What size battery for 400W of solar panels?

Divide panel watts by system voltage: 400W on a 12V system pushes up to ~33A in full sun. A 100Ah LiFePO4 battery absorbs that comfortably and stores about a day of typical motorhome use; step up to 200Ah if you want two days of autonomy or run an inverter. On a 24V system the same panels only push ~16A, so cable losses drop and a 24V 100Ah battery (2.56kWh) is the sweet spot.

Either way, set the MPPT controller to a lithium profile – Redodo MPPT controllers pair directly with their Bluetooth batteries, so you can verify charge current from the app. This is the exact maths behind our current Oxford install: 400W of panels on a Redodo MPPT, waiting on the battery.