A PSU efficiency curve plots the relationship between power supply efficiency and the amount of electrical load it's carrying. The x-axis shows load percentage (from 0% to 100%), whilst the y-axis shows efficiency as a percentage. Real-world power supplies don't convert electricity at the same efficiency across all load levels; they typically peak somewhere in the middle range, usually between 40% and 80% load.
Why this matters for your buying decision: if you buy a power supply rated for far more wattage than your system uses, it will operate in the inefficient left side of the curve most of the time. This wastes electricity, generates unnecessary heat, and increases your energy bills. Conversely, running a PSU near its maximum rating pushes it to the right side of the curve, where efficiency also drops and reliability suffers.
When comparing power supplies, look for models where the peak efficiency occurs near your expected load level. A 750W PSU used by a system drawing 400W will operate near peak efficiency, whereas a 750W PSU powering a 200W system sits well away from the sweet spot.
You'll find efficiency curves in detailed reviews and technical specifications from reputable manufacturers. The 80 Plus certification system (Bronze, Gold, Platinum, Titanium) mandates minimum efficiency standards at 20%, 50%, and 100% load, but doesn't tell you about the shape of the entire curve. Cross-reference multiple sources when making a choice, especially for workstations or servers where your load profile is predictable.