Hello, the minimum suggested PSU for the GT 710 is 200 Watts. But wattage is not always the deciding factor. Which Make/brand 240 Watts PSU do you have ? Is it an OEM PSU ?
Being a single-slot card, the NVIDIA GeForce GT 710 does not require any additional power connector, its power draw is rated at 19 W maximum. 240 Watts should suffice, but like I said before, wattage number alone never makes a good or a reliable/safe PSU. The Quality of the PSU is more important.