Power Supply Calculator

Choosing the right power supply unit (PSU) for a PC build or electronics project requires calculating the total power draw of all components plus a headroom buffer. An undersized PSU causes system instability, crashes, or permanent damage. An oversized PSU is wasteful and unnecessary.

Total System Wattage formula: Total Draw = CPU TDP + GPU TDP + RAM Wattage + Storage Wattage + Peripherals + Motherboard

Recommended PSU Wattage: PSU Wattage = Total Draw / 0.80

Dividing by 0.80 keeps the PSU at 80% load or less when the machine is working hardest.
That is a safety margin, not an efficiency target: it leaves room for the brief transient spikes a modern GPU throws, and it keeps the unit out of the range where its fan runs flat out and its capacitors age fastest.

What each variable means:

• TDP (Thermal Design Power): the maximum sustained power draw of a CPU or GPU (watts); listed on manufacturer spec sheets; represents real-world peak gaming/rendering load
• RAM: DDR4/DDR5 sticks draw ~2–5 W each; 4 sticks = 8–20 W total
• Storage: SATA SSD: ~2–4 W; NVMe SSD: ~5–10 W; HDD: ~6–10 W spinning + 2 W idle
• Motherboard: typically 30–80 W depending on VRM quality and features
• 80% rule: running a PSU at 80% rated load keeps temperatures manageable, reduces wear, and ensures peak transient loads (GPU spikes) are covered

Where efficiency actually peaks. Not at 80%. Every 80 Plus curve is measured at 20%, 50% and 100% load, and on essentially every unit made the 50% figure is the highest of the three. A supply sized so that your typical load sits near half its rating is running at its best point, which is a separate question from the 80% peak-load ceiling above. In practice both pull the same way: size for peak with headroom and your idle and browsing load lands comfortably in the efficient middle.

Worked example, a high-end gaming PC:

• CPU (Intel Core i9-14900K): 125 W TDP (boosting to 253 W)
• GPU (NVIDIA RTX 4080): 320 W TDP
• RAM (32 GB DDR5): 15 W
• NVMe SSDs (× 2): 15 W
• Motherboard: 50 W
• Cooling + fans + RGB: 20 W Total Draw: 545 W (at spec TDP) or up to 673 W at peak boost

PSU at 80% load: 673 / 0.80 = 841 W minimum → choose an 850 W or 1000 W 80 Plus Gold or better PSU.

80 Plus efficiency ratings: 80 Plus Bronze: 82–85% efficient | Gold: 87–90% | Platinum: 90–92% | Titanium: 92–96% Higher efficiency = less heat and lower electricity cost over years of operation.