Pinellas Park police said 51-year-old Terence Edwin Phillips was staying with the victim and her family because his Port Richey home didn't have power.
The teen told investigators her grandfather came into her room in the early morning hours on Saturday and sexually battered her in her bed.
Police responded to the house that same day to speak with the victim.
After gathering evidence, Pinellas Park police arrested Phillips on Thursday, charging him with two counts of felony sexual battery.
He is being held at the Pinellas County Jail on $200,000 bond.