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jaybell
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March 9, 2024
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Solar battery capacity - how to work out what's needed

  • March 9, 2024
  • 12 replies
  • 1703 views

I’m trying to work out what capacity batteries I need for an existing  solar PV array….my average consumption in the  5 month October-February period is 12kW per day and for the other seven (summer) months its 7kW/day. 

generation works out at 2000 kW per year obviously heavily loaded to April-September.

so what suggestions for the battery capacity ( assuming a 20% margin )?

    Best answer by BPLightlog

    Updated on 19/05/25 by Ben_OVO

    Hi all, sorry - busy day

    I will start by saying, whatever you determine, you can always use more and now with retrofit batteries getting a vat benefit (lower rate), it’s a good time , although no one is sure how long this is for. 


    As for capacity, it will partly need to be coordinated by the inverter as that can set which batteries and what/when you can add. 
    Another element is if you are going to let the batteries ‘lull’ over the worst winter months or if you might charge from the grid (if you can get a better rate overnight - there are tariffs around).

     

    Besides your consumption (a good starting measure), it’s also good to work out a ‘max’ draw .. that is what are you going to switch on at the same time (kw). The system (check your inverter/control) will have a maximum output it can deliver and therefore even with full batteries and solar generation, you might have times when you’ll draw from the grid as well.

     

    Take everything into consideration and get an estimate figure. Many batteries add in 2.4kW chunks. Depending on your budget, 4.8kW is a reasonable starting point but 9.6kW is better and so on. We began with 4.8 but now have 14.4 (I’d still like more 😬)

    12 replies

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    juliamc
    Rank 20
    juliamc
    Rank 20
    March 10, 2024

    I definitely need help as a fog descends every time I approach this subject. 

    Rather than looking at my daily usage to work out what battery and inverter I need, I thought I might try to calculate how long a full charge would last on a given day, with various sizes of kit. Does this make sense ?

    My heaviest electricity usage day this winter this was 18th Jan. As an experiment I ran the heat pump all through the night, and with other usual stuff too I used 48 kWh, and the outside temperature went from -7 degC to +3 degC

    This graph is from my energy bill - I’m on Octopus Go so have 4 hours at a cheap rate (red line):

    18th Jan 48 kWh consumption

     

    I’m not intending to buy battery storage to get me through this amount of use of course !!! and this would have been a very low solar day too.

    The 16th Feb was an unusually mild day, outside temps between +9 degC and +14 degC:

    16th Feb 15 kWh consumption

    I’d hope a battery would get me through this kind of day…

      BPLightlog
      Super User
      BPLightlogSuper User
      Super User
      March 10, 2024

      I definitely need help as a fog descends every time I approach this subject. 

      Rather than looking at my daily usage to work out what battery and inverter I need, I thought I might try to calculate how long a full charge would last on a given day, with various sizes of kit. Does this make sense ?

       

      Yes, that’s a decent view to have a look at

      My heaviest electricity usage day this winter this was 18th Jan. As an experiment I ran the heat pump all through the night, and with other usual stuff too I used 48 kWh, and the outside temperature went from -7 degC to +3 degC

      This graph is from my energy bill - I’m on Octopus Go so have 4 hours at a cheap rate (red line):

      18th Jan 48 kWh consumption

       

      I’m not intending to buy battery storage to get me through this amount of use of course !!! and this would have been a very low solar day too.

      The 16th Feb was an unusually mild day, outside temps between +9 degC and +14 degC:

      16th Feb 15 kWh consumption

      I’d hope a battery would get me through this kind of day…

      Again, yes, you might assume so. 
      Unfortunately all of the calculations are a little on the guesstimate side as you never know exactly what solar activity will give, nor exactly what draw will be required. 
      Given the OE Go pricing you could also recharge batteries at that time of course and it would be a huge system to cover every possible eventuality. Most don’t look to fully cover the worst day so perhaps also look at an average during say October. 
      But as long as you have your system (particularly the inverter) which allows more batteries to be added later (not all do) then I’m sure it will be a significant benefit. 
      Don’t forget that any good installer will also look to plan best coverage depending on location, shading etc

      Bring me Sunshine ~ E&E {Solar PV, Battery Storage, Hybrid EV, ASHP and Home Assistant automation}
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