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What size PV system/number of panels do I need?

  • 26-07-2021 9:57pm
    #1
    Pieterblurb
    Registered Users Posts: 61 ✭✭


    Hi All,

    I am gathering quotes for a PV system and one of the basic starting points is the size of the system or how many panels we should install. I don't get a good explanation of the sales people for their assumptions to calculate this.

    Based on last year's usage + recently purchased EV I would estimate our yearly usage to be 8,400 kWh/year. This would be average of 23 kWh/day. Based on 3,5 hours sunhours/day average a system of 6.6 kW would be required. Is this calculation and these assumptions correct?

    2 of the 3 system installers insist on 4.5 to 5 kW systems? Am I missing something?



Comments

  • #2
    graememk
    Moderators, Home & Garden Moderators Posts: 6,278 Mod ✭✭✭✭graememk


    First thing I'd say, is get an energy monitor for the house. Then you'll see your base load to run your house - will give you a far better picture of the actual energy use/profile.

    Then put as many on as you can! - Build for the bad days and don't worry about the export on the good.

    Getting into it a bit more,

    For trying to estimate usage with ev, Id remove the EV completely from any calculations - you'll be charging that on night rate. (If your not, get the day night meter in now). Anything you do charge from solar will be a bonus. (I put 50kwh into the car at the weekend, only really saved myself €2.50)

    Questions to ask yourself:

    • How many panels will actually fit?
    • What's your usage during the day like?
    • Is there people in the house during the day?
    • Can high power loads be put on during the day when solar is generating lots?
    • Look at the Solar quotes thread, I know its long, but it will show the trend of what has happened and what sort of quotes people are getting now.
    • Don't be tunnelled into a battery system, do get quotes for just solar panels and inverter too.

    without grants, its very hard for battery systems to pay for themselves. Best case scenario, you have a 4.8kwh battery, you'll get out maybe 4kwh a day, for maybe 2/3 of the year (220 days), at a day rate of say 17c - best case would be €150/yr saved.

    But some people have got a battery system installed with the extra grant for it for only a little bit more than a straight solar system.

    The rated wattage of the panels are in a Lab under ideal conditions, Some panel manufactures also give a real world figure - usually about 25% less!

    Energy prices are rising, but costs/payback have to be done on what the prices are now, not what they could be in the future. And if/when they do rise, That will accelerate your payback.

    TL:DR

    As many as you can afford, but dont over pay (rough guide, if you can get below 1k/kwp (kilowatt of panels), its a decent quote)



  • #3
    Pieterblurb
    Registered Users Posts: 61 ✭✭Pieterblurb


    Thanks for the great explanation @graememk. I am not too concerned about the payback of the system, but would be concerned about overpaying for it so the guide price is useful. I am going through the solar quotes thread indeed 😊.



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