Sizing a solar lighting system

Off-grid solar lighting balances three things: the load (fixture wattage × run hours per night), the battery (enough to cover the night plus reserve for cloudy days), and the panel (which must recharge the battery during available sun hours). Start from the nightly energy need in watt-hours, size the battery for that load times the days of autonomy you want (often 2–3), and size the panel to replace a full night’s draw within your location’s daily peak-sun hours, with margin for losses.

Battery chemistry matters: lithium (LiFePO₄) tolerates deeper, more frequent discharge and lasts longer than lead-acid, which shouldn’t be drained below about 50%. Add a charge controller (MPPT or PWM) sized to the panel and battery, and remember winter is the limiting season — size for the shortest days and worst sun the location sees, not the annual average.