You make it seem so simple, but like many things in Mexico it isn't. Not with CFE and not with CESPE.
We have a small solar installation that normally offsets most of our electricity usage. The month after prepaying a few hundred pesos our bill was double what it normally is. The summer solar credits we had built up were missing and no new credits were added. I was getting ready to hire someone to have the solar panels checked when our credit balance ran out.
To my surprise the next bill was back down to the normal amount and the solar credits we had been missing showed up all at once - hundreds of KWHs worth. The only thing we can figure is that CFE's accounting software is unable to handle the combination of bill prepayment and solar credits at the same time. Or maybe the problem was unique to our account, who knows?
That was a better experience than CESPE's handling of our water bill. CESPE took the prepayment, pocketed it, and the pesos never showed up in our account. We had the receipt but it wasn't worth the time and effort to try to get them to credit the ~$15 we had prepaid.
We learned our lesson. We now pay exactly what we owe every month. With a BBVA account it should be easy going forward.