The formula
Daily solar energy = Panel watts × Peak sun hours × System efficiency
Days to charge = Energy needed ÷ (Daily solar energy − Daily loads)
System efficiency covers heat, wiring, controller and battery charging losses: about 75% with an MPPT controller and 60% with PWM on typical panels.
Worked example
A 200 W panel with an MPPT controller in 4 peak sun hours produces about 600 Wh a day. Charging a 100 Ah 12 V battery from 40% to 100% needs 720 Wh, so about 1.2 days of sun with nothing else running.
Frequently asked questions
How long will a 100W solar panel take to charge a 100Ah battery?
From 50%, a 12 V 100 Ah battery needs 600 Wh. A 100 W panel makes about 300 Wh a day in 4 sun hours, so roughly 2 days with no loads.
Can solar overcharge my battery?
Not with a charge controller. It stops or reduces charging when the battery is full.