To find the right battery size, convert watt-hours to amp-hours (Ah) using the formula: Battery Ah = (Total Wh ÷ Battery Voltage) Now consider depth of discharge (DoD) —most lithium batteries can safely discharge up to 80-90%, while AGM is about 50%. When building a solar power system, batteries are key, whether you're preparing for off-grid living, seasonal blackout protection, or daily load balancing. But how do you know which battery size best meets your energy needs? This guide walks through essential terminology, step-by-step sizing. Finding the right battery size ensures you maximize your solar energy storage, allowing you to use that clean energy even when the sun isn't shining. Charge controller specifications, 3. Start by calculating your energy needs using watt-hours.