Battery bank sizing recommendations

¹ Ave. daily PV generation (kWh) assumes a minimum insolation for Binghamton NY of 2.2 hr/day of "full sun" (1000W/m²) in December, a maximum of 5.8 hr/day in June, and an average of 4.1 hr/day annually, a year-to-year uncertainty of +/-11% (NREL data), and an overall PV system efficiency factor of 0.70.

² Recommended battery capacity (kWh or Ah) is set equal to twice the average daily PV generation. For longest life, sealed maintenance-free batteries (aka VRLA, gelled electrolyte, AGM, etc.) should not be discharged to greater than 50% of their total capacity rating (at the 24hr discharge rate). As a rule-of-thumb, recommended battery capacity (in kWh) turns out to be approximately 3x the PV system rating (in kW).

³ Min. recommended battery capacity (kWh or Ah) is the largest battery bank size that the PV system can recharge from 50% depth-of-discharge (DoD) on an average mid-winter day. Min. recommended battery capacity (kWh or Ah) assumes no other means of battery charging other than the PV system. A battery bank of this size or smaller would be fully charged (on average) by the PV system every day of the year without other charging sources. This is usually the best choice unless expected daily energy usage is more than 1/2 of the min. battery capacity.

⁴ Max. recommended battery capacity (kWh or Ah) is the largest battery bank size that the PV system can recharge from 50% depth-of-discharge (DoD) on an average mid-summer day. In general, other charging sources (such as an auxiliary generator or utility grid) will be needed at other times of the year to bring the battery bank to a full state of charge at regular intervals (at least once per week is recommended). Without other charging sources, in order for this battery bank to remain fully charged at times of the year other than the summer months, energy usage must be equal to or less than that provided by the PV system on any given day. This can be difficult to achieve, especially during winter months.

⁵ Ave. recommended battery capacity (kWh or Ah) is the largest battery bank size that the PV system can recharge from 50% depth-of-discharge (DoD) on an average day. In general, other charging sources (such as an auxiliary generator or utility grid) will be needed during the winter months to bring the battery bank to a full state of charge at regular intervals (at least once per week is recommended). Without other charging sources, in order for this battery bank to remain fully charged in winter months, energy usage must be equal to or less than that provided by the PV system on any given day.

⁶ Battery bank voltage is the nominal value, not the fully-charged voltage. 48V values are highlighted due to better economics and efficiency of higher voltage battery charging systems utilizing 48V DC-to-AC inverters.

⁷ Estimated cost of batteries includes batteries, cabling, conduit and enclosure(s) - all of which are required for code compliant systems.