A common rule of thumb for sizing batteries to solar power relates nominal panel output in Watts to bank capacity in Ah. It is not gospel. It is not a commandment. It is a quick heuristic to understand how your system is biased.

1:1 == 200W solar + 200Ah battery bank. This generally considered the minimum, though it will likely work out fine in sunny areas with light loads. It may not be enough in the winter when the sun is low and daylight short. Power rationing may be necessary.

2:1 == 200W solar + 100Ah of battery. This slightly overpaneled configuration brings in extra power for heavier loads or for use in areas without much sun.

3:1 == 300W solar + 100Ah of battery. This significantly overpaneled configuration generally yields an excess of power, useful in rain, low winter sun, overcast conditions, etc. 'Dwellers in the Pacific Northwest¹⁾ may need a 3:1 ratio to meet basic needs.

1:2 == 100W solar + 200Ah of battery. Also known as overbanking), this configuration can work fine in certain configurations:

• solar charging is augmented with alternator charging, or shore power charging, or;
• the battery bank is lithium or some other chemistry unaffected by partial state of charge.

Outside the above configurations, underpaneling can contribute to battery murder.

Different battery types have different charging requirements:

• lithium - no minimum, charge at any rate up to the max
• flooded lead-acid: minimum C/10 (10A per 100ah of capacity)
  • minimum ratio with MPPT = 1.25w:1ah
  • minimum ratio with MPPT = 1.5w:1ah²⁾
• AGM lead-acid: minimum C/5 (20A per 100ah of capacity)
  • minimum ratio with MPPT = 2.5w:1ah
  • minimum ratio with MPPT = 3w:1ah³⁾