If I have an opamp intended for +-15 V and I want to use a single power supply rather than +- rails, what is the maximum voltage I can use? Is it 15 V or 30 V?
30 V It's the voltage between V+ and V- Notice though common mode limits for the inputs. Some opamps can handle signals down to ground and some don't. Chck the parameter "common mode range" in the datasheet.