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Capacity requirement check, please (Solar needs)

  • 15-10-2020 5:57pm
    #1
    Moderators, Society & Culture Moderators Posts: 15,520 Mod ✭✭✭✭ Manic Moran


    Greetings.

    I need a sanity check on a system I'm looking to add onto my house for electrical supply. The goal is cost reduction, not total self-sufficiency. Had a chap come out to the house, he plugged various numbers from my electricity bill into the computer, and came back saying I needed a 13.5Kw system, but unless I'm doing something wrong, I almost never exceed 8kw draw.

    This is the worst day of usage of the year, each bar is a 15-minute-period measured in KwH of electricity consumption in my house.
    WjjquXh.png

    Unless I'm going insane, in order to get an idea of electrical draw at any instant, I take one of those 15-minute periods, take the KwH usage, and multiply by 4. So, at 6pm, my house would be drawing about 8.1Kw at any moment. Or, in other words, an 8Kw system would cover daylight electricity usage except when I need to buy a couple KwH in the peak of the evening. A 13.5KwH system seems to be utterly excessive.

    That's worst, about three months of the year look like that. This is "average", about 5 months.
    WeYKcmn.png
    (Least rarely breaks 1.5KwH over an hour's usage)

    Now, to make things even more complicated, the power grid around here works a bit like a battery on a monthly basis. If my panels make more power than I need at any time, it gets sent up the line into the grid, and I get 'credits' for the KwHs I've put in. Which means that in peak period, I get "back from that credit" any excess KwH I need to draw back in. (Or power usage during the night, when there's no solar being created). However, at the end of the month, the balances all reset. If I've 'credit', they don't carry over, so the fact that I'm likely making a lot more power than I need in February (It's sunny here) cannot be used to compensate for the heavy draw in July. On the other hand, the cost of installing panels just for the sake of covering the heavy use months seems inefficient.

    As a result, I'm thinking that given the power draw, I really want something around a 7.5-8kW system, or am I likely overestimating the production in daylight?

    Is my thinking about right, here?


Comments

  • Posts: 0 [Deleted User]


    Average total usage per day?


  • Moderators, Society & Culture Moderators Posts: 15,520 Mod ✭✭✭✭ Manic Moran


    Seems to be about 80kWh per 24-hour period in autumn. Peak months is about 110kWh, about 30kWh in the low-usage months.


  • Registered Users Posts: 2,691 ✭✭✭ graememk


    Seems to be about 80kWh per 24-hour period in autumn. Peak months is about 110kWh, about 30kWh in the low-usage months.

    80kwh in a day!? I only break 50 when charging my ev.

    Are you sure your calculations are correct or do you have a very high load all the time?


  • Posts: 0 [Deleted User]


    Seems to be about 80kWh


    Wow! I'd have a hard time keeping up to you.


    I'd recommend an 9kWp system for daytime offset or 20kWp with a 60kWh battery.


    I guess your chap was putting sugar on it.

    13.5kWp(ish) would suit your load profile. Souper ballparked, I'm not crunching numbers for free.ninety nine.


  • Registered Users Posts: 6,380 ✭✭✭ Alkers


    You say that you almost never exceed an 8kW draw, I'm guessing you're not in Ireland but having 8kW of panels doesn't mean they will provide 8kW all of the time, it will depend on their orientation to the sun, sunlight intensity etc, hence the installer has chosen the higher rating.


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  • Posts: 0 [Deleted User]


    Expect 80% rated as a nominal max in optimal conditions.

    Rule of thumb if you hang 1kWp you get 3kWh > 6kWh per day.
    The long formula is...














































    ...much longer.


  • Registered Users Posts: 2,945 ✭✭✭ blackbox


    You say your objective is to save money. Did the supplier indicate how many years the 13.5kW system would take to break even?


  • Posts: 0 [Deleted User]


    Solar is cheaper than utility.
    It's the bits in the middle that extend the payback.


  • Moderators, Society & Culture Moderators Posts: 15,520 Mod ✭✭✭✭ Manic Moran


    graememk wrote: »
    80kwh in a day!? I only break 50 when charging my ev.

    Are you sure your calculations are correct or do you have a very high load all the time?

    It’s a 380m2 house. Keeping it at a reasonable temperature in central Texas (four months of continuous 30-35 degree days) with eight tons of air conditioning capacity tends to draw a lot of power and really skews the average. Nov through March tends to be closer to 30kWh a day.
    You say your objective is to save money. Did the supplier indicate how many years the 13.5kW system would take to break even

    27 years. Which is what prompted me to ask the question. I get that electricity is cheap from the grid around these parts, but also we get a hell of a lot of sun to compensate. I’m suspecting an error in the calculations. I was expecting 6-12 from various readings.


  • Posts: 0 [Deleted User]


    Payback ought to be really fast. You'll have no problem qualifiying for the highest rate (import offset).
    Shop around. Non grant systems are better value, less chancers.

    You can happily throw 6kW at it and never worry about giving power away for free.


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  • Registered Users Posts: 2,820 ✭✭✭ air


    Improving the insulation on your residence is likely to be a lot more cost effective & environmentally friendly than installing PV.
    That said if the panels go on the roof they should provide some shade at least.


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