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How much battery storage do I need?

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  • How much battery storage do I need?

    Hi all

    I wanted to setup a solar PV and battery system to run 2 side by side fridge freezer combo and another load. They are both operate at 240 volt.
    The fridges use about 115 watts per hour as measured by a watt meter over a 2 day period. The other load uses 300 watts but is on for only 6 hours per day.
    Total load is 4560 watt hours a day.

    I did my calculations using a 12 volt battery as my basis but would like to step it up to 48 volts if it is feasable.
    I allowed for: 2 days of autonomy.
    4 peak sun hours a day. (Brisbane Australia - could be more, but am conservative)
    0.67 system inefficiencies.
    battery temp multiplier.
    80% DOD for lithium batteries (this could change to Gel with 20% DOD depending on costs and feasibility again).
    I came up with a theoretical 4641Ah battery at 12 volts.
    My question is do I realistically need a battery that big if the loads can be run directly off the PV during the day when the battery is fully charged and only require battery power overnight.

    I look forward to any real world words of advice.

    Last edited by Schnipper; 03-09-2019, 07:45 PM.

  • #2
    My experience with battery storage is that it is most cost effective to use the smallest lithium battery that will cover your maximum overnight useage, and top up with a generator as required.

    It will be simple to run the numbers for your system.

    If you use new components you will be looking at around $15k aud for that size system.

    Example system: prices in AUD
    Victron easy solar 24/3000 $3200
    BYD BBox 5kwh $4600
    5kw PV and racking/ cabling $4500
    cabling/fuses/enclosures etc $1000
    Dunlite 3.6kva autostart genset $1800

    Of course there are a million options, that should give you a good starting point of what sort of system you need.

    You can save by using 2nd hand gear, just beware that you should be looking at pricing your energy on a per kwh rate over the life of the system.

    Last edited by tom rickard; 03-08-2019, 03:18 AM.


    • #3
      Thanks for the reply Tom

      I already have 2 kW of used solar panels I picked up for a reasonable price, waiting to be put to good use!

      And I have 6600 kW PV grid tied on my roof, but I want to experiment with an off grid project.

      I just feel that I might be over-sizing my battery calculations too much. I was looking at a site recently that recommended about 8 x 200 Ah 12 volt Gel batteries, for a 4-8 kWh daily load with 3 days redundancy. kWh-tubular-gel



      • #4
        You need to be clear about what you are trying to achieve.

        If you are looking at shifting load from your grid connect then you are best off getting the smallest batteries that will support your intended load. Don't worry about autonomy, it is irrelevant.

        If at the end of the sunlight hours your batteries aren't fully charged, top them up using your grid connect.

        The numbers are easy to crunch to see what is economical.