CPU Undervolting Battery Savings Calculator

Definitions and units (so inputs match the model)

• Battery capacity (Wh): watt-hours, an energy amount. A 60 Wh battery can theoretically supply 60 W for 1 hour.
• Power (W): watts, a rate of energy use at a moment (or averaged over time).
• Energy (kWh): kilowatt-hours, used for electricity billing. 1 kWh = 1000 Wh.
• Baseline runtime (hours): how long the laptop lasts on battery under the same workload you’re evaluating.
• CPU power at load (W): average CPU/package power during that same baseline workload.
• Undervolt reduction (%): expected percentage reduction in CPU power due to undervolting (e.g., 10% means CPU power becomes 0.90×).

How the calculator works (model)

The core idea is:

1. Use battery capacity and baseline runtime to infer the laptop’s average total system power during the baseline workload.
2. Apply the undervolt reduction only to the CPU portion of that total power.
3. Compute a new estimated total power draw, then divide battery capacity by that power to get a new runtime.

Step 1: Baseline total power

If your battery has capacity C (Wh) and baseline runtime is R (hours), the average total system power is:

Step 2: Apply undervolt to CPU only

Let CPU power during that baseline workload be P_c (W). Let the undervolt reduction be r as a fraction (e.g., 15% → 0.15). Then CPU power decreases by:

ΔP = P_c × r

Estimated new total system power becomes:

P_t,new = P_t,base − ΔP

Step 3: New runtime and gain

Estimated new runtime:

R_new = C / P_t,new

Estimated runtime gain:

Gain = R_new − R

Energy and cost savings (plugged-in time)

If you use the laptop H hours/day on average, then annual energy saved (assuming the same workload and the undervolt benefit applies during those hours) is approximated by:

kWh/year ≈ (ΔP × H × 365) / 1000

Annual cost savings at electricity rate e ($/kWh):

$/year ≈ kWh/year × e

Interpreting your results

• Extra battery time is usually modest unless CPU power is a large fraction of the total draw. On light tasks, the display, Wi‑Fi, and background activity can dominate, so undervolting may only add minutes.
• Gains scale with CPU share: if your baseline system draw is 12 W and the CPU is only 2 W, even a large percentage reduction won’t move the total much.
• Cost savings are often small for a single laptop because the wattage change is small and usage hours are limited. The more compelling benefit is frequently lower temperatures/fan noise and more unplugged time.

Worked example

Suppose:

• Battery capacity C = 60 Wh
• Baseline runtime R = 5 hours
• CPU power at load P_c = 6 W
• Undervolt reduction r = 15% = 0.15
• Daily use H = 4 hours/day
• Electricity rate e = $0.15/kWh

Baseline total power:

P_t,base = 60 / 5 = 12 W

CPU reduction:

ΔP = 6 × 0.15 = 0.9 W

New total power:

P_t,new = 12 − 0.9 = 11.1 W

New runtime:

R_new = 60 / 11.1 ≈ 5.41 hours

Gain:

≈ 0.41 hours ≈ 24–25 minutes

Annual energy savings:

kWh/year ≈ (0.9 × 4 × 365)/1000 ≈ 1.31 kWh

Annual cost savings:

≈ 1.31 × 0.15 ≈ $0.20/year

Quick comparison: how reduction % affects runtime (same baseline)

Using the same baseline values from the example (60 Wh, 5 h baseline, CPU 6 W), here’s how different undervolt reductions change the estimate.

Assumptions & limitations

• Workload consistency: Baseline runtime and CPU power must represent the same kind of use. If you measure CPU power during a heavy benchmark but the baseline runtime is from web browsing, results will be misleading.
• CPU-only change: The model assumes undervolting only reduces CPU power. In real use, changes to temperature can also affect boosting behavior, fan power, and (indirectly) other components—sometimes improving, sometimes worsening net draw.
• Nonlinear power behavior: CPU package power may not scale linearly with voltage changes across all frequencies and power states. Some systems clamp power (PL1/PL2), and undervolting might change sustained clocks rather than power in a simple proportional way.
• Battery health and reporting: Aged batteries may have less usable Wh than the rated spec. Power telemetry can also be noisy or averaged differently across tools.
• Screen and GPU dominance: If display brightness or a discrete GPU dominates total power, CPU undervolting can have a small effect on overall runtime.
• Stability and safety: Too much undervolt can cause crashes, freezes, data corruption, or failed wake-from-sleep. Always stability-test changes and revert if you see errors.
• Platform constraints: Some laptops/BIOS versions restrict or disable undervolting (e.g., due to security mitigations). In those cases the achievable reduction may be near zero.

How to choose reasonable inputs (practical tips)

• Battery capacity (Wh): use the manufacturer spec or the OS battery report/design capacity. Prefer “full charge capacity” if available.
• Baseline runtime: time a typical session from 100% to a low-battery warning at a consistent brightness and workload.
• CPU power at load: use an average over several minutes while doing the same workload; short spikes can exaggerate results.
• Reduction %: if you don’t have measurements, start conservative (e.g., 5–10%) and update after you observe real power deltas.

FAQ

Does undervolting always increase battery life?

Not always. If the CPU isn’t a major contributor to total system power during your workload, the total draw barely changes. Also, instability can cause reboots or inefficiencies that negate any gains.

Can undervolting reduce performance?

If stable, undervolting typically preserves performance. In some cases it can even reduce thermal throttling. If unstable, performance can degrade due to errors, crashes, or conservative fallback behavior.

Why are the dollar savings so small?

A few watts over a few hours/day is a small number of kWh per year. The main payoff is often comfort (heat/fans) and extra unplugged minutes.