Operating Temperature versus Temperature Rating

(Last edited 7/25/2026)

Power supplies can perform differently depending on the temperature at which they are operating at. When a power supply is rated for it's total output wattage, it is rated to do so at a particular temperature. Anything beyond this temperature may take away from the power supply's capability. A power supply that is rated to put out 550W at 25°C or 30°C (room temperature) may only be able to put out 75% of that at 40°C or 50°C (actual operating temperature). This difference is called the "de-rating curve". A normal operating temperature for a power supply is 40°C.

When a power supply can do full output at 40° or 50°C, it will likely be in its marketing as this is an advantageous bulletpoint to have. If it's not advertised, the power supply may be rated at only room temperature. Here are some examples:

The Cougar graphic shows that "de-rate" we were talking about in the first paragraph. I'll get into that after I describe how the above differs from "operating temperature".

"Operating temperature" is exactly has the term describes. It's simply the range of which the power supply can operate within. No guarantees that it can put out its full output power at the highest temperature. Just that it will operate at that highest temperature. It's not unusual for even the cheapest power supplies to have an operating temperature range of 0° to 50°C. In fact, I would be very worries if any electorinc couldn't operate at 50°C (122°F) is uncomfortable... but not unusually hot.

Seasonic is pretty upfront with the difference between operating temperature and rated temperature for maximum output and including the "de-rate" in the marketing. The below is from the product page for the Focus power supply:

Here, it describes that the power supply's operating temperature is 0 to 50°C. But it's only guaranteed full power output up to 40°C. And that expectations of output beyond 40°C should decrease 80% at 50°C. "Lineraly" meaning that the line from 40°C to 50°C that defines the power output capability is a straight line.

In the Cougar marketing, we can see their orange line dropping a little bit at 40°C. I suppose this implies that up to 40°C, their product can actually do more than what its rated for. After 40°C, the curve drops off a little steeper. This isn't quite "linear". Though.

I'll make my own de-rate graph using the data from the Seasonic Focus marketing. I'll use a 1000W for nice round numbers. We know we can do 1000W up to 40°C. We know we can do 80%, or 800W, at 40°C. We know that the de-rate is linear (a straight line). That gives us the graph below:

Also keep in mind that MTBF, DMTBF, the fan curve shown on the box, and other specifications of the PSU, may be measured at different temperatures as well. Not necessarily 40°C or 50°C and quite often 25°C.

One should only expect this fan curve under these loads if the PSU is only sucking in 25°C air.