Need help on deciding how many KWH I trully need for in grid installation.

Yes I am very new, and planning to buy (finance) a system. But I have received different quotes from three different companies. Each company has given (suggested) me different sizes to be installed, from 6k to 10k systems.

I have started to purchase LED lighting for the entire house. Hopefully that would cut down half usage on lighting.

If someone can help me out on correct size that I would need. I would put the information that I believe I need to help me out.

Total usage for this past 12 months (year)was 15,770 KWH, total cost was $2,193.94 .

I live in Yuma County, Arizona. The information on sun length is as follows.
Location 85349
Weather Data Source YUMA MCAS, AZ (TMY3)
Latitude 32.65° N
Longitude 114.62° W

Date Sunrise Sunset Length Dawn Dusk Length
Today 05:59 19:29 13:30 05:32 19:55 14:23
+1 day 05:59 19:28 13:29 05:33 19:54 14:21
+1 week 06:03 19:21 13:18 05:38 19:47 14:09
+2 weeks 06:08 19:13 13:05 05:43 19:38 13:55
+1 month 06:18 18:53 12:35 05:54 19:17 13:23
+2 months 06:38 18:13 11:35 06:14 18:37 12:23
+3 months 07:02 17:42 10:40 06:37 18:08 11:31
+6 months 07:28 18:18 10:50 07:03 18:43 11:40
Notes: Daylight saving time, hours don't change.

I have Tile Roof slope East to West.

I have been told that 7 hours to 5 hours a day would be used to calculate.

Also if anyone knows what the Cost/Watt installed would cost?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 28,184 admin
    Re: Need help on deciding how many KWH I trully need for in grid installation.

    Before choosing your solar power--Really nail down the conservation stuff.

    LED (and other conservation) gives you 2x the payback. First there is the actual reduction of power usage... And then the second time (in summer) with less heat your A/C has to move outside--Reduction in A/C power bills.

    Other things--Do you have natural gas, propane, or electric water heater? In a warm to hot climate, heating water with a desuperheater on the A/C system can give you virtually "free" hot water during hot weather.

    Or, you cant get heat pump based water heaters (or stand alone heat pump water heater) that will be ~2-3 times more efficient than a pure resistive electric water heater--Plus it gives cold/dehumidified air out as "waste".

    Once you have done a fair amount of conservation (including checking the amount of insulation in your attic and around your cooling ducts) and reviewed the efficiency of your A/C or Heat Pump system (if your A/C is over ~10 years old, a new system may be much more energy efficient)--Then you can estimate (or measure) your (hopefully reduced) power needs.

    And, you need to check with the utility and see what sort of metering plans they have. That can make a huge difference in the rate of return (I have Time of Use metering, I get paid $0.30 per kWH for summer afternoon power generation and "buy" off peak power for ~$0.10 per kWH). And I "bank" my power--I use more power than I generate during Winter, and use the "banked" power from Summer.

    I am not quite sure I understand your "hours of sun"... What we typically use here is Hours of Noon Time Equivalent Sun (Noon-time sun is around 1,000 Watts / square meter). Using the Solar Electric Handbook for Yuma AZ, fixed array, tilted 57 degrees from vertical (or 33 degrees from horizontal), we get:

    Jan 4.78
    Feb 5.42
    Mar 6.17
    Apr 6.46
    May 6.78
    Jun 6.73
    Jul 6.28
    Aug 5.96
    Sep 5.75
    Oct 5.56
    Nov 5.00
    Dec 4.58

    So, if you had a 6 kWatt array in July, it would produce (average year):
    • 6.0 kW array * 0.77 Panel+inverter derating * 6.28 Hours of Noon-Time sun per day = 29 kWH per day
    • 29 kWH per day * 31 days July = 899 kWH for the month of July

    You can do that calculation for each month and see how that overlays your power usage and your utilities "Net Metering" rate plan.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 28,184 admin
    Re: Need help on deciding how many KWH I trully need for in grid installation.

    I should add, the above was for a south facing array. You can do the same calculation with an East or West facing array (or even do both East and West, and adding the daily output together).

    You can also adjust Array Tilt to favor summer or winter production too...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jebattyjebatty Solar Expert Posts: 56
    Re: Need help on deciding how many KWH I trully need for in grid installation.
    Before choosing your solar power--Really nail down the conservation stuff.
    Ditto. As one who just did what you want to do for an array installed October 2013, with our all electric house (but wood stove for primary heat) my wife and I already had done lots to reduce our grid power consumption: CFL bulbs, turn everything off when not being used (especially lights), power strips for things that stay "on," for example. Then I averaged the last couple of years of grid usage from the info on our electric bill. Our average usage still was about 12,000 kwh/year.

    Next step was wishful and hopeful thinking: we set a goal to reduce usage another 25% to 9000 kwh/year (includes nearly 6000 kwh/year for supplemental electric heat in the basement). Next step was to use PVWatts to find out how much PV I needed to supply 9000 kwh/year on average for my location. The result was a 6.5 kwh fixed array system, but I made an error in using PVWatts and somewhat undersized my system. No matter, as our goal has remained not to use more kwh on average than we produce.

    Regardless of the PVWatts error, the system we installed is coming close to the 9000 kwh/year production for its first year of operation. See 6-5k-System.

    To tackle the electric heat usage, I am building a small active solar hot air collector using the downspout design and will have that in place by the heating season. We also have switched out nearly all of our lighting to LED's. Careful sale shopping plus utility rebates reduced our per bulb cost to $2.00 each. Our goal remains to make the house usage carbon net-zero: a clean, fresh breath of air for everyone.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Need help on deciding how many KWH I trully need for in grid installation.

    15,770 kW hours per year.
    Divide by 12 = 1314 kW hours per month.
    Divide by 30 = 43 kW hours per day.

    Hence why you are hearing about conservation first.

    If you were to try to offset all that all year 'round with the equivalent good sun averaging 5.5 hours per day you'd need a system that can produce 7.8 kW - definitely 10kW array territory.

    If you can reduce your consumption first then the payback for any system is improved.

    Dividing your annual cost by the total kilowatt hours used shows an electric rate of fourteen cents per kW hour. Pretty hard for solar to compete with that, even with tax credits and other financial voodoo. I'm not sure solar makes any sense from a fiscal point of view in this case. Conservation nearly always does.
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Need help on deciding how many KWH I trully need for in grid installation.

    Here in Arizona, the best you can do is about 1,600 hours per year of effective full sunlight including all the losses. With your east/west orientation, you are down to about 1,430 hours. 15,770kwH / 1430 = 11.0kW. Figuring in some conservation reduces that and the last 10% of your usage is generally not cost effective to do with solar - puts you at about 9Kw array size which can be handled by a 7 or 7.6kW inverter and is the max you can backfeed through your 200A service (without upgrading it).
    I don't know about the Yuma area, but around here the going rate for this size GT system on a tile roof is about $2.65/watt
  • LuzaLuza Registered Users Posts: 3
    Re: Need help on deciding how many KWH I trully need for in grid installation.

    Thanks I will study the information, advise I was given here and do some adjustments. I understand I must find a way to conserve energy first. Currently I have electric water heater, and electric stove. I Need to do research on those to eliminate them. A/C kills us down here with power consumption in the summer usually 3 to 4 months a year. I have learned more about power consumption in the last two weeks than I though I would know.
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