The "credits" are just the kW-hours themselves. For example, if in the first billing cycle, the solar generated 200 kW-h more than we used, those 200 kW-hours get "banked." Then, say, the next cycle we use 300 kW-h more than we generated. Instead of having a bill for 300 kW-h, they apply the 200 kW-h from our "bank," and only charge us for 100 kW-h. So, basically the rate they pay us is whatever the rate is when we actually use the kW-hours.
Note, even in the months when the solar generation is higher than the usage, we don't get a $0 bill. There is a minimum charge, usually 50 kW-hous per 2-month billing cycle. We've had several bills for $56 pesos. And, no, the minimum charge kW-hours don't get added to the "banked" amounts.