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

Contract
Loss chain · POI → cell
Warranty & power side

The energy waterfall

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

  1. HV transformer (POI is on the HV side)→ 402 MWh

    The offtake meter sits on the HV side, so the step-up transformer is the first loss you buy back — 99.5% on the discharge leg.

  2. AC cabling→ 403.2 MWh

    MV/HV AC runs and terminations between the transformers.

  3. Auxiliary / ancillary loads→ 411.4 MWh

    Thermal management, controls, and standby draw ~2% of throughput — a serious model books this as its own line, not a footnote.

  4. MV transformer→ 415.6 MWh

    The medium-voltage step-up between the PCS and the collection bus.

  5. PCS conversion→ 421.9 MWh

    The inverter's own conversion loss on the discharge leg.

  6. DC cabling→ 423.2 MWh

    Cable runs and fusing from the racks to the PCS.

  7. Usable SoC window (DoD)→ 427.5 MWh

    The BMS reserves headroom at the ends of the SoC range; only 99% of installed energy is dispatchable.

  8. Production-to-BOL cell fade→ 429.6 MWh

    Cells lose a little capacity between the factory and first commissioning.

  9. Cell one-way efficiency→ 442.9 MWh

    The cell's own round-trip loss, counted on the discharge half.

  10. Degradation to year 20 (SOH 70%)→ 632.7 MWh

    Size so the fleet still clears the contract in year 20, once it has faded to 70% of BOL — unless you augment mid-life instead of overbuilding on day one.

632.7 MWhDC nameplate to install (1.58× contracted) — sized so it still delivers in year 20
442.9 MWhif you only sized for day one (1.11× — the loss chain alone, no degradation)
0.16 Cdischarge C-rate against the installed 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, after the full HV-transformer-to-cell loss chain, a 99% 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, the overbuild analysis, 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/