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Small home off-grid Solar thermal design and required batteries

ylekyote | Posted in Mechanicals on
Hello. I’d like help with this off-grid heating solution if someone has time.  I am building a 960 SF two story, off grid building. I’m in construction zone 5 (western Colorado).  I have one 115v/20amp circuit I can tie into for urgent uses but usually I want to be able to run the home on no more than 8 deep cycle batteries providing about 160 minutes Reserve Capacity each, or 65 amp hours each.
In Summer, I will only heat domestic tap water for daily bathing, by-hand clothes washing, and washing dishes. 
In Winter, I have the above plus I want to have one or two water-to-air heat exchangers to help warm the Southeast end of the house, which is closest to the point of use (sinks and tub). But I also have two small interior wood stoves designed into the house floor plan to do the rest of the interior air heating, during the really cold times.
For the domestic hot water to the 3 taps (bathtub, bathroom sink, kitchen sink) I want to have at minimum a 40-50 gallon hot water storage tank that is heated by a combination of four means, but I’m unsure if there is a way to do it.  Below I’ll list what I have on hand. 
I’m wondering if any/all of these will be useful? If so, how should I configure these to most efficiently heat my home in winter while providing hot water also, and in Summer providing hot tap water but not adding too much interior heat (home will Not have air conditioning).
Here’s what I have…
1.  I have 12 each, 75-watt Siemens brand, model SP75 (Pro 4 JF) solar panels. They are 23 years old but in good condition/undamaged and worked good 8 years ago when last used.  I want to have these keep my batteries as full as possible so I don’t have to use on-grid electricity or LPG generator to charge them.
2.  I have 32 each, 8-foot long evacuated thermal heating tubes mounted on an aluminum frame that connect to a big square, insulated 400 gallon thermal battery tank (via insulated copper or PEX pipes) which has two liquid-to-liquid heat exchangers on it.  It’s an open “drainback” system. I thought about placing this on South side of home, painting it black, and using a high power solar panel (and maybe one local battery) to heat a coil as well as power a closed loop circulation pump from the tank heat exchanger to inside the house, and to the water heater. This way it would only heat and pump during the day (when sun is shining) when the tank is naturally hotter and it will sit idle at night when the sun is down.
3.  I have a outdoor wood boiler that has two hot water pump ports on it. This would only be burned in the winter usually, for obvious reasons, to make hot tap water.
4.  I have a 250 gallon LPG propane tank connected to a 12.5kW Kohler generator about 140′ dig away. The generator also energizes the 115v/20amp circuit I mentioned earlier, in case of power outages. The generator, propane tank and AC outlet are located in the same shed area about 140 feet from my new home site.
My lighting is LED and basic in each room. 
My AC appliances are:
An 8-amp 11 cubic foot refrigerator/freezer.
Water pressure pump (cistern to the taps and hot water tank). 
4 floor box fans. 
A LPG oven. 
Maybe a 42″ flat screen LCD TV. 
A few personal care and small kitchen appliances used infrequently, and a portable radio/CD/MP3 player-speaker, 2-3 cell chargers.
There is NO clothes washer, clothes dryer, or automatic dishwasher.
I estimate my load will be less than 4,000 watts surge/startup, even if I turned everything on at once. Usually under 2,300 watts (only if water pump and fridge turns on at same time).
I’m also wondering if I should make the water pump a DC type but I’m unsure if that’s feasible. The battery bank will be outside on a patio in an insulated/vented wooden box, about 20 feet wire run from the water pump and cistern.
With these four tools what can I do, and how, and why, to provide power to my small home and also heat the tap water and climate as well in winter?
Also, are 6-8 common deep cycle batteries, with about 65-70 amp hours each, enough for my minimalist power needs? 
Thank you very much!

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  1. brad_rh | | #1

    It looks like your requirements are simple enough that this is feasible, but what you're proposing is a really complicated system. My 1st suggestion is to get rid of the wood fired boiler. An inside wood stove will be a much more efficient backup heat source.

    What is the voltage of those batteries? Need that to know that to know the watt-hr capacity of the batteries. What is your expected energy use per day (kwh/day)? Use pvwatts ( to get the estimated production from your solar panels.

    1. ylekyote | | #2

      The batteries are 12v.

      The wood boiler I have connected to my home is much better than the two stoves I had trying to heat my homes (2,850 SF) interior. Plus no smokey smell all the time, safer, used the same amount of wood per winter, etc. I wish I had it connected to my
      gas hot water tank also...I need to do that next modification.

      I have no idea on energy use as this primarily a greenhouse and only used as living quarters part-time like for guests and family. There would only be those appliances I mentioned running as needed. Fulltime the hot water tank and medium fridge/freezer. Wood boiler and a circulation pump or two from it during winter.

  2. Expert Member
    Akos | | #3


    You should step back a bit and figure out what you must have there and what is nice to have. Trying to integrate everything in your list just because it exists might not be the best solution, most likely you'll end up with something that mostly works and needs constant maintenance and adjustment.

    First I would work on your grid feed. If you have 240V at the source, you can rewire to feed the house with 240 and use an auto transformer at the house side to generate 120V+120V. This would significantly increase your grid power, it will make life a lot simpler on the other side.

    For the PV/inverter side, both Outback and Schneider grid tie units can be configured to limit grid current and make up for peak loads from the batteries. If you can get the service up to 220v (4.4kW) plus even a smaller inverter of 5kW it should be plenty of power to run a house. Even squeeze by a smaller mini split, much better/efficient for heat than trying to tap heat from a hot water tank.

    For batteries, you really want solar batteries. The construction of these is different than deep cycle batteries. Even then, you do need to significantly oversize, depth of discharge is what kills lead acid. You can go with off the shelf deep cycles but don't expect a lot of years of service out of them.

    Your typical house load is somewhere around 5 to 10 kWh/day, so if you want to account for a couple of day to account for cloudy days, that is a much larger battery than the 5kWh you are proposing (closer to 20kWh to cover a day with reasonable DOD). It is much better to stick to some grid power and only use the batteries to make up for peak load, it will be much smaller, but even then I would look at around 10kWh batteries.

    For the solar thermal, not sure how much benefit it is to add it in. Maybe a simple one is to mount the thermal panels low on the side of the house in near vertical orientation (to limit over production in the summer). Go with a much larger DHW tank and install it above the thermal panels feed it with a side arm in thermosyphon from the panels. The outdoor wood boiler can also connect a similar way to the tank. You can then use the tank to run your two indoor air coils.

    This all sounds pretty complicated, a mini split combined with the wood stove is much simpler.

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