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Glossary/CPU/Power limit (PL1 / PL2)

Power limit (PL1 / PL2)

PL1 is the package power a CPU may draw indefinitely and PL2 is the higher short-window boost budget, both enforced independently of temperature.

Also known as PL1, PL2, package power limit, PPT

Measured by SystemCheck

Part of the four-stage run: a 60-second sustained CPU load reporting throughput, decay and consistency; a memory stage reporting bandwidth and latency; a calibrated GPU raster load reporting frame time, pacing and decay; and a path tracer reporting samples per second. The paragraphs below say which metric carries this term.

Intel expresses this as PL1 (long-duration power), PL2 (short-duration turbo power), and Tau (the window PL2 is allowed to last). AMD uses PPT as the equivalent package power ceiling. When the chip hits these limits it drops frequency exactly as it would for heat, but the trigger is watts, not degrees.

This is the most misdiagnosed cause of poor sustained performance. A user sees the clock collapse, assumes a cooling problem, buys a larger heatsink, and sees nothing improve, because the machine was at 65 W on a PL1 the OEM set to 65 W, at 70 °C, nowhere near a thermal limit. Laptop and prebuilt vendors set conservative PL1 values routinely, and some drop PL1 further on battery power.

SystemCheck measures the decay this produces but cannot distinguish it from a thermal cause, because a browser can read neither package power nor temperature. The diagnostic procedure is to run SystemCheck's CPU stage, note the decay, then watch package power and core temperature in HWiNFO64 or Ryzen Master during a repeat run. Power flat at a round number with temperature well below 90 °C means a power limit; temperature pinned at the limit means heat.

Related terms

Thermal throttlingThermal throttling is a processor reducing its clock speed and voltage automatically once it reaches a temperature limit, to avoid damage.Read moreCurrent limit throttlingCurrent limit throttling is a CPU cutting frequency because amperage through the voltage regulators hit a ceiling, even when temperature and wattage are fine.Read moreTDPTDP is the amount of heat in watts a cooler must dissipate to keep a processor at its base clock under a sustained standard workload.Read moreThermal decayThermal decay is the measurable fall in performance from the start of a sustained load to its steady state as the system heats and hits its limits.Read morePower deliveryPower delivery is the chain from the power supply through the motherboard's voltage regulators to the chip, and any weak link caps sustained performance.Read more
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