Sorry for the trouble you encountered. As long as you rent energy on the Energy Rental platform, you will be charged for the rental cost regardless of whether you use the rented energy. For more information about security deposit refound, you can check FAQ here https://docs.justlend.org/developers/justlend-dao-testnet/energy-rental
If you rent, you will pay according to the time of holding, because the energy you rented has been occupied. If the energy has been used, the energy need to takes time to restore to full, the time-fee will basically not be felt, I think