Newbie Off-Grid Solar questions

WalabyWalaby Registered Users Posts: 4
First off, Im a newbie and still researching and learning.  My intent is to learn as much as I can, but also probably hire someone to do my installation work. I think I have a fair sense of when it is time to hire a professional, but I like to know enough to make sure I get the right support.
So, I want to build a small off-grid solar system for my shop.  The shop currently has a 200 AMP service coming from the main house.  Intent when installed was to use that shop 200 AMP to feed things like my RV and eventually another shop.
I want to have a 240 Volt system, as my well pump is connected to the shop, and I also have a 240 Volt air compressor that will eventually be installed.  I've been looking at 120/240 V inverters.  I was thinking maybe something along this line
I plan on starting out small and maybe expanding over time.  Start off with maybe this inverter, 10x280W panels (used) and a couple of 100AH batteries. 
Doing the research I have thus far, I understand that neutral/ground bonding is required at the first entry point, which is great if I connect to the house.  Since my house has the first entry point, and I plan on having a disconnect at the panel in my shop, when the shop is disconnected from the grid (via either disconnect switch or throwing the main breaker feeding the shop), do I need to make sure the inverter feeding the shop has neutral/ground bond? Im assuming yes, since the inverter is now the first point of entry for the shop.
The way the grid connection is wired currently is I have 400 AMP service to the meter on side of house.  I have 200 amp feed off that for the main house, and another 200 AMP feed to the shop. The main breaker for the shop feed is at the house (on outside wall, next to meter).  I had originally planned on just throwing that breaker to disconnect the shop, but Im thinking a disconnect is the proper way.
Also, is it acceptable to ground the inverter to the shop's panel grounding bus?  Im assuming that the ground from the house to the shop via the existing connection is still present even if disconnected via disconnect switch or breaker thrown?
Hope this makes sense and appreciate insight and learning opportunity.
Thanks
Mike

Comments

  • PhotowhitPhotowhit Solar Expert Posts: 5,878 ✭✭✭✭✭
    Hi Mike, Welcome to the group!

    What are your goals?
    If you're trying to save money, a grid tied system without batteries is likely your best bet.

    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.
  • WalabyWalaby Registered Users Posts: 4
    edited August 5 #3
    Hi
    Not just trying to save money (although if that helps, great).  Initial goal is (a) learn by doing in a controlled application and (b) use as a blueprint for powering a future larger shop potentially 100% solar powered.
    Thanks
    Mike
  • BB.BB. Super Moderators, Administrators Posts: 32,741 admin
    edited August 5 #4
    Welcome to the forum Mike.

    First is to understand the off grid solar power is expensive. For a reasonable system, in a sunny location, and used 9+ months a year--You are looking at $1-$2+ per kWH with all costs, battery new/replacements, etc. over 20 year life. Add a backup genset, and just for fuel costs, that is another $1+ per kWH (not including genset, oil, maintenance, etc.).

    As Photowhit asks--Your goals are very important. And looking at costs and for a "balanced design" (power needs, surge current, costs, etc.)--Selecting the "right" loads/equipment is critical too. More or less, you want the most efficient/smallest loads you can use for your needs. It is almost always cheaper to conserve energy than it is to generate it.

    For example, motors (and heating/cooling) are power hogs. Using equipment that has "inverter" (inverter fridge, inverter A/C) or using VFD (variable frequency drives) both can be more efficient (depending on motor type/design) and the inverter/VFD hardware generally reduces power needs (reduced RPM during low power operations) and surge current (I/VFDs are typically "soft start"). Much more "solar friendly".

    I like to start with "back of the envelope" calculations. And there are two ways to go--One is to estimate/measure your energy needs and design a system to meet those... Or the second is to start with a major component, size the rest of the hardware around that component, and then estimate how much energy it will produce and see if it will meet your needs/goals.

    Generally, I like to start with the battery bank. It is the "heart" of the system. The battery has to supply all the energy needed (bad weather, over night, surge loads, etc.). The rest of the system is designed to "keep the battery bank "happy")--Enough solar to keep battery charged, Large enough battery to support the loads, etc.

    Anyway, in your case, you picked the size of the solar array. The math is the same and using simple rules of thumbs, we can do a quick paper system and size the balance of system and estimate how it will perform.

    I will take a punt on the other questions (sizing of main panels, disconnects/transfer switches, grounding). Those are all important issues to address--But lets start with the basic solar power system first to avoid talking about too many design requirements at once.

    Your "starting points"--2,800 Watts of solar, a 6 kWatt AC inverter, A couple 100 AH batteries, and 200 Amp service around Atlanta Georgia?

    Since a major goal is to keep the battery bank "happy", I will start with the 2,800 Watt array. And size an off grid system around the array. This is going the limit the sizes/capacity of the rest of the system (smaller A/C inverter, smaller battery bank--A major expense. And less overall energy harvest).

    Using rules of thumbs to size a system quickly... A 2,800 Watt array to size the battery bank. For a full time off grid system, the battery bank should be charged at least, at 10% to 13% rate of charge:
    • 2,800 Watt array * 0.77 panel+charge controller deratings * 1/0.13 rate of charge * 1/48 volt battery bank = 346 AH @ 48 volt battery bank nominal
    • 2,800 Watt array * 0.77 panel+charge controller deratings * 1/0.10 rate of charge * 1/48 volt battery bank = 499 AH @ 48 volt battery bank suggested maximum for off grid application
    To pick even numbers... Lets design with a 400 AH @ 48 volt battery bank (for example, a "starter bank" of 8x 6 volt @ 200 AH "golf cart" deep cycle flooded cell batteries in series x 2 parallel strings for 48 volt @ 400 AH using a total of 16 "golf cart" batteries). You can choose other battery types (such as AGM, Li Ion, etc.)... But the general design will be similar.

    And such a bank, powering your loads for 2 days without sun (bad weather), to 50% planned discharge (for longer battery life) will support daily loads of:
    • 48 volts * 400 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 4,080 Watt*Hours = 4.08 kWH per day for two days of bad weather
    And a fixed mount 2,800 Watt solar array around Atlanta Georgia will produce:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Atlanta
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 56° angle from vertical:
    (For best year-round performance)

    JanFebMarAprMayJun
    3.55
     
    3.96
     
    4.86
     
    5.24
     
    5.36
     
    5.00
     
    JulAugSepOctNovDec
    5.18
     
    5.01
     
    4.82
     
    4.92
     
    3.96
     
    3.52
     

    Toss the "bottom" three months (assume backup genset if needed) we get an average of 3.96 hours of sun per day in February. The average harvest would be:
    • 2,800 Watt array * 0.52 off grid AC system FLA batteries * 3.96 hours of sun (Feb) = 5,76 6 Watt*Hours per day February harvest
    Now, that assumes you use every WH harvested per day--Some days will be more, some will be less, and you need power when you need it.. And will never use 100% of your harvested power per day. Generally a good "derating" for "base loads" (loads that must run every day such as lights, refrigerator, work computer, etc. vs optional loads such as clothes washing, vacuuming, etc.) would be to use 50% to 65% of planned harvest:
    • 5,766 WH per day (Feb) * 0.50 base load derating = 2,883 WH of base loads per day (50% Feb)
    • 5,766 WH per day (Feb) * 0.50 base load derating = 3,748 WH of base loads per day (65% Feb)
    And there is sizing the battery bank... More or less, with FLA batteries the AC inverter's Watt rating should be around 1,000 Watts per 100 AH of battery bank maximum (@ 48 volts). For a 48 volt @ 400 AH FLA battery bank:
    • 400 AH * 1/100 AH per 1,000 Watt inverter = 4,000 Watt AC inverter max
    • 4,000 Watt max inverter / 2 = 2,000 Watt Inverter "comfortable" size for FLA battery bank
    The max continuous output of a 4,000 Watt @ 120/240 VAC inverter is around (assuming srt(2) for 120 VAC max current):
    • 4,000 Watts / 240 VAC = 16.7 amps max continuous @ 240 VAC
    • 4,000 Watts * 1/240 VAC * srt (2) or 1.414 = 23.6 amps (estimated) max continuous per 120 VAC leg
    Looking at the battery bank of 4,080 WH per day (no sun) or the 2,800 Watt array in February of 5,766 Watts, you are looking at supporting a 4,000 Watt load for around 1 hour
    • 4,080 WH per day battery power / 4,000 Watt inverter load = 1.02 hours per day from battery
    • 5,766 WH per Feb Day harvest / 4,000 Watt inverter load = 1.44 hours per day from Feb solar array
    And a 4,000 Watt inverter will usually surge 2x rated power--Or 2*4,000 Watt inverter = 8,000 VA (or Watts) max for a few seconds to a few minutes (depending on inverter brand/model).

    The above design(s) are just based on rules of thumbs for an "average" home/cabin. Your shop needs will be different (air compressors are difficult to start, take lots of energy to run) vs a drill press, mill, or lathe (with soft start/VFD) which may start easy(ier) and run for longer (or shorter) periods of time during cuts/repositioning work.

    Anyway--Some guesses and "numbers". Your thoughts?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,186 ✭✭✭✭✭
    100% solar powered in Georgia climate, with a 200 amp service? Probably well over $150,000.

    Nothing wrong with doing something small to learn.
    I never read anything about backing up power in your posts, and that would be a reason for a battery, or a genset for that matter.

    Read all the posts here again and as Bill said, Your thoughts?


    "we go where power lines don't" Sierra Nevada mountain area
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • WalabyWalaby Registered Users Posts: 4
    edited August 5 #6
    Clearly a lot to unpack with Bill's thorough response - I thank you sir, I have some homework to do for sure.
    WRT the 200 AMP service, I guess I wasn't clear.  I have no intention of trying to achieve anywhere near 200AMP from this little setup in my shop.  I only provided the 200 AMP info so responders could see what the current grid setup is.  No way I would expect to support that level of need, nor do I need that for what Im trying to do.
    Couple additive comments
    1.  Rationale for 240 V is I would like to power my well pump, which is 240.  I plan on measuring the draw, but obviously well pump doesn't run continuously.
    2.  Also plan to run LED lights (two or three) and three or four 120v outlets.
    3.  If capacity allowed, I would like to run the 240 V compressor, but maybe I leave that for later.  Not a necessity.
    4.  My rationale for the larger inverter is my "assumption" (yes, I know what assUMptions do) that if I went larger than I needed initially, it would allow for expansion as I learned more and added more panels.  If that's not the way to look at, thanks for the clarity.
    I have the 10x280 watt panels already, so just trying to put them to some sort of use and live and learn.
    I have to digest Bill's response and clearly do some re-cyphering.
    Thanks
    Mike
  • PhotowhitPhotowhit Solar Expert Posts: 5,878 ✭✭✭✭✭
    It use to be very hard to "grow" a system, but easier today with LiFePo4 batteries. While still better to have batteries all the same age it's easier to rationalize buying/expanding lithium battery bank.

    Another nice thing about lithium batteries is they don't suffer from voltage drop with high energy demands, so a system with your initial design, might well run your compressor on sunny days with no problem. The battery bank would be engaged to 'kick off' running the compressor and the solar support might run it without serious draw from the battery bank.

    Looks like the inverter has a built in 120 amp MPPT charge controller. That should support a very good size array, roughly 6000 watts, but might see if it can be 'over paneled' Closer to 8000 if it can. I know nothing of the brand, but says it's a low frequency inverter, typically more dependable and higher surge, but it doesn't state the surge...
    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.
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,186 ✭✭✭✭✭
    I was just responding to what I read because it is possible to do this. I kind of have that experience...

     " The shop currently has a 200 AMP service coming from the main house.  Intent when installed was to use that shop 200 AMP to feed things like my RV and eventually another shop.

    Good Luck to you!
    "we go where power lines don't" Sierra Nevada mountain area
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • WalabyWalaby Registered Users Posts: 4
    edited August 7 #9
    I was just responding to what I read because it is possible to do this. I kind of have that experience...

     " The shop currently has a 200 AMP service coming from the main house.  Intent when installed was to use that shop 200 AMP to feed things like my RV and eventually another shop.

    Good Luck to you!
    Yeah, I can see why you came to that conclusion... just wasn't my intent.  My intent was to at least share what my grid connectivity was, so the original question regarding neutral/ground bonding had the full context.  Sorry for that.
    Rather than try to tie into the existing grid panel and have a disconnect switch,  think I will have a separate panel, fully grounded and ground/neutral bond in inverter.  Move circuits from grid to solar based on capacity.
    LiFePo4 batteries were actually what I've been looking at using.
    Thanks
    Mike
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