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

Mr_Matthew_LMr_Matthew_L Posts: 1Registered Users
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.

Comments

  • solar_davesolar_dave Posts: 2,280Solar Expert ✭✭✭
    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: http://www.builditsolar.com/
  • solarvicsolarvic Posts: 954Solar Expert
    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. :Dsolarvic:D
  • icarusicarus Posts: 5,070Solar Expert
    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,

    Tony
  • ThomasThomas Posts: 291Solar Expert
    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.
    Always troubleshoot with adequate sunlight.  Hi Ho Hi Ho
    2.3kW [10] Enecsys SMI-240-60 micro inverters

  • BB.BB. Posts: 25,246Super Moderators admin
    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.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • waynefromnscanadawaynefromnscanada Posts: 3,009Solar Expert ✭✭✭✭
    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.
  • plongsonplongson Posts: 108Solar Expert ✭✭✭✭
    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).
  • SteveKSteveK Posts: 277Solar Expert ✭✭✭✭✭✭
    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.
  • BB.BB. Posts: 25,246Super Moderators admin
    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:roll:)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ThomasThomas Posts: 291Solar Expert
    Re: I want to run my water heater on solar power (Newbie)

    Wayne, RE; your panel early use.
    How would your solar hot water panel do at a startup temperature of ..say.. 67'F? (instaed of allready hot).
    Always troubleshoot with adequate sunlight.  Hi Ho Hi Ho
    2.3kW [10] Enecsys SMI-240-60 micro inverters

  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: I want to run my water heater on solar power (Newbie)
    Thomas wrote: »
    Wayne, RE; your panel early use.
    How would your solar hot water panel do at a startup temperature of ..say.. 67'F? (instaed of allready hot).

    thomas,
    can i ask you what the heck you are talking about? from what i see your post is nonsensical. you post too many posts that are nonsense and i did tell you about that. in all fairness you must explain what you mean here, but if it's still nonsense i may need to give you a reprimand.
  • waynefromnscanadawaynefromnscanada Posts: 3,009Solar Expert ✭✭✭✭
    Re: I want to run my water heater on solar power (Newbie)
    niel wrote: »
    thomas,
    can i ask you what the heck you are talking about? from what i see your post is nonsensical. you post too many posts that are nonsense.
    Agree Niel. I gave up reading or responding to anything he posts. Only thing that bothers me is that some new form member, not knowing the situation, may be misled or confused by his nonsense. We are no longer amused.
  • solarvicsolarvic Posts: 954Solar Expert
    Re: I want to run my water heater on solar power (Newbie)

    Wayne from Canada. I never thought much about installing a solar waterheating panel for lots of reasons. i will study up on it and start a new post with my questions. :Dsolarvic:D
  • waynefromnscanadawaynefromnscanada Posts: 3,009Solar Expert ✭✭✭✭
    Re: I want to run my water heater on solar power (Newbie)
    solarvic wrote: »
    Wayne from Canada. I never thought much about installing a solar waterheating panel for lots of reasons. i will study up on it and start a new post with my questions. :Dsolarvic:D

    I think you'll be very surprised at what you find and how well they work. Mine is of the simplest kind, designed and built from scratch by myself, to directly heat the actual water I use. No antifreeze, no heat exchanger, no pumps, no drainback, no electric power required. I just have to look after it so it doesn't freeze, otherwise the panel could be ruined. But it's about 7 years old now, so I must be doing something right. lol Freezing is not a problem from mid spring to mid fall, I just activate it and forget it. Highest tank temp I've seen was after three or 4 days full sun, 176F. Definitely have to use care, use one's head, so as not to get scalded. The panel is located just below the storage tank (tank inside of course, and insulated with over a foot of additional insulation to hold the heat in) so the heated water circulates by convection from the panel to the tank, no pump required. However, on sub-freezing nights or sub-freezing weather without sun, I must never forget to drain the panel, which by design I can drain from inside the house, no need to go out in the storm. Normally in winter I leave the panel drained and heat the water from the wood stove, but a couple of days this wk, the empty panel had reached 210F, setting off the high temp alarm, so I figured what the heck, might as well take advantage of it :)
  • icarusicarus Posts: 5,070Solar Expert
    Re: I want to run my water heater on solar power (Newbie)

    Vic,

    I have built a number of small(ish) flat plate collectors. The last on I built is built is a simple copper loop. It is a plywood box, covered with recycled patio door glass (available free or nearly free) covering ~200' of 1/2 copper, sitting on a piece of scrape 1/8" plate steel as an absorber. A simple Azel differential controller (~$100) turns a Grunfos pump on and off with an adjustable Delta T. The pump circulates water into a cast off hot water heater tank, that serves as a pre heat tank for a demand gas water heater. The whole thing cost me ~$500 is ~3'X12'. In the grey Pacific coast, it contributes ~80% of the the hot water load for a family of 2. (~30% in the winter, 200% in the summer) The Azel controller has an anti freeze mode so when the panel temp nears freezing, the pump kicks on to keep the collector from freezing. This has kept the system from freezing with temps down near 0F, although if it is that cold, a antifreeze, heat exchanger system might be in order, or simply drain it. I also installed automatic drain valves so that by simply closing the supply valves, and opening an air vent drops the pressure so the drains open without having to climb on the roof.

    Like we have all said, solar hot water is a much more cost effective install that PV, especially if someone were considering using PV for hot water!

    Tony
  • solarvicsolarvic Posts: 954Solar Expert
    Re: I want to run my water heater on solar power (Newbie)

    I had originaly bought an outside wood furnace and have the pipe buryed in the ground. Never used the stove because I got the gaswell. I have the side arm already installed on a 40 gal. electric waterheater I could preheat before going to my geo spring hybred waterheater. Since my hybred waterheater only used 490 kwh the first year I am wondering if I would save much by the time I run a circulating pump ect. I am gridtied and got thru january with 9 kwh in my account. In feb I should start accumulating in my account again. :Dsolarvic:D
  • Lee DodgeLee Dodge Posts: 112Solar Expert ✭✭✭✭
    Re: I want to run my water heater on solar power (Newbie)

    I have a 25 ft^2 solar hot water panel that circulates antifreeze through a heat exchanger in a 50 gallon stainless steel tank. It supplies most of the hot water requirements for one person. I need to add instrumentation to better quantify its performance. The solar storage tank is following by a tempering value to combine the outlet from the solar tank with cold water to achieve an outlet temperature of about 120 F which is then fed to a natural-gas fired tankless water heater. The panel has a collection efficiency of about 40% to 55% depending on the outside temperature and the water temperature.

    I also have a 3.15 kW grid-tied solar PV system that has high collection efficiency panels, but those are more like 17% or 18% collection efficiency, so very poor compared to the solar hot water panel. The PV system provides about 180% of the electrical needs for the house. I would not want to try to use that high-grade energy in the form of electricity for heating water with a resistance heater.
  • ThomasThomas Posts: 291Solar Expert
    Re: I want to run my water heater on solar power (Newbie)

    I will try to explain my last question to you. I was asking how would Waynes panels work if the water in the tank was not allready hot, at 125'F he said in his post. Thats all. I have solar hot water panels that are thermal only from a site on the internet, I have been warned about advertising for anyone else here, I forget that this is a sponsored site. I am happy with the 3 panels I have in series, today the out put side was at 110'F(stagnant), it has to get to 125'F+10'F to start pumping while my elements are on. I was just asking to see how well his worked in the wintertime like now, I may want to change to a different panel, or system, like anti-freeze or something...like I have seen on the internet. I use no anti-freeze or any-thing like it, how much different/better is/are those systems anyway? And last but not least, are all the evacuated tube hot water systems heat exchange/ anti-freeze?
    Always troubleshoot with adequate sunlight.  Hi Ho Hi Ho
    2.3kW [10] Enecsys SMI-240-60 micro inverters

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