The BESS sizing calculator. The waterfall, interactive.

Enter a contract — power, duration, guarantee year — and watch it walk backward through every real-world loss to the DC nameplate you'd actually have to buy. The same method as the sizing guide, with your numbers — and the same math behind our computed analysis of the BOL overbuild.

Assumptions

The energy waterfall

Contract: 400 MWh usable at the POI (100 MW × 4 h)

  1. One-way conversion losses (PCS + transformer, DC→POI)→ 421.1 MWh

    Delivering 400 MWh at the POI means discharging 421.1 MWh from DC — the 95% one-way chain eats the rest. (Round-trip ≈ one-way², so this corresponds to ~90% RTE.)

  2. Auxiliary loads (thermal management, controls, standby)→ 434.1 MWh

    Auxiliaries consume ~3% of throughput. A serious model books this as its own line, not a footnote.

  3. Usable SoC window (DoD)→ 456.9 MWh

    The BMS holds reserves at both ends of the SoC range; only 95% of installed energy is dispatchable.

  4. Degradation to year 20 (SOH 70%)→ 652.7 MWh

    The contract must still be met in year 20, when the fleet has faded to 70% — unless you plan augmentation instead of day-one overbuild.

652.7 MWhDC nameplate at BOL (1.63× the contracted energy)
0.15 Cdischarge C-rate against the BOL pack
105.3 MVA → 30 × 3.6 MVAPCS fleet: converters are sized in MVA, not MW — at PF 0.95, 100 MW needs 105.3 MVA (108 MVA installed)
The guarantee-point sentence

400 MWh usable at the POI, deliverable at 100 MW, in year 20, assuming 95% one-way efficiency, 3% auxiliaries, a 95% SoC window, and 70% SOH.

First-pass educational estimate. Real projects add temperature/altitude derating, cycling-dependent degradation curves, augmentation strategies, and vendor-specific capability data — exactly what the sizing guide and the full course teach.
Use it in your work

Link or cite this tool

Free to use and link. Citation: BESS Sizing Calculator, BESS.courses — https://bess.courses/tools/bess-sizing/