Here’s a clear, step-by-step write-up you can use or share. Knowing your power supply unit’s (PSU) wattage is essential for upgrading components, troubleshooting crashes, or building a new PC. Here are the only reliable methods to find out. Method 1: Check the PSU Sticker (Most Accurate) The physical sticker on the PSU itself lists the continuous wattage — not just peak power.