Moving to BIOS
This section of the guide will cover proprietary power saving settings (like ASUS' EPU) and industry-wide settings, like C-States, voltage controls, power limits, and more. All settings covered here will be located in BIOS. If you're looking for a guide on overclocking, you can find one of those over here.
BIOS: ASUS Motherboards - EPU Power Saving Mode
ASUS boards have an in-house power-saving solution that won't be found on competing brands, though other boards often have their own take on similar features. ASUS has built what they call an "Energy Processing Unit," or EPU, into their modern motherboards.
The EPU monitors system load across various critical devices (CPU, chipset) and then regulates voltages and power delivery to those systems accordingly. If the EPU doesn't think your CPU requires full voltage to deliver a reliable clockrate at your present load, it'll decrease power delivery to better conserve power. This setting is recommended to be left disabled if overclocking, as it will introduce instability and variability to the system. If not doing higher OCs, enabling the EPU will seriously mitigate power consumption when under minimal load.
BIOS: EIST (Enhanced Intel SpeedStep Technology) - On or Off?
EIST has previously been a subject of debate on overclocking and enthusiast forums, but as a concept, is pretty straight-forward. In theory, EIST automatically monitors and adjusts VCORE (CPU voltage) and BCLK/multipliers (frequency), aiming to reduce power consumption and lower the thermal signature of the processor. When the system demands greater processing capabilities, EIST allows more voltage to flow to the CPU and increases frequency accordingly.
In practice, EIST intentionally throttles the CPU when load is low; for the average consumer, the impact is unnoticeable, but for performance users, it could directly affect benchmark results. For this reason, most reviewers (including GN) and enthusiast overclockers disable EIST when running benchmarks. In everyday and gaming (non-performance) use, EIST is good to leave enabled to help minimize power draw.
BIOS: CPU Load-Line Calibration Levels - Overclocking vs. Power Saving
LLC, or Load-Line Calibration, is a BIOS feature that helps prevent or minimize voltage drops to the CPU during load changes (called vdroop). When a CPU modulates voltage as it transitions from idle to load (or vice versa), the voltage drops during the transition; this could potentially destabilize a highly-volatile overclock, and so Load-Line Calibration was introduced. Higher levels of LLC will help control vdroop with much greater accuracy, but also increases power draw by maintaining a higher, more stable voltage.
A high LLC setting can threaten CPU life under extreme OC conditions, but lower LLC settings will give the CPU greater tolerance for vdroop and voltage modulation. If the CPU isn't being put through any serious overclocking procedures and doesn't need a constant, high-voltage supply, LLC can be set to lower levels to conserve power and decrease thermals. Consequently, this can extend the life of the CPU.
BIOS: CPU Power Phase Control Settings
We often talk about power phase design when comparing motherboards (guide here). High-quality power phase design can seriously impact a motherboard's ability to maintain a stable overclock. We don't talk about it much in a power saving sense, though.
The CPU power phase control setting in BIOS can often be set to various levels of extremes. Maximum power phase control (often called "Extreme") will ensure all power phases dedicated to the CPU are maxed-out constantly. This serves as an important tool for serious OCs, again, but isn't desirable for power conservation or mainstream/gaming (non-OC'd) builds. If you've got a down phase option in BIOS, set it to the "optimized for power saving" setting. BIOS varies so widely between boards that we can't be comprehensive here, but leave a comment if you can't find this setting or are confused by it.
BIOS: DRAM Power Phase Control
Similar to the CPU power phase control setting (see above), except for the phase(s) dedicated to DRAM instead.
BIOS: Power Decay Mode
With Power Decay Mode enabled, the CPU's integrated VRM passes lower voltage to the CPU to diminish power consumption at idle times. This is similar to some of the other at-idle BIOS settings, and further conserves power when the CPU doesn't demand high frequencies and voltage.
BIOS: C6 & C7 Power States
Intel C-States are similar in top-level concept to Windows S-states, or sleep states. In Windows, S3 and S4 represent sleep/suspend and hibernate, respectively, with various other S-states preceding them (S0 is full operation, S0iX is Intel Active Idle, etc.).
C-States are much lower-level, though, and directly interact with the CPU through firmware. Like Windows, C0 means full operation and higher numbers (like C7) represent a "deeper" sleep of the hardware. C6/C7 power states in Haswell CPUs tend to conserve roughly 10-13% of the wattage.
Not all power supplies support C6/C7 power saving states due to efficiency requirements on behalf of the PSU (power supply must be able to wake the CPU from >1W draw). Check for compatibility on your PSU.
BIOS: Idle Power-In Response
This setting goes hand-in-hand with Power Decay Mode. As the name suggests, it's the speed or latency with which the integrated VRM reduces power levels when the CPU idles. A "fast" setting means the CPU will drain power more quickly when idling, which saves power, but also introduces instability to the clock when overclocking.
That covers most of the relevant BIOS options. If we missed one that you're curious about (or know about), leave a comment below! Oh, and don't forget that underclocking is always an option if you're building a specific-purpose HTPC and need lower power draw.
That should get you started off pretty well on reducing the power draw footprint of your PC. If you've got any suggestions or questions, post 'em below! Our forums often get more detailed and faster responses, so also consider posting there.
- Steve "Lelldorianx" Burke.