To determine the appropriate battery capacity for a 400-watt solar panel system, you need to consider several factors, including the amount of energy storage required, the type of battery, and your usage pattern.

Energy storage requirement

The first step is to determine how much energy you want to store from your solar panels. This depends on your energy consumption needs and the duration you want the battery to supply power when the sun isn’t shining. Let’s assume you want to store enough energy to power your home during the night or cloudy days.

Battery type

There are various types of batteries commonly used for solar energy storage, such as lead-acid, lithium-ion, and others. Lithium-ion batteries are popular for solar systems due to their higher efficiency, longer lifespan, and deep discharge capabilities. However, they can be more expensive upfront compared to lead-acid batteries.

Depth of Discharge (DoD)

The depth to which you discharge the battery also affects its longevity. A higher depth of discharge will require a larger battery capacity to avoid excessive strain on the battery and maximize its lifespan.

Keep in mind that this is a simplified calculation, and actual system design may involve other considerations like efficiency losses, temperature, and charge controller specifications.