Battery Sizing Exercise

Problem:

Use the table below to specify a battery bank for the following photovoltaic system. The occupants of a remote home near Ojai, California are designing a photovoltaic system to meet their 1,080 watt hours per day AC electrical load. They have decided on a 12-volt direct current system and feel they need two days of autonomy. The maximum depth of discharge they desire over that two-day period is 50 percent. The occupants have tentatively selected the Model A battery from XYZ Manufacturer, a 6-volt battery rated at 200 amp hours. The occupants will keep the battery(s) in a conditioned space that will be maintained at 77°F.

Solution:

  1. To start, de-rate for inverter efficiency by dividing the AC Average Daily Load (1,080 watts) by the standard inverter efficiency figure (90% or 0.9).
  2. Multiply the resulting Average Amp-Hours/Day (100) by the Days of Autonomy (2) and divide by the Discharge Limit or DOD (50% or 0.5) and divide again by the Battery Ah Capacity for the specified battery (200). The resulting figure is the number of batteries in parallel (2).
  3. Next determine the number of batteries needed to achieve the system voltage by dividing the DC System Voltage (12) by the Battery Voltage (6). Then multiply this number (2) by the number of batteries in parallel (2) to determine the Total Batteries needed (4).

Source: “PHOTOVOLTAICS - Design and Installation Manual” by Solar Energy International.

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