yes, the supply voltage is usually about 13.5 volt, but it could occasionally reach peaks of 16 - 17 volt.
could you tell me more about the modified bias? I'm a novice so I think I'm missing something...
This subject has been discussed several times - also in this forum. And there a lot of papers dealing with single supply in the internet (search for opamp single supply circuits)
Nevertheless, I like to give you some information:
Normally, opamps are designed for double supply - which means: operating point in the middle at 0 volts.
This quiescent voltage at the output (without any input, and forgetting about parasitics like offset) allows the input voltages referenced to zero.
In your case, with single supply, the quiescent dc voltage at the output should be 50% of the opamp supply (at the opamp terminal). Thus, the output voltage can swing to both sides equally.
But this requires that both input dc voltages also at 50% supply. Thus, you need a voltage divider that produces this bias for the non-inv. input. The inv. input gets this bias automatically due to 100% feedback. In order not to disturb this bias, your input signal to be buffered must be coupled through a suitable capacitor.
OK?