Batteries store the electrical energy produced by the solar array during the day for use at night or during cloudy periods. Battery bank sizing depends on two factors: the building’s daily energy demand (which determines how much storage is needed for each day of no solar production) and the number of days of autonomy desired (typically 2 to 5 days, depending on the site’s climate and the criticality of the loads). Lithium iron phosphate (LiFePO4) batteries have become the preferred choice for most off-grid applications in recent years, replacing lead-acid batteries as costs have fallen dramatically. Lithium iron phosphate batteries offer higher energy density, longer cycle life (3,000 to 6,000 cycles versus 500 to 1,500 for lead-acid), higher usable capacity (90 to 95 percent depth of discharge versus 50 percent for lead-acid), and faster charging. Their higher upfront cost is typically offset by their longer service life and higher usable…