A smartphone with a built-in projector gives you a screen anywhere. But powering that projector module costs energy, and the drain is real. Most users notice the battery dropping faster during projection. The light source, display processing, and heat all pull power at once. On a standard 5,000 mAh phone, that can mean losing a noticeable chunk of battery every hour. The drain is manageable when you understand what causes it. Battery size, brightness settings, and content source all play a role.