This report on PC energy consumption and cost was possible not just with our time at desks and lab runs, but as a result of helping hands from many notable persons. Indeed, it is our prime duty to extend our heartfelt appreciation to Evan Mills, Senior Scientist from Lawrence Berkeley National Laboratory with all the possible understanding, guidance, support and experience.

## recommended PSU selection process

In a nutshell, OuterVision ^{®} Power Supply Calculator analyzes dozens of power supplies per each certification category, compares their efficiency, unit price, overall PC energy cost, and payback period. Based on these analysis, we provide recommendation to our users with the best bang for the buck PSU per each certification category. The user has an option to compare the recommended PSUs, adjust electricity cost to see PC's overall energy cost per year. Figure 1 shows the calculation details and recommendation by the engine.

## power supply efficiency

PSU supplies power from the electrical outlet to PC's components. It is not a straightforward process as it looks, because PC's components' use direct current (DC) and electrical outlet supplies alternating current (AC). The primary function of power supply is to covert the AC into DC as required for the PC's components. However, during this conversion some of the input power is dissipated as heat. Thus not all of the AC input is converted to DC output.

The efficiency of a power supply is its output power divided by its input power. The less power wasted into heat, the more efficient the PSU is. In simple terms, the efficiency number tells us how well the PSU converts the input AC power into output DC power.

Over the last decade, 80 PLUS certification program certifies PSU with its efficiency rating, with the intention to promote efficient energy use. Now a days, 80 PLUS is seen as an industry standard for PSU efficiency rating.

To qualify for 80 PLUS, a power supply must achieve at least 80% efficiency at three specified loads (20%, 50% and 100% of maximum rated power). For example, an 80 PLUS PSU should generate at least 500W of output power from roughly 625W of input power from the electrical outlet.

Even within 80 Plus certification, there are tiers of even more-efficient PSU, including 80 Plus Bronze, Silver, Gold, Platinum, and Titanium certifications. It starts with a base of 80 PLUS and getting better is bronze, silver, gold, platinum, and titanium. Figure 2 shows the efficiency metrics for each tier at the three specified workloads.

The higher the certification tier, the more efficient PSU will be. Though high rated PSU will save more money on energy cost, they tend to command high price. Hence, our goal is to provide recommendations on the best bang for the buck power supplies based on the energy cost, efficiency level, and payback period.

## Energy Consumption and Cost of an Electronic Device

Calculating the energy cost of an electronic device is fairly simple as shown in the following equation:

Where

E_{kWh/day} |
kilowatt-hours (kWh) per day |

P_{W} |
Power in Watts (W) |

T_{day} |
utilization hours per day |

Annual energy consumption E_{kWh/year} is equal to the daily energy consumption E_{kWh/day} times number of days the electronic device is used per year, as shown in the following equation:

Annual energy cost COST_{year} is equal to annual energy consumption E_{kWh/year} times electricity cost per kWh COST_{kWh} as shown in the following equation:

Computing annual energy cost is simple as shown above, but holds good only for simple electrical or electronic devices. Calculating energy consumption for PC is more complex, and it is detailed out in the next section.

## PC Energy Consumption and Cost

A PC is more complex compared to a regular electronic device such as a desktop monitor for example. Each PC component (e.g. CPU, motherboard, memory, video card) has different power requirements under a given mode of operation (active gaming vs web browsing vs video streaming vs idle, etc.). For example, Intel Core i7-6700K processor may consume up to 80W under heavy gaming and only 4W while idling.

To calculate PC's daily energy consumption, we need to capture its complete duty cycle for time spent in gaming, web browsing, video streaming, standby, idle, etc. For the sake of simplicity, we broke these into two cycles as follows:

- Game Time, which captures time spent on gaming, video editing, 3D rendering that may account 100% power consumption cycle

- Non-Game Time, which will be the rest of the cycles that roughly account for 60% of PC's power consumption

PC's daily energy consumption E_{kWh/day} in kilowatt-hours (kWh) per day is defined by the following equation:

Where

E_{kWh/day} |
PC's daily energy consumption |

P_{100} |
PC's power consumption at 100% load level |

P_{60} |
PC's power consumption at 60% load level |

𝜂_{100} |
PSU's efficiency at 100% load level |

𝜂_{60} |
PSU's efficiency at 60% load level |

GAME_{time} |
time spent on game cycle |

NonGAME_{time} |
time spent on non-game cycle |

P_{display} |
PC monitor's energy consumption in kilowatt-hours per day |

PC_{time} |
overall PC usage time per day |

As you can see in the equation above, we provide our customers with an option to select monitor(s) to get an overall energy consumption picture, even though, monitors do not draw power from the PSU.

PC energy cost or PC electricity cost can be calculated using the same equations as for a regular electronic device provided above. First, we need to calculate Annual PC energy consumption E_{kWh/year} and than multiply it by electricity cost per kWh COST_{kWh}.

## weighted average efficiency

Since we use different efficiency levels for PC's daily energy consumption calculation, we need weighted average efficiency 𝜂_{WA} for a uniform efficiency representation of efficiency level. This weighted average efficiency is used by our comparison engine for recommended power supply selection. The weighted average efficiency is calculated using the following equation:

## payback period

The power supply payback period estimates how long it will take for user to "break even" on more efficient PSU investment compared to a control unit. For the comparison engine, we considered the lowest certification tier based on calculated PSU load as a control unit. We do not recommend PSUs below 80 PLUS White rating and thus the lowest certification tier in our selection is set to 80 PLUS White. The payback period is calculated using the following equation:

Where

C_{unit} |
cost of the PSU in higher certification tier |

C_{control} |
cost of the PSU of control unit (lowest certification tier, in this case) |

EC_{unit} |
PC's energy cost with PSU in higher certification tier |

EC_{control} |
PC's energy cost with PSU of control unit |

At eXtreme Outer Vision, we understand that when it comes in selecting a power supply for your computer, your choices are many. Our service will make your choice for a power supply more convenient and time efficient. We offer wattage, amperage, and energy cost recommendations and provide you with the best reference to a power supply unit that meet your demands. All our suggested power supply units are produced by well-known manufacturers. We are confident that we are the best at what we do. Our recommendations to you are of the highest quality. You won't be disappointed!

As always, your comments and suggestions are important to us. We definitely would like to hear from you. Please submit your questions, comments and suggestions in the form below.