C20 / C5 / C100 (discharge rate)
Standard discharge rate: C20 = discharge the battery in 20 h. It is the reference used to publish nominal Ah.
The Cxx rate indicates how many hours it takes to fully discharge the battery: C20 = discharge in 20 hours (the standard rate for automotive batteries), C10 = 10 hours, C5 = 5 hours, C100 = 100 hours (typical of solar batteries). The Ah capacity published by the manufacturer ALWAYS refers to the C20 rate unless stated otherwise. The same battery model delivers less Ah the faster it is discharged (Peukert effect): a 100 Ah battery (C20) delivers about 70-80 Ah if discharged at C5, and 50-60 Ah at C1.
That is why solar batteries are published at C100 (higher usable capacity) and starting batteries at C20 (realistic rate). When comparing two batteries with capacities published at different rates, convert them to the same rate before deciding.