How Much Power Does Your Graphics Card Need?

Actual Power Consumption And Current Requirements

Now that it’s clear what the efficiency of a power supply affects, and that all the test measurement values are taken at the socket, you can figure out the actual power consumption by subtracting the losses due to the inefficiency of the power supply. Since the graphics cards have very different load levels, the average used is 82.4% (based on manufacturer statement).

Formula for the calculation:

Actual Consumption in Watts = Measurement at Socket in Watts x efficiency of power supply

300 Watts = 364 Watts x 0.824

This step is necessary because to determine the power supply class, only the actual power consumption of the graphics card is important. The previous value, with efficiency, is only crucial for the calculation of electricity cost.

Unfortunately, now another very important factor comes into play: the current. At these high values, many older power supplies fail when a new high-end graphics card is installed. Not all 500 watts are created equal, especially when it comes to cheap power supplies. Since many components are powered on the 12 V line, the power supply also needs sufficient current (amperage, A).

Let’s take the Radeon HD 4870 as an example. AMD states “500 watt power supply” as a requirement on the packaging. According to the actual measurements, though, the graphics card doesn’t need more than 150 watts even under full load. Now we’ll calculate in 150 watts for the CPU, motherboard and drives, 300 watts should be enough. If we assume from this a 300 watt load on the 12 V line, this means 25 A is required for this rail.

Formula for the calculation:

Current Strength (Amps) = Watt : Volt25 A = 300 Watts : 12 Volts

Cheap power supply 500 watts with 17 A on the 12V line.

As you can see from the stated power rating, you can very quickly run into trouble with this 500-watt power supply, because despite the 500 W figure, it only provides 17 A on the 12 V line—at least 20 A to 25 A is necessary for a current standard PC with the HD 4870. So even if AMD sets 500 watts as a requirement, you can not assume all cheap power supplies will suffice based solely on this parameter.

With current brand name power supplies, things are different: pretty much every 500-watt brand name power supply will provide 25 A on the 12 V line. Even older brand name power supplies with fewer watts achieve the required targets.

Brand name power supply: 433 watts and 33 A on the 12 V line.

If you want to be on the safe side with your power supply, you should determine the maximum power consumption of all components, and then calculate the maximum current needed. Especially when running SLI or CrossFire, the current requirements can quickly become more important than the wattage class with high-end graphics chips (such as the GeForce Ultra 3SLI with 38.8 A at 465.6 watts).