How Much Power Does Your Graphics Card Need?

Power Supply Efficiency

Now that we’re clear on the power cost of a graphics card, we can use this as the basis for a somewhat simplified theory for the selection of a power supply class. You need to differentiate between two values: the power measurement at the outlet—which is crucial for the overall electricity cost calculation—and what the actual power supply load is that is produced by the components.

At this point we must generalize somewhat, since we can’t guarantee the product quality of each power supply manufacturer based on specifications. The following statements are relevant to brand name power supplies, but fluctuations in quality are always possible. As always, you’re better off avoiding cheap power supplies with a poor track record for performance.

As an example, we take a 500 watt power supply, and plug into it components drawing 500 watts of power; the power supply is therefore at 100 percent load, theoretically. But the efficiency factor now determines how much the power supply absorbs from the electrical outlet and dissipates into heat. If the power supply in this example has an efficiency of 85%, this means 15% of the power taken from the outlet is wasted. So while we use 500 watts, the power meter shows 588 watts (since 588 * 85% = 500). These additional watts of power must not be included in the power supply choice; they are only relevant to the electricity cost.

A few notes:

1. You should easily be able to use the wattage specification of the manufacturer: a 500-watt power supply can be loaded with 500 watts.
2. The measured value at the outlet is higher and not the actual load of the components.
3. Test values are almost always measurements at the outlet.
4. Current brand name power supplies run at more than 80% efficiency with higher loads; lower efficiencies exist mainly with loads below 100 watts.
5. The higher the efficiency, the less energy is converted into heat, so costs remain low.
6. The lower the efficiency, the more energy is converted into heat, and the more cost increases, since more energy is required at the outlet.

In this table we see once again the differences between power supply (load by the components) and measured outlet (electricity cost). On the left is the power supply watt information; on the right is the theoretical number of watts, which the meter would display under full load. 85 percent efficiency is assumed.