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What This Calculator Does
This tool estimates your Tesla battery's true usable capacity using data from your vehicle's own displays — no third-party hardware needed. It uses three different calculation methods, so you can see how they compare and understand the confidence level of each.
The Core Idea: SOC Window
Every charge cycle tells you something. Between the end of your last charge and the start of this one, your battery moved from one state-of-charge (SOC) to another. That difference is the SOC window:
Example: 90% → 7% = 0.83 window (83% of the pack was consumed)
If you also know how many kWh the car consumed over that window, you can estimate what the full 100% pack would hold:
Example: 42.5 kWh ÷ 0.83 = 51.2 kWh estimated full-pack capacity
Larger SOC windows (≥ 70%) give more accurate results because BMS percentage rounding errors (±1%) matter less relative to the larger window.
The Three Methods
| Method | Formula | What It Measures | Best For |
|---|---|---|---|
| BMS Display | (End Miles ÷ End SOC%) × 100 ÷ Original Range × Nominal kWh | Tesla's internal model of the battery — what the computer thinks | Reference; tends to overstate real capacity |
| Observed (Primary) | kWh Used ÷ SOC Window | Actual energy delivered per SOC unit — driving + parked losses combined | Warranty evaluation; most conservative & directly measurable |
| Drive-Adjusted | kWh Used ÷ Drive Fraction ÷ SOC Window | Isolates drive energy, then scales to full pack — requires optional inputs | Sensitivity analysis; typically 3–5 kWh higher than Observed |
The Observed method is the primary metric because it requires no subjective split between driving and parking — it uses only what the car's displays show directly.
Battery Stack Breakdown
The color-coded battery visual shows where every kWh of the original nominal pack went. Think of the full bar as "100% of what the battery was designed to hold."
| Color | Component | What It Means |
|---|---|---|
| Drive | Drive Energy | kWh actually used to move the car |
| Park | Parked Standby Loss | Energy lost while the car sat idle (vampire drain) |
| Phantom | SOC Reconciliation | Gap between logged Drive+Park SOC and observed window — near zero is normal |
| Uncharged | SOC Not Accessed | Portion of pack left unused this cycle (e.g., if you stopped at 7% instead of 0%) |
| Degradation | Lost Capacity | Capacity that no longer exists — gone due to battery aging |
Without the optional Drive/Park SOC inputs, Drive and Park are shown combined as "Energy Used."
Cycle Quality
Not every cycle produces equally reliable results:
- ≥ 70% SOC window: Large qualifying cycle — most reliable. BMS rounding errors are small relative to the window.
- 50–69% SOC window: Moderate reliability — usable but has slightly higher uncertainty.
- Below 50%: Too small for a reliable estimate — the 1% BMS rounding creates too much relative error.
Warranty Context
Tesla's battery warranty for most models guarantees at least 70% of original capacity for 8 years or 150,000 miles. For a 75 kWh pack, the threshold is 52.5 kWh. If your Observed estimate consistently falls below this threshold, you may have grounds to request a formal battery evaluation from Tesla before your warranty expires.
This tool does not replace Tesla's official diagnostic — it helps you document consistent, reproducible evidence using your car's own data.
Important Caveats
- All estimates use the BMS's 1% SOC granularity — inherent uncertainty of ±0.5–1 kWh.
- The Observed method assumes Tesla's kWh display includes ALL energy since last charge (driving + parked). This is our best understanding, but not officially confirmed.
- Temperature, driving style, and short trips affect efficiency — not capacity. These methods measure delivered capacity, not efficiency.
- A single cycle result is less meaningful than an average across many large cycles.