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#Does an Oversize PSU Waste Power?

“Does an Oversize PSU Waste Power?”

A Corsair brand PSU in a black PC.
Corsair

When it doubt, it’s recommended you size up your PSU, but does sizing up your PSU waste power? Here’s a look at PSU power utilization.

Practically Speaking, There’s No Difference

If you’re new to PC building or simply haven’t stopped to think about it before, it would be easy to assume that the power wattage values listed for PSUs represent absolute power loads, much like the wattage rating on a space heater represents the actual wattage the device pulls to heat a room.

While a PC can certainly feel like a space heater sometimes, the PSU wattage rating is for the maximum load, not the absolute load.

Corsair RM850X 850W PSU

The Corsair RMx series offers reliability, fully modular connectors, and a 10-year warranty.

The PSU will supply only as much power as your PC requires and no more. If you put a monster 1600W PSU in a build that only requires 200W under load, then that’s all the PSU will deliver. You could swap the 1600W PSU for a 500W PSU, and you would detect zero difference between using the computer with the bigger PSU and the smaller PSU.

Further, you’d struggle to notice a difference in your electric bill. Even with your computer hooked up to a device like a Kill A Watt to measure the wattage pulled at the socket, you’d still scarcely notice the difference between the two.

Technically Speaking, There Are Inefficiencies

If you want to move away from talking about differences, you would actually notice—the ones that would have an impact on your electric bill—and into the technicalities of PSU efficiencies, there is actually a difference between using a large and small PSU for a given build.

PSUs have an efficiency curve. When they are significantly underloaded or, at the opposite end, put under maximum load, they are less efficient. Peak efficiency is around 50% of the rated load.

How efficient or inefficient it might be is typically described using the 80 Plus certification terminology. 80 Plus certification indicates that a PSU is at least 80% efficient at converting AC electrical energy into the DC electrical energy your PC uses.

Above the minimum 80 Plus Basic certification, you have Bronze, Silver, Gold, Platinum, and Titanium. At 80 Plus Basic, you get 80% efficiency at 50% load, and it climbs all the way to 94% at the Titanium level.

When the load drops to 20%, the efficiency of an 80 Plus Basic PSU stays at 80%, but at the premium end of the PSU scale, the efficiency of a Titanium unit drifts down to 92% from 94%. A 2-3% loss of efficiency for anything above the Basic certification is expected. You can read more about 80 Plus certification and the related specs here if you want to dig in deeper.

So what does this mean, practically speaking? Let’s compare two identical builds but change the PSU to show how the actual power draw changes based on PSU efficiencies.

Let’s say we have a PC build that requires 250W of power. For the sake of simplicity, we’re going to say the 250W draw is steady and doesn’t fluctuate wildly with load demands just so we can do some basic back-of-the-envelope calculations.

If we put a 500W 80 Plus Gold PSU in this build, then the 250W draw happens to hit the 50% point for the wattage rating dead on. A Gold-rated PSU is expected to be 87% efficient at a 20% load, 90% efficient at a 50% load, and 87% efficient at a 100% load.

That means our 500W PSU, at 50% load, requires 10% more energy from the wall than it delivers to the components. 250W * 1.10 = 275W. We lose 25W in the conversion process.

Now let’s say we keep the same build and swap out the 500W PSU for a larger 1000W PSU, still with an 80 Plus Gold rating.

A 250W load on a 1000W PSU is only 25% of the rated capacity and knocks our efficiency down to 87% or so, from the previous 90%. So now our conversion overhead is 13%, or 250W * 1.13 = 282.5. We lose 32.5 watts.

The difference between the two overhead losses is only 7.5 watts. Although that’s enough energy to power an LED lightbulb, it’s not a whole lot of energy. In fact, at 8 hours of use per day, 365 days a year, and 12 cents per kWh, it works out to the $1.31 extra on your annual power bill.

So, when in doubt, there’s little risk in sizing up your PSU beyond paying a premium for a higher wattage rating you may never need. But, especially if you’re purchasing a quality unit, you can use it not just for the PC you’re building today, but for the one you’ll build five years from now.

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