Measure the load first
Use the freezer label or, preferably, a plug-in watt meter to understand normal consumption. Compressor startup can briefly require more power than steady operation.
Estimate the energy you need
Runtime depends on usable battery watt-hours divided by the freezer average watts. Because freezers cycle, a measured daily energy figure can be more useful than a single running-watt number.
Leave room for real-world losses
Inverter losses, temperature, door openings and other connected loads reduce practical runtime. Treat estimates as planning figures, not guarantees.