There is a 3,349 mAh battery in the iPhone 15, while the iPhone 15 Plus has a 4,383 mAh battery, both just a bit higher-capacity than the batteries from the respective prior-generation models.
The 15-hour battery life is due to what iGarden calls an AI-inverter technology, which is supposed to enhance battery efficiency. Allegedly, this new technology also helps the machine be lighter.