Size of system for grid tie?

peteoldmanpeteoldman Registered Users Posts: 5 ✭✭
I am new to this and am looking for the correct size system. I average 1200 kw per month.I am in NW Iowa and they bank my access power . I would like to get about 90% off my usage.I have bids from 4 contractors with different sizes from each from 7kw to11 kw .What would be the best solution? I am doing a multi pole mount and have no shade problems .Also will do seasonal tilt. Any help will be greatly appreciated


  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Size of system for grid tie?

    Using PV Watts, say the nearest site with similar weather patterns is Mason City, using 1kW (1,000 watt) array, and the rest defaults:

    A fixed array:
    "Station Identification"
    "Lat (deg N):", 43.15
    "Long (deg W):", 93.33
    "Elev (m): ", 373
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.770"
    "AC Rating:"," 0.8 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 43.2"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 9.0 cents/kWh"

    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.80, 100, 9.00
    2, 4.57, 104, 9.36
    3, 4.57, 111, 9.99
    4, 5.02, 114, 10.26
    5, 5.58, 124, 11.16
    6, 5.75, 119, 10.71
    7, 5.83, 124, 11.16
    8, 5.92, 128, 11.52
    9, 5.20, 112, 10.08
    10, 4.83, 112, 10.08
    11, 3.18, 74, 6.66
    12, 2.97, 76, 6.84
    "Year", 4.77, 1299, 116.91

    You would get (20 year averge) of 1,299 kWH per year per 1kW of array.

    For you, then we would look at:
    • 1,200 kWH per month * 12 months * 1/1,299 kWH per year per 1kW array = 11.1 kW array

    For most areas, a 10 kW maximum array fits in the "personal" solar that many PUC/Utilities carve out for home GT solar systems.

    If you are going to tilt your array 2-4 times a year, you can run the above calculations 2-4x with a different tilt and see how much that gains you (may not be that much unless you have a lot of snow on the ground during winter--cold weather and reflected light can do wonders for production).

    Before going down the solar GT road--Have you addressed all possible conservation measures yet? Insulation, double pane windows, high efficiency Heating/Cooling ground or air sourced heat pumps, energy star appliances, no computers/entertainment systems left on 24 hours per day, using laptops vs desktop computers, etc.?

    Also, you are aware that pure Grid Tied solar power systems cannot supply emergency power if your utility power fails (ice storms, etc.). You will still need alternative power (generator, off grid or hybrid solar power, etc.) for emergency power.

    And, regarding your power bill... In California, we must use TOU (time of use) billing. For me (an earlier adopter), I get paid ~$0.30 per kWH for summer afternoon power I generate. And pay ~$0.09 per kWH for off peak power.

    If you have flat rate billing (i.e., $0.10 per kWH)--it is easy to see how much GT solar may, or may not save you.

    TOU billing (i.e., peak from 11AM to 9PM summer), your choices becomes much more complex... Can you time shift your power usage or not (i.e., my older rate plan, I could time shift summer power outside the noon-6pm peak power window--Newer plan has peaks from 10am to 9pm--much more difficult to time shift power usage).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Size of system for grid tie?

    I'll be the Repeating Parrot here and say: Conservation is your best investment.

    1200 kW hours per month or 40 kW hours per day is of a size where some investigation is warranted to see where it's all going and if maybe some of it can be saved. Even with a GT system installed, reducing your consumption is beneficial: the less you use, the more you sell back to the utility.
  • TnAndyTnAndy Solar Expert Posts: 249 ✭✭✭✭✭
    Re: Size of system for grid tie?

    I'd want to know what "bank my excess" means. If that means the months you produce more than you need, they carry it forward to months you produce less, what happens if you constantly run a surplus....or at least have a surplus at the end of the year. Our utility cuts a check end of December if we have an excess, but I've heard some places, you simply loose it. You need to check it out with YOUR utility. If you do loose it, scale back the system so you don't run too much surplus, if any......hardly worth it to give it away !
  • solar_davesolar_dave Solar Expert Posts: 2,382 ✭✭✭✭
    Re: Size of system for grid tie?

    The key to sizing grid tie is to understand how your utility will handle your back feed. Does excess in your low consumption month roll forward to the higher consumption months? Do you get a dollar credit each month or annually? Do ou have Time of Use plans and how does the utility account those plans?

    In California it is done with direct dollar credit, in Arizona it is kWh credit within the TOU when it is generated, other states buy back at a generation avoidance rate , read low amounts.

    Understanding the utility rate plans is key to successful sizing.
  • peteoldmanpeteoldman Registered Users Posts: 5 ✭✭
    Re: Size of system for grid tie?

    In Iowa the power company credits my acc. forward infinetly and I understand it is on time of use with reverse meter up to a 10 kw system. I can also request a check if my acc. goes in the black. I have geo thermal heat and that has added quite a bit to my bill as dec. jan. feb. and mar.of 2010&11 I used 9592kwh. it was very cold all last winter and my geo was my only heat . Alliant energy just did an audit last month and were fairly pleased with my home as far as my energy use. I still have a few things to work on such as a solar air heater I am working on to help during the day so hopefully that will help cut my use. I have shut offs for thetv. andcomputer for the ghost loads and cfls in all the main lights. My water heater is electric but the geo has a desuper heater in it so
    it heatsmost of my water. Thank you all for all your help
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