tell me if this is correct or am I just not smart enough to figure this out?

    Thank you and please be nice, I'm trying to learn all this out, and this is the first place I have ever asked a question.

  2. #2
    Join Date
    Apr 2010
    Near Phoenix

    Default Re: I want to run my water heater on solar power (Newbie)

    Solar electric is a very poor way to heat water, you are better off looking at solar thermal panels to do that. There are several DIY site and this kind of project lends it self to home brew systems for low cost.
    Take a look at this site:
    Last edited by solar_dave; February 14th, 2012 at 21:17 PST.
    My TED 5000 system
    Sticking it to the power company one watt at a time!
    60 Ningbo Electric 175 watt panels and 12 Canadian Solar 180 watt panels with 2 PVP 5200 Inverters

  3. #3
    Join Date
    Feb 2010
    South of Erie, North of Pittsburgh PA

    Default Re: I want to run my water heater on solar power (Newbie)

    This isn,t a good idea heating water with a regular electric waterheater. It will cost you too much money. I have one of those General electric Geo spring waterheaters which has energy rating of 2.3 and I used about 490 kwh for year of waterheating. My neighbor has 4 290 watt solar panels he has wired to a dc water heating element on a 40 gal waterheater. He preheats water in it and pipes the preheated water to a grid connected 80 gal electric waterheater. He told me he gets the water to about 110 f on a sunny day. Says his electric bill went down about $30 a month. I tryed to discourage him on doing that but he insist it works. One of the moderators here probably will help you figure what you need for your plan but you will probably need to give more info about water usage ect. If you have the grid, use that to heat water if not propane would be lots cheaper as a battery system to power a waterheater would be many thousands of dollars. solarvic
    16 KC 158G & 3 KD185GX-LPU panels on Fronius IG PLUS 3.0-1 inverter and 14 SHARP NDU3A & 1 KD185GXU panel on FRONIUS IG-3000 inverter. All mounted on pole top racks. Retired and enjoying it!!

  4. #4
    Join Date
    Jul 2007
    Quetico, Ontario

    Default Re: I want to run my water heater on solar power (Newbie)

    How often can we say it? This is a bad idea! Solar water heat (direct thermal) is very much less expensive, and very much more efficient than PV water heat. Running a water heater off of batteries is a doubley bad idea.

    Sorry, experiment somewhere else,

    Please note, being a moderator does not add any weight to my opinions 300 watts Siemens/BP panels,plus a Sun 90,, making ~400. ~30 amps into Rogue MPT-3024, 450 ah of Trojan T-105, Morningstar ts300 inverter, a Tri-Metric meter.a collection of antique generators, plus 2 Honda eu-1000i's (also a BS2512 IX controller) and assorted other stuff!

  5. #5
    Join Date
    Nov 2011
    Baton Rouge, Louisianna

    Default Re: I want to run my water heater on solar power (Newbie)

    If you have the money, build it, put the panel on the roof, I am sure there would be some other use for it after a few solar-hots baths.

  6. #6
    Join Date
    Mar 2006
    SF Bay Area (California)

    Default Re: I want to run my water heater on solar power (Newbie)

    Roughly, a full off grid power system will cost around $1 to $2+ per kWH. Using a heat pump type water heater will be 2x or more efficient than a pure resistive water heater... However, below ~55F, the heat pump types start losing efficiency (check specifications for details).

    Guessing you are in the Nashville Tn area--Your average lows are not too bad--but your record lows (-10 to -17F in winter) would probably force you to use an antifreeze closed loop thermal hot water system for heating (unless you could shut it down during winter). Which makes the whole thing a bit more complex and costly (plus more maintenance).

    If you can build/install a solar thermal system (open or closed loop), it will cost much less money to install and operate (although, like anything with plumbing, there will be leaks and maintenance).

    Is there some reason you want a pure electric hot water system? With a battery bank, it does make it very expensive (like you have worked out).

    There are a fair number of posts about solar hot water/solar thermal systems (and links) in the Working FAQ thread.

    Last edited by BB.; February 15th, 2012 at 9:17 PST.
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  7. #7
    Join Date
    Oct 2010
    Nova Scotia canada

    Default Re: I want to run my water heater on solar power (Newbie)

    As others have said, electrically heating your water from PV electric is grossly inefficient and incredibly expensive. Definitely NOT the way to go. Do check into direct solar water heating though. All my domestic hot water is supplied from a simple water heating panel throughout spring, summer and fall. Normally in winter it's heated off the wood stove. But even yesterday, Feb 14, since we had clear skies I activated the system, it was -6C outside, and when activated at 11:20 AM my 60 Cdn gal hot water tank was reading 126F, and when I shut it down at 3:35 PM it was 133F and hot right to the bottom. And I've only got a small panel, just over one sq mitre.
    1900 watts PV, (1000 watts PV feeding MidNite Classic 150; 900 watts PV + 160 watts micro hydro both feeding into a single shared Morningstar TS-MPPT-60) ; Xantrex Pure Sine 1800/12 for heavy loads; Xantrex Pure Sine 1000/12 on 24/7 for everything else; six Rolls Surrette 2 volt L16 @ 12 volts.
    Domestic hot water totally provided by the sun 8 months out of every year via thermal panel.

  8. #8

    Default Re: I want to run my water heater on solar power (Newbie)

    Hey Bill, How do you calculate the size of a system? The output of panels? The wattage of the battery bank?

    To OP, anything induction is a NO-NO. For example, we NEVER use the heated dry cycle of the dishwasher because of the amp draw. I even hot wired 180 vdc straight from the panels to that option button on the dishwasher to electrocute anyone who forgets...LOL (JUST KIDDING, but a good idea).
    10-Astronergy 235 watt panels
    Fixed 180* south face-No tracker
    Midnite Solar Classic MPPT Charge Controller 250V
    Mini-Magnum MS4448PAE Power Center
    16(ea) S-530 Surrette Batteries 800aH @ 48VDC 38,400 watt hrs.
    Lat/Long 37.8 N 113.1 W

  9. #9
    Join Date
    Sep 2010
    Shrewsbury, MA.

    Default Re: I want to run my water heater on solar power (Newbie)

    In a pinch when the power goes out, and we manage to have a 4.5 hour day with full batteries, we have run our Nyle Geyser heat pump water heater off our PV's to make 50 gal of 125F water from cold. There is no way we could run it 24/7 as we produce about half (or less) of what we would need on a monthly basis.

    Heat pump water heater we use will draw 500-700W at 120V and is a little easier to manage versus resistance elements. Still not agreeing to heat water using PV.
    Last edited by SteveK; February 15th, 2012 at 7:35 PST. Reason: or less
    6-180W CETC's., Midnite Solar Classic 200, Exeltech XP-1100E, Victron 24V-350W Inv, Energizer 24V 450aH FLA. Honda EU2000i Tri-Fuel. Victron BMV-602. Meanwell PB-1000-24. In Reserve: 2-180W CETC panels, Rogue MPT-3024, Exeltech XP-1100E

  10. #10
    Join Date
    Mar 2006
    SF Bay Area (California)

    Default Re: I want to run my water heater on solar power (Newbie)

    Calculating the size panel/heat required... I have not done the calculations before, so below is a guess/estimate. But because solar thermal panels are not as expensive, you can oversize a bit without too much extra costs.

    Say, 40 gallons of water per day, raise temperature from 40F to 130F:
    • 130F-40F = 90F rise
    • 90F rise * 8 lbs per gallon * 40 gallons per day = 28,800 BTU per day
    • 28,800 BTU per day / 3.4 BTU per Watt*Hour = 8,470 WH per day

    So, the standard solar PV + Battery estimate would be... 4 hours of sun minimum (9 months of year typical for many regions in North America), 0.52 end to end system efficiency for off grid system, battery for 1-3 days and 50% maximum discharge (longer life). Nominal calculation:
    • 8,470 WH per day * 1/0.52 system eff * 1/4 hours of sun per day = 4,072 watt array minimum
    • 8,470 WH per day * 1/48 volt battery bank * 1/0.85 inverter eff * 2 days no sun * 1/0.50 max battery discharge = 830 AH @ 48 volt battery bank nominal

    Note, the above calculations are rough numbers (8lbs per gallon for water, and 3.4 BTU/WH) but should be close enough for government work.

    The above is assuming a resistance heater--If you go with a heat pump in a mild climate, it would use about 1/2 the power (or less)--so array+battery bank would be about 1/2 the size.

    Same calculation for solar thermal... Typical thermal system is around 60-80% efficient:
    • 8,470 WH per day * 1/0.60 eff * 1/1,000 WH per sq.meter * 1/4 hours per day = 3.5 sq meters
    • 3.5 sqmeter * 10.7 sqft/sqmtr = 37.7 sq ft

    So a 5ft by 8ft thermal solar array should be more than enough for a typical 9 month a year 40 gallon per day (with pretty cold input water) system.

    The typical rule of thumb for solar thermal is around 1/2 to 1 sq ft of solar panel per gallon of hot water per day... Which seems about right (my estimate above is towards the 1 sq ft per gallon of water). Too large of panel and you may boil the water during hot summer weather in the solar loop. You might mount the panel with a "steeper slope" so that it gets less energy during summer and more during pre and post winter time when there is less sun and colder weather/water.

    -Bill "my swag" B. (scientific wild a$$ guess)
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

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