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Solar sizing

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  • Solar sizing

    Hi, I am at the point of deciding whether to move forward with a PV system or not. I have spent a lot of time reading various articles and posts, contacted my utility company and county regarding a owner installed system. What I would like to find out from some experienced users is realistically what size the system needs to be. Then I can determine costs and see if the payback would be worth it to me. I am in north central Florida and my utility allows for a net metering system. So the way I understand it, they will never pay for extra power over the course of a year only issue a credit that will equal $0.00 if I produce more than I use. And I will pay for any power that I use from them because I produced less than I needed, but at the standard residential rate. I have a great south and west facing area with no shade, and have determined the best way to tie into my homes electrical system. So basically I need to determine how big of a system I would need to reach the $0.00 per year goal without going to far over. This is my daily kwh energy use average from the past 2 years from my utility company.
    Month 2016 2017------- Month 2016 2017
    Jan 49 53 July 70 65
    Feb 48 49 Aug 87 71
    March 44 51 Sept 72 62
    April 45 51 Oct 61 61
    May 53 62 Nov 49 43
    June 63 65 Dec 50 49

  • #2
    Read "Solar Power Your Home for Dummies".

    Run PVWatts after reading all the help/info screens a few times. Take your annual usage after any conservation reductions and divide it by what the model generates for annual output in kWh per STC installed kW. That'll be a decent 1st approx. of the system size you'll need.

    Reduce your electrical usage as much as possible. That'll make a smaller system possible.

    Get very familiar with your rate tariffs to the point where you can verify/replicate your prior bills.

    You may find a 100 % off set of a bill may not be the most cost effective solution, depending a lot on how you're charged for power.
    Last edited by J.P.M.; 01-10-2018, 11:58 AM.


    • #3
      I believe you are saying, your average daily use ranges from 44 KwH to 87 KwH. Guessing here that a huge
      part of that is going to heat/air conditioning. If so, look into the equipment efficiency. Recent equipment may
      use a third or less the energy of older types, effective for both heating and cooling. Bruce Roe


      • #4
        I have had an energy audit done. Yes mostly cooling but already have a geothermal unit in place, so not much savings to be had. Also have a hobby shop that accounts for some energy use as well. So I want to size off my current numbers. Not a 100% sure why I had the 87 KwH that month. So I have been using 50-70 as my average numbers thinking that seemed like a realistic average. I will try the PVWatts calculator again.


        • #5
          1) Figure what your yearly kWh usage is or will be after any changes in appliances/economizing.
          2) Use PVWATTS2 online at the Nrel site to determine what size array is needed in your location to produce that many kWh/year.
          3) Figure about 90% of usage is the "sweet spot" of cost effectiveness for most rate plans.
          - mostly depends on how well your usage pattern matches your solar generation pattern. The better the match the higher you can go.

          Your friendly local solar installers do this sort of figuring all the time - get a few quotes from whoever is listed on SolarReviews in your area and see how well your numbers match.
          A good strategy is often to put in an oversize inverter and a undersize array and then add on more PV panels later if needed.
          BSEE, R11, NABCEP, >1200kW installed