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Sizing a Solar PV system

  • 24-04-2022 12:57am
    Registered Users Posts: 3


    I'd appreciate opinions on my method for sizing a suitable PV system for a house.

    Step 1 : Look at your last 12~16 months of electricity bills to get an idea of what your average annual usage is. Lets call this ANNUALTOTAL

    Step 2 : If you have a day/night meter then calculate how much is used on night rate. Lets call this NIGHTRATE (this is = 0 if no night-rate)

    Step 3 : Look at your day-rate usage (or 24 usage) over the months of Nov-Jan. These are the months when PV production si at its lowest and these winter loads cannot really be met by PV. I usually take out 75% of this usage from the target PV production. Lets call this WINTERUSAGE

    Step 4 : The target PV production is now ANNUALTOTAL - NIGHTRATE - (WINTERUSAGE*0.75)

    Step 5 : Start playing with PVGIS (can't put in link :( )to see what size system (playing with size, orientation and roof-slop) will give you this production

    Thanks / Colm


  • Registered Users Posts: 2,730 ✭✭✭yankinlk

    Winter is a bit longer, February is winter for sure. I think you are on a good track tho.

    One thing I say is , what is the goal? If its to save money first.... then 1. Switch providers every year 2. Move to say night meter and shift loads to cheaper night rate. Develop these two habits first (plus pay attention to wasting of electricity and base load).

    Then, if you want to add solar from here, you are in a better place to start.

    I think an easier formula is to look at what is your monthly consumption of electricity in the summer months, ignore winter months and base the system size to cover that.

    Do you have any plans to add a battery to help store night rate in winter months to use during the day? That's another worthy calculation.

    Good luck, you are definitely doing the right thing calculating first!

  • Registered Users Posts: 3 carbontracking

    @yankinlk indeed I meant to include February in those winter months.

    My thinking is that you wouldn't size exactly for your summer consumption, you'd oversize somewhat for the months of May/June/July/August

    You're getting feed-in tariff (or at least will) for the export so there's les of a "loss".

    however by oversizing for those months you'll get higher production for the other 8 months where you know every kWh you produce will be gobbled up, thereby saving daytime rate elec from the grid.

    It's a question of getting the balance, keep the export to an acceptable level, not drastically oversizing the system, maximizing overall self-consumption.