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Battery Bank Sizing Worksheet

Use this worksheet to determine the size of the battery bank needed to provide power during periods without electrical generation.  This is called "days of autonomy."  If you depend on solar array generation only, the periods of concern are during the night and during cloudy weather.  If you have other sources such as wind, hydro, or engine-generator backup your storage needs will be less.   You need the total amp-hours per day used by all the AC and DC loads in your system from the last line of the System Loads Worksheet to begin this calculation.

  1. Your average daily amp-hour requirement from the last line
    of the System Loads Worksheet.

  2. Maximum number of consecutive days without power generation that you need
    to supply your loads (days without solar, wind, hydro, engine-generator, etc.)

  3. Multiply line 1 by line 2.  This is the amp-hours or storage you need.

  4. Maximum depth of battery discharge you wish to subject your battery bank to.
    Each discharge takes a certain amount of life out of the battery.  The deeper
    the discharge, the greater the loss of battery life.  Do not exceed 80% (0.80).

  5. Divide line 3 by line 4 and write result here.

  6. Select the proper temperature correction factor from the list below for the
    coldest temperature your battery bank will reach.  Consider such factors as:
    heat and insulation in the battery room and if the heat will be lost when you
    rely on the battery storage.  Keep in mind that the battery plates and
    electrolyte temperatures are the important parameter here.  Brief dips in air
    temperature of only a few hours will not immediately cause an equal
    temperature dip inside the battery cells.
        80 F    27 C    1.00         40 F     4 C     1.30
        70 F    21 C    1.04         30 F    -1 C     1.40
        60 F    16 C    1.11         20 F    -7 C     1.59

        50 F    10 C    1.19

  7. Multiply line 5 by line 6 and show result here.  This is the TOTAL battery
    capacity required in amp-hours.

  8. Enter the amp-hour rating of the proposed battery cell you want to use.

  9. Divide line 7 by line 8 and round up to the next whole number.   This is the
    number of battery cells wired in parallel required.

  10. Divide the nominal system voltage (12, 24 or 48 V) by the battery cell
    voltage (usually 6 or 12 V).  This is the number of batteries in series required.

  11. Multiply line 9 by line 10 and enter result here.  This is the TOTAL number of
    battery cells needed for the bank to meet voltage and amp-hour requirements.