The primary aim of the study is to compare the operative times following ureteroscopic lithotripsy for renal stones with Thulium Fiber Laser using low and high power settings. Secondary aims are to compare the results of low and high power settings in terms of stone free rates, laser time, laser activation patterns, intraoperative and postoperative complications. Patients ≥ 18 years with renal stones (8-25 mm) scheduled for ureteroscopic lithotripsy at the Day Surgery Clinic at Haukeland University Hospital in Bergen, Norway, are eligible for inclusion in the study. After written consent and inclusion, patients are randomised to laser lithotripsy using either low power settings (Group 1: 4-6 Watt, short pulse mode) or high power settings (Group 2: 16-18 Watt, short pulse mode). The ureteroscopic procedures are performed in general anaesthesia using a standardised technique. All patients are followed up with a low dose CT scan 3 months post endoscopically to assess stone free status as well as a consultation at the outpatient clinic. Results and data for the two randomisation groups are compared according to the aims of the study.