Own your power.
Peakwise (峰值) is a free calculator that tells a Taiwan factory in 60 seconds whether on-site battery storage pays back, using the real Taipower tariff and the MOEA subsidy.
Try it live · See it in action · Highlights · More by juw12
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Free, in your browser One HTML file. Nothing to install, no sign-up, and your numbers never leave the page. |
Real Taiwan numbers Taipower's October 2025 three-stage high-voltage tariff, MOEA storage subsidy and current BESS prices. |
Check any quote Shows the maximum fair price per kWh, so you know if an integrator's offer pays. We don't sell batteries. |
Open the calculator at juw12.github.io/peakwise/calculator/
Taiwan just made peak electricity expensive. Storage can now pay for itself.
Since Taipower's October 2025 tariff update, a high-voltage factory pays NT$9.39/kWh at the summer peak and only NT$2.53/kWh off-peak. That NT$6.86 spread, plus the MOEA subsidy of up to NT$5M per MWh, turns a behind-the-meter battery from "nice to have" into a 6-year payback.
Most factory owners never run the numbers because a consultant quote takes weeks. Peakwise takes your contract capacity, peak load and monthly bill, and tells you in a minute whether a battery is worth a site visit.
Unlike the calculators run by battery sellers, Peakwise needs no login and no meter-data upload, says plainly when a battery is not worth it, shows the downside (cost +20 %, savings −20 %, no subsidy), and gives a shareable link you can send to your boss.
- Open juw12.github.io/peakwise (中文 or English, auto-detected; switch any time).
- Click Open the calculator and enter your contract capacity, peak load, monthly kWh and working days.
- Read the recommended size, payback, IRR and 15-year NPV, then Print / save PDF.
A sample Chinese report is in docs/sample-report.pdf.
- Arbitrage: charge off-peak and discharge at peak on working days. Summer uses the peak spread and the rest of the year uses the half-peak spread. A cycle only runs if its margin beats battery wear.
- Sizing: inverter power is capped at the smaller of contract capacity and peak load, and storage is sized for 2 hours.
- Losses: 88% round-trip efficiency, 90% usable depth, 2%/yr fade and 1.5%/yr O&M.
- Money: NT$12.5M/MWh installed, MOEA subsidy of NT$5M/MWh (≤50%, 1–10 MWh), 15-year life and a 6% discount rate.
- Excluded on purpose: Taipower dReg ancillary-service revenue, because clearing prices are near zero after the market filled up.
Reference case: a 2 MWh / 1 MW system from model/taiwan_pilot.py, with net capex of NT$25M, or NT$15M after the subsidy:
| Tariff | Subsidy | Payback | IRR | NPV (NT$M) |
|---|---|---|---|---|
| 2023 tariff | no | never | — | −21.2 |
| 2023 tariff | yes | 10 yr | 2.8% | −2.5 |
| Current (Oct 2025) | no | 10 yr | 4.7% | −1.8 |
| Current (Oct 2025) | yes | 6 yr | 14.8% | +8.2 |
| Storage TOU (proposed) | yes | 5 yr | 20.6% | +15.6 |
(Low demand-charge saving case. The model also reports "none" and "high" cases.)
This is a screening tool, not an engineering study. Real projects still need a site survey, a grid-connection review and a binding integrator quote.
- Bilingual calculator with Python-parity self-test
- Printable Chinese report
- Upload a Taipower bill (PDF) to auto-fill the inputs
- 15-minute load-profile import for exact peak shaving
- Solar + storage combined case
- Integrator directory with verified quotes
python tests/test_model.py # model regression tests plus calculator parity check
python model/taiwan_pilot.py # full scenario table (JSON + chart in model/taiwan_pilot_output/)
# web: no build step; open the file directly, or serve docs/
python -m http.server 8000 -d docs # then visit http://localhost:8000/calculator/#selftestThe #selftest hash runs the browser engine on the Python reference case and prints its results as JSON. tests/test_model.py checks that they match.
Released under the MIT License © 2026 Juwen Lie. Contributions are welcome under the same license; see the contributing guide and Code of Conduct.