Successful Tiny House Solar Setup

Steven Lake
Steven Lake Solar Expert Posts: 402 ✭✭
in General Solar Power Topics #1
http://tinyhouseoverthehill.blogspot.com/2013/04/solar-system-design-and-installation.html

Stumbled onto this while following one of my favorite tiny house groups. It's a successful tiny house setup using a very simple solar electric setup. :) Just thought I'd share this with you as I know you like examples of these kinds of setups and applications.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #2
    Re: Successful Tiny House Solar Setup

    Very systematic approach.

    The batteries are oversized - 32A charging current for 660AH bank will make it hard to keep them charged.
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  • Andy Roberts
    Andy Roberts Registered Users Posts: 7 ✭
    #3
    Re: Successful Tiny House Solar Setup
    NorthGuy wrote: »
    The batteries are oversized - 32A charging current for 660AH bank will make it hard to keep them charged.

    This was my first project. My logic is that 32A * 5.8 hours full sun / day = 185.6 Ah/day * 24V = 4.45kWh/day. Daily need is 3.6kWh/day so the batteries should receive the charge they need on a daily basis, but will hold enough charge for 2.5 days without sun and also keep the batteries above 50% DOD, assuming conservative use on cloudy days.

    In the interest of learning, I'd like to know what changes you'd suggest for this project.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Successful Tiny House Solar Setup

    Welcome to the forum Andy.

    Unfortunately battery charging is not a linear thing: you don't get 'X' Amps for 'Y' hours. You get a curve of Amps which goes down over time as Volts comes up. That last 20% takes longer than the first 30%, so to speak.

    As such you shoot for a higher peak current which gets you enough time to fully charge the batteries in the available hours of daylight.

    So to do a little standard analysis ...

    3.6 kW hours per day loads (With inverter draw? Efficiency derated?) / 24 Volts = 150 Amp hours. To give a practical reserve that needs to be 25% of capacity, so the battery bank would be 600 Amp hours. In that respect your 660 should be fine.

    Recharging at a 10% peak rate: 66 Amps (maxes out most controllers, MidNite Classic or OB FM80 would handle it) @ 24 Volts = 1584 Watts, less derating of panels: 2057 Watt array.

    Let's see how that stands up to he Icarus formula:
    2057 * 5 hours equivalent good sun (always be conservative at this) * 0.52 over-all efficiency = 5.3 kW hours AC per day. More than enough.

    As such you could probably sneak the current down to 60 Amps max, use an FM60 controller, and a slightly smaller array around 1870 Watts. That would work out to 4.8 kW hours per day.

    This may sound like overkill, but the sun does not always shine brightly every day, or for the same amount of time. Nor are loads consistent day-to-day. In general, people never regret having more panel than they need but they always regret having less.

    Immediately you will not see any problem, but over time a very small deficit charging factor will work its way in there and the result will be to lose time off your batteries' lifespan.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Successful Tiny House Solar Setup

    Nice article and design...

    Using our basic rules of thumbs to start with...

    660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,243 Watt array minimum
    660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,486 Watt array nominal
    660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,231 Watt array maximum cost effective

    The normal "break even" point for a home in much of the US is about 4 hours of sun minimum for 9 months of the year (winter, use a genset for make up power):

    960 Watt array (yours) * 0.52 over all system eff * 4 hours of sun per day = 1,997 Watt*hours (AC Useful) per 4 hour day...

    I have to go right now--But that is where I would start.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
    Re: Successful Tiny House Solar Setup

    rather interesting place. gives new meaning to mobile home too.;)

    btw, i see what appears to be insulation on the ground wire. the weeb needs to have contact with the wire and not the insulation. a small razor knife should do the trick so you don't have to cut the wire itself.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #7
    Re: Successful Tiny House Solar Setup

    In this case, I would go backwards.

    First, I would figure out that I cannot install more than 4 panels because there's no room. After reconciling this with the loads, I would figure that I really need more. I'd try to get very efficient solar panels. SolarWorld now makes 270W panels (same size as yours), which would give you 120W more compare to what you have.

    Then, I would look at batteries. IMHO, they need a certain current to charge well, which is about 10% of their capacity or more. A 330AH array (same as yours, but one string) would fit this criteria.

    I remember when I lived in Colorado, it was always sunny. The combinaton of panels/batteries would probably work fawlessly all year around except for 30-40 days. I would figure that I can survive one cloudy day if it is not too dark. With fast-changing Colorado weather that would be enough for me.

    I would also buy one of these cute 1000W Honda inverter generators, which I would use when it's too cloudy or when there's a problem with the system.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Successful Tiny House Solar Setup

    Actually most FLA's need 5% of their 20 hour capacity in current to charge well, but that is net: while loads are drawing at the same time you do not have this minimum figure if you calculate for it to begin with.

    Example:

    220 Amp hours of 12 Volt battery capacity requires 11 Amps. You could theoretically get that from 192 Watts of panel. But if there are loads drawing even 1 Amp then the net is 10 Amps in to the battery, or 4.5% not 5%. Thus we have the rule-of-thumb target of 10% which leaves a margin for supplying loads while still making the minimum rate+.

    I agree with NorthGuy about working backward in this case. It is like with an RV: only so much roof real estate, so spend the money on as much monocrystaline panel as can fit. Then work out a practical battery bank size from that. Then limit the loads to what it can supply.

    But that last one is the hard part, especially if you're not alone: "OFF" is an alien concept to some people. :p
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  • Andy Roberts
    Andy Roberts Registered Users Posts: 7 ✭
    #9
    Re: Successful Tiny House Solar Setup
    Cariboocoot wrote: »
    Unfortunately battery charging is not a linear thing: you don't get 'X' Amps for 'Y' hours. You get a curve of Amps which goes down over time as Volts comes up. That last 20% takes longer than the first 30%, so to speak...

    Let's see how that stands up to he Icarus formula:
    2057 * 5 hours equivalent good sun (always be conservative at this) * 0.52 over-all efficiency = 5.3 kW hours AC per day. More than enough...

    This may sound like overkill, but the sun does not always shine brightly every day, or for the same amount of time. Nor are loads consistent day-to-day. In general, people never regret having more panel than they need but they always regret having less.

    Cariboocoot,
    Thanks for the feedback. I have a couple questions:

    Regarding the charge current vs. time curve, do you have a reference curve or formula that you can point me to?

    What is the Icarus formula? The overall efficiency in that formula, is that including inverter efficiency, availability, MDOD, Temp & Discharge Rate, Coulomb efficiency and de-rating factor? If so, that number in my calculations is 50.7%.

    You can imagine that with this project, roof space is at a premium. I could have fit two more panels on the coupola, but I was weary of putting two panels at a different angle that the other four. Plus, we were reserving that area for solar thermal in the future.

    Andy
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  • Andy Roberts
    Andy Roberts Registered Users Posts: 7 ✭
    #10
    Re: Successful Tiny House Solar Setup
    NorthGuy wrote: »
    ...IMHO, they need a certain current to charge well, which is about 10% of their capacity or more. A 330AH array (same as yours, but one string) would fit this criteria.

    NorthGuy,
    Thanks for the advice. You are correct in the limited roof space. I actually started by calculating load, then battery bank then sizing the solar array with the intent of seeing how close that would come to the needed roof space. Per my calculations, the 960W was sufficient to meet the needs (also I did not see a 270W SW panel when I purchased the eqipment). I was unaware of any rule-of-thumb about batteries needing a current of 10% of their capacity. Even looking through my reference material I cannot find any such reference - do you have a source I can review to improve my next design?

    Andy
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  • Andy Roberts
    Andy Roberts Registered Users Posts: 7 ✭
    #11
    Re: Successful Tiny House Solar Setup
    niel wrote: »
    btw, i see what appears to be insulation on the ground wire. the weeb needs to have contact with the wire and not the insulation. a small razor knife should do the trick so you don't have to cut the wire itself.

    Niel,
    You can't tell from the pictures, but I did just that. I removed the insluation locally where the weeb camps attached to the ground wire.

    Andy
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  • Andy Roberts
    Andy Roberts Registered Users Posts: 7 ✭
    #12
    Re: Successful Tiny House Solar Setup
    BB. wrote: »
    Using our basic rules of thumbs to start with...

    660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,243 Watt array minimum
    660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,486 Watt array nominal
    660 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,231 Watt array maximum cost effective

    The normal "break even" point for a home in much of the US is about 4 hours of sun minimum for 9 months of the year (winter, use a genset for make up power):

    960 Watt array (yours) * 0.52 over all system eff * 4 hours of sun per day = 1,997 Watt*hours (AC Useful) per 4 hour day...

    Bill,
    Thanks for the feedback. I don't see the difference in your three calculations other than the result, perhaps a typo? Where does this formula come from?

    For this project the house is only to be used in the summers in Colorado so the design month is late April where the insolation is 5.8kWh/m^2-day. Plugging that into your formula I get 2,895Wh per day. My calculations came up with 3,834Wh/day. I'll have to spend some time checking the differences.

    Andy
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #13
    Re: Successful Tiny House Solar Setup

    Sorry Andy--Got too rushed... I have corrected the original and the copy you have made (should be 5%, 10%, 13%--I wrote 5% three times).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #14
    Re: Successful Tiny House Solar Setup

    We use 50% (Tony's / "Icarus") to 52% Raw panel to useful 120 VAC useful power as our overall system efficiency estimate.

    The details do matter--The 50% is a good starting estimate--But if you use power during the day, have a large inverter on 24x7, how the batteries are charged, etc. all affect the actual power efficiency.

    Measuring/logging specific gravity will tell you a lot about how well the batteries are being treated.

    Using a Battery Monitor could be helpful too (Victron is another good brand too).

    Some reading about batteries:

    http://www.windsun.com/Batteries/Battery_FAQ.htm
    http://www.batteryfaq.org/
    http://batteryuniversity.com/

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #15
    Re: Successful Tiny House Solar Setup
    Andy Roberts wrote: »
    I was unaware of any rule-of-thumb about batteries needing a current of 10% of their capacity. Even looking through my reference material I cannot find any such reference - do you have a source I can review to improve my next design?

    This is a very common recommendation. Look, for example, at the page 13 in this document.

    As I understand, with low charging currents you get very long bulk stage, and absorbtion starts only when batteries are almost fully charged. As a result, batteries don't get enough bubbling and electrolyte doesn't get mixed well enough. There could be other reasons.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #16
    Re: Successful Tiny House Solar Setup
    NorthGuy wrote: »
    This is a very common recommendation. Look, for example, at the page 13 in this document.

    I see your using Trojan, NorthGuy's link points to Trojan specs, who specifies wanting 10-13% of capacity for charging.

    The Specs for Solar World 240 watt panels contain NOCT values or Normal Operating Cell Temperature values which are a better place to start if 'real' operating panel output is desired. A NOCT output would be around 175 watts (or about 73% or STC rating of 240 watts) and this is before some loss in wiring and the charge controller. Though your likely cooler in Colorado much of the time.

    Bills 77 percent incorporates more than this though, including some charging before and after the hours of 'full sun'
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #17
    Re: Successful Tiny House Solar Setup

    I'd pretty much disagree that you need an inverter large enough to "to meet the maximum power draw from the house" calculating as every thing at once. Pretty much what will run at a given time. That's what breakers are for.

    And use is what you calculate for long term storage, what your Kil-A-Watt meter says you actually use, rather than peak use. You need the peak use, for sizing inverter size, but calculating a laptop that has potential to use 160+ watts and hour, makes for an unrealistically large storage, likely the same is true of hot plate usage, which would be 800 watts on high, but likely used at half that or less most of the time.

    I don't understand the refrigerator calculations, it only runs 4 hours on thermostat or do you only allow it to cool for 4 hours a day? 330 watts is a pretty large load, I don't think my 13 year old full size fridge(in house, not cabin) draws 180 watts, about 1200 watt hours a day last fall, running on thermostat. My guess is you could live with 1/2 of your battery bank, if you conserved on cloudy weeks, as I recall Colorado usually has short cloudy periods in the summer. My system in Missouri started with about 1000 watt array for the first couple years, and I used a small fridge and a window air conditioner (for 3-4hours) during the summer. I eventually increased the array to 1700 watt so I could run the 'air' longer during the day. Tiny cabin 10x16, 6" thick walls for insulation, with a sleeping loft (winter time and air stratifying in summer)
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    #18
    Re: Successful Tiny House Solar Setup

    The only thing that stands out is the shadow from one of the support struts on the ventpipe crossing the rightmost panel. During the season, is the sun low enough that even the pipe itself might cast a shadow on the panels? Maybe what I'm seeing are just reflections and not shadows...
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  • Andy Roberts
    Andy Roberts Registered Users Posts: 7 ✭
    #19
    Re: Successful Tiny House Solar Setup
    Photowhit wrote: »
    I don't understand the refrigerator calculations, it only runs 4 hours on thermostat or do you only allow it to cool for 4 hours a day?

    Photowhit,
    The 4 hours for the refrigerator is a ficticious number only used to get the right daily energy consumption value while leaving the max power draw in for the inverter calculations. I cannot argue the 330W value, you'll have to take that one up with Norcold.

    I've received some very good feedback on this forum. Reflecting on my first RE project I was probably overly conservative with the battery bank, so in the future that's an area for improvement. I've studied battery construction and failure modes enough to know that the worst thing you can do is over-dircharge them, so I wanted to make sure they lasted a long time and didn't die from sulfation in the first two years.

    Thanks,
    Andy
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  • Andy Roberts
    Andy Roberts Registered Users Posts: 7 ✭
    #20
    Re: Successful Tiny House Solar Setup
    PNjunction wrote: »
    The only thing that stands out is the shadow from one of the support struts on the ventpipe crossing the rightmost panel. During the season, is the sun low enough that even the pipe itself might cast a shadow on the panels? Maybe what I'm seeing are just reflections and not shadows...

    PNjunction,
    The struts aren't a problem in the summer when the house will be used. If it were an all year residence the struts, as well as the top of the vent pipe would have to be modified or moved to eliminate shaddowing.

    Andy
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #21
    Re: Successful Tiny House Solar Setup

    With a decent charge controller, over charging is not likely... Probably the most common form of battery homicide is "deficit charging".... A very "large" battery bank and a "small" solar array. And that is a common cause for sulfation problems.

    Taking measurements with a Hydrometer (logging temperature corrected specific gravity), a Kill-a-Watt type meter for per appliance daily load measurements, battery monitor, etc. are all on the path to better battery health.

    Regarding the refrigerator--you have different numbers to worry about--Typical full sized energy start refrigerator numbers: 120 watt average (compressor running, but only running ~25-50% of the time), 600 watts internal defroster heaters running, >1,000 watt compressor starting surge, 1.5 kWH per day, etc...

    You can add VA (Volt*Amps) measurements (related to Power Factor) for inverter/wiring design and sizing.

    In the "old'en" days... When people needed "more energy" for their off grid systems, they tended to add more battery bank capacity. That is almost always the wrong thing to do--but back then batteries were cheap and solar panels were expensive. Today, the panels are "cheap" and the batteries are "expensive"--So the right way, was and still is, to add more solar panels to "under powered"/over subscribed off grid power systems. In general, the owners will be happier and the batteries will probably last longer.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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