For a 6V battery, a solar panel with an output of around 6V to 12V is ideal. Ensure that the panel's wattage is sufficient to meet the charging needs of your battery based on its size and capacity.
How much does a 4 hour battery system cost? Figure ES-2 shows the overall capital cost for a 4-hour battery system based on those projections, with storage costs of $147/kWh, $243/kWh, and $339/kWh in 2035 and $108/kWh, $178/kWh, and $307/kWh in 2050 (values in 2024$).