600,000 Watt Hours Per Day (maximum) Off-grid system design help

fm.109fm.109 Registered Users Posts: 33 ✭✭
Hello to the professional community of NAWS. I am once again back to bring up a challenge which one of my friend has proposed lol. This is a project which we are trying to pull off and yes financial resources wont be a problem in this case however the system size being so big in comparison to what I have been querying about earlier, i feel that it is time to discuss this in a more elaborate manner in order to get the best out of this system.

I wanted to start this discussion off by asking what should be the flow chart of items to be used for example from panels to AC output, what items would be involved in a system this size and what should be taken care of when implementing this.


  • fm.109fm.109 Registered Users Posts: 33 ✭✭
    I am attaching this list of items and the number of hours of use. feel free to modify values in order to reduce the usage per day (maximum)
  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    I would guess you are doing too many "worst case" assumptions. At ~600 kWH per day, in California, that would cost close to USD $5,400 per month in utility power... Each of the guess rooms uses more power than a full off grid home (lighting, TV, ceiling fan, etc.).

    Just to give you some numbers to size a 600,000 WH per day system using our standard rules of thumbs. You can price the stuff out locally and see what it would cost you (just back of the envelope calculations).

    Battery sizing--2 days storage and 50% maximum discharge:
    • 600,000 WH per day * 1/0.85 inverter eff * 1/48 volt battery bank * 2 days storage * 50% maximum discharge = 58,824 AH @ 48 volt battery bank
    Then there is charging the battery bank. Two calculations: a) 5% to 13% rate of charge and b) simply your energy usage and hours of sun at your location:
    • 58,824 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 225,364 Watt array minimum (seasonal/weekend use)
    • 58,824 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge =450,729 Array nominal (full time off grid)
    • 58,824 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge =585,948 Array "cost effective maximum"
    Next, based on where you are located:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 65° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec
    Toss the bottom 3 months (use some generator backup)--5.23 hours of sun for February "break even month":
    • 600,000 WH per day * 1/0.52 typical off grid system eff. * 1/5.23 hours of sun = 220,620 Watt array (February)
    Unless you are near 100% occupancy and energy usage--I would guess the system is 2-4x larger than the nominal "actual" energy usage would be.

    If you have any possibility of getting utility power--I would certainly start there. Run for a few months or year--And see what actual power usage is. Or--Run a diesel genset and measure your power usage for a few weeks/month. Again, accurate measurements are probably going save you money.


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,190 ✭✭✭✭✭
    His numbers for refrigeration do not show cycling, they are 24 hours which is wrong.  The numbers for cooling certainly are not from decent mini-split heat pumps and a fast guess 2/3 higher than reality. Do the project in phases and as Bill wrote measure first and pick the best appliances/heat-pumps.  Start with a generator and quickly add the battery/Inverter charger. Then do the solar.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • fm.109fm.109 Registered Users Posts: 33 ✭✭
    Well you guys are right about the system being massively oversized and for this reason I am trying to acquire a copy of the yearly utility bill that the family is paying there at the moment. It would provide me with a picture of yearly electric consumption on a month to month basis that would certainly add a more realistic maximum value to this calculation. However while talking to the family this is how they explained their usage lol which is quite interesting knowing that the entire mansion along with guest house and other areas are heavily used almost all the time hence it would be best i believe if i can get a copy of their bill. They want the house to be off-grid completely and in that scenario i suggested that they change their current CFL to LED and also acquire more efficient Air conditioning units. I am not sure about whether there are any air conditioners that would work best with solar however if you guys do know of any such air conditioner then do let me know about it as it could really help shave off unnecessary watts.
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,190 ✭✭✭✭✭
    It is beyond ridiculous to go offgrid from an existing utility. I have heard a few reasons where it made sense and have consulted on them.
    Changing from CFL to LED is a minor savings of energy. 
    The other thing is living offgrid if it is going to be successful requires the owner or a household maintenance person understand and be able to manage energy. I do this for a few clients and it can be difficult from a distance. Much better if they have the ability to learn.
    Visit my webpage below or talk with the store here.
    Good Luck!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    If you really actually mean that you need a  600kWh/d system then you need some serious serious professional help. Thats enough to power a medium sized village.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,

  • fm.109fm.109 Registered Users Posts: 33 ✭✭
    I completely understand where you are coming from Dave and I agree going completely off grid from an existing utility line is kind of ridiculous however, there is a slight complication in this area (well for most parts of pakistan) and that is the grid is not reliable at all. There is a ridiculous amount of maintenance work on the grid almost every day causing blackouts for roughly 12 to 14 hours a day (sometimes even up to a whole day). Apart from this maintenance work we have general load shedding hours which is on a cycle of 2 hours on and 1 hour off. This has pretty much frustrated the people living there lol. They spent a considerable amount of money on their house and their small farm there which for them is not giving them any form of mental satisfaction. Hence they are exploring the idea of going solar by acquiring energy efficient items and replacing them with their current outdated tech. Even though the family does not have a financial concern but still they would like the best possible solution as to how solar can be the most effective part of their house. They already have generators for backups but it is not a reliable form of backup either as it can break down probably on crucial occasions like family events and all. Which is why I want to know what would be the best way to implement this plan and get a rough idea as to how much it can cost.
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,190 ✭✭✭✭✭
    The most inexpensive energy is the energy you don't use. By conserving and using the best heat-pumps and refrigeration you can make this happen.  If you have 4 hours a day of reliable grid you can design a battery only inverter/charger and easily solve this problem. At that point you can add solar if it makes sense. The 4 + hours of grid will charge the battery.

    Probably the most important thing is a local source of quality batteries. If you have a reliable importer who is local that will be best.
    I can drop ship from California anywhere in the world but it is better for you if the importing is someone local who knows the customary rules that always seem to change at the wrong time.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    You should find a knowledgeable engineer to design and oversee the installation/qualification of the system. And not all engineers are created equal. Some of the discussions on an off grid system that is smaller than what you are looking at:



    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • fm.109fm.109 Registered Users Posts: 33 ✭✭
    Yes not all engineers are equal and that i completely agree! Would 4 hours be enough to charge that much amount of batteries? Wont that be a bit harsh on the battery bank?

    I am also looking online for people or case studies on 200kw installations and have found some too hence I am contacting them as well to see how their system is responding since installation and what lessons have they learned up till now. Hey BB can you also help out if battery banks should be in series only or can they also be used in parallel. I remember that too many batteries in parallel would require fuses and added wiring cost plus maintenance.
  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    You are talking about a very large installation... You will be limited in your selection of batteries (deep cycle, industrial, Li Ion, NiFe, etc...)--Chemistry, size of cells (AH @ 2 volts vs AH @ 4/6/etc. volts) etc... In general, I prefer larger AH batteries (l.e., for a "golf cart battery" you can get 1x cell @ 2 volts & 600 AH, or 3x cell @ 6 volts and 200 AH, or 6x cell @ 100 AH and 12 volt, etc.--All "batteries" store the same amount of energy E=Volts*Amp*Hours and are about the same size and weight).

    When you are working with Flooded Cell Lead acid batteries--Every parallel string is another set of cells to monitor for specific gravity/water levels (1x 48 volts battery = 24 cells; 2 strings is 48, 3 strings is 96, etc.). And every string is another set of wiring to check.

    From an over all point of design--Roughly you can do back of the envelope calculations taking about a 10,000 AH @ 48 volt battery bank--How many cells/batteries, how they are wired, etc. will not dramatically affect the overall cost of the system (for a starting estimate).

    Once you have a paper design and cost "approval"--Then start picking the specific hardware needed to implement (x number of brand z inverters, y number of xx AH * zz voltage battery bank--Forklift or other deep cycle, etc.).

    You will (hopefully) find a number of products that can meet your needs. And then it will be a trade-off between cost, reliability, and easy of use...

    Regarding reliability, cost, and ease to purchase/use/maintain--The old engineering saying is you can optimize any two variables. (i.e., you want reliable+ease--You will pay a lot for it. If you want low cost and high reliable, the system may not be easy to maintain, etc.).

    Your proposed/estimated system is way large than the typical rules of design we use here... What will be optimum for you--It will take a lot of experience and research to figure it out for you. What is available locally, costs, engineering, support, etc... are all going to be different answers for you than somebody in the US or Europe.

    And you will have secondary "issues"... The link I gave to the larger 3 phase system--All sorts of wiring/circulating current/grounding/etc. issues. With large industrial batteries--They can have high leakage current... You could end up with upwards of 2% of the battery energy "lost" to self discharge (that means your array may need to be ~20% larger to make up for the losses). Costs of distilled water for flooded cell batteries may not be small, etc. You may be looking at 120 to 240 VDC input AC inverters and battery chargers--240 VDC and 1,000's of amperes of short circuit current can be very scary if something goes wrong, etc...

    Just to give you an idea of what just a couple of cells of a large battery can do with a short circuit (submarine battery):


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