Battery payback

Try it: battery-payback.larsvanbraam.nl, or go straight to the sample household.

Which home battery pays for itself on your own meter data, once saldering ends in 2027? The page replays a year of hourly grid use and solar surplus through several home batteries (HomeWizard, Marstek, Zendure, Anker, EcoFlow, Sessy), each at every size it comes in, ranks them by profit over their lifetime, and shows what the chosen one saves per year, month and day.

Built with Vite and TypeScript, as a static site with no server code. The only runtime dependency is Vercel Analytics.

Disclaimer: Battery Payback is a free, independent tool. All figures are estimates from past use and expected prices, not financial advice, and come without any guarantee. No rights can be derived from them, and the maker accepts no liability for decisions based on them.

Your data stays in your browser

First-time visitors get a start screen with three choices: fetch their data, upload a file, or explore a made-up sample household (also linkable as /sample). Returning visitors go straight to their stored results. The two ways to load your own data:

• From their energy supplier, with a bookmarklet. They drag a button to their bookmarks bar, log in to their supplier's site and click the bookmark. The bookmarklet runs there, in their own logged-in browser: it fetches the past year of hourly usage from the supplier's own API, opens this site in a new tab and hands the data over with postMessage. If that tab can't be reached (a popup blocker, for example), the data downloads as a CSV to upload instead. This site's server never sees a login or a meter reading, and the supplier only sees the same requests its own site makes. Clicking the bookmark again later fetches only the new days.
• By uploading a CSV with timestamp,import_kwh,export_kwh, one row per hour (ISO timestamps with an offset, for example 2026-06-01T12:00:00+02:00).

Whatever is loaded is kept in localStorage, so the page shows the visitor's own numbers next time. They can download it as CSV or forget it from the "Load your data" dialog.

Supported suppliers: Greenchoice. See Adding a supplier.

Develop

npm install
npm run dev

Open http://localhost:5173. In development the supplier list also has a Demo supplier that serves the sample household from http://127.0.0.1:5173, a different origin, so you can try the whole bookmarklet flow without an account: drag its button to your bookmarks bar, open http://127.0.0.1:5173 and click the bookmark.

npm test          # Vitest: simulation, Greenchoice parsing, dataset merging, message validation, routing
npm run typecheck # type-check only
npm run build     # type-check, then build to dist/
npm run preview   # serve the built dist/ locally
npm run sample    # regenerate public/data/sample-hourly.csv

Node 22.18 or newer is needed (engines in package.json).

The share image and icons in public/ are rendered from scripts/og/; scripts/og/README.md has the commands. The screenshot in this README is the live sample page:

"/Applications/Google Chrome.app/Contents/MacOS/Google Chrome" --headless --hide-scrollbars \
  --window-size=1440,1280 --virtual-time-budget=10000 \
  --screenshot=docs/screenshot.png https://battery-payback.larsvanbraam.nl/sample

Paths

Path	Shows
/	The start screen, or straight to /results when this browser has stored data
/sample	The results for the made-up sample household
/results	The results for your stored data
/import/<supplier>	Where the bookmark opens the tool to hand over your data

Old links with #sample or #import=<supplier> are rewritten to these paths. The routing is src/router.ts, using the browser's History API.

What visitors can change on the page

Nothing here is stored except the loaded data; every setting starts from its default on each visit.

Setting	Default	What it changes
Prices from 2027	Eneco, 1 year fixed	Picks a row from tariffs-2027.csv
Grid price, Sent back earns	From the picked tariff	Overrides the price per kWh bought and the net value per kWh sent back
I have a HomeWizard P1 meter	Off	Drops the meter price for batteries with homewizard_p1 = 1
Ranking sort	Profit	Click Price, Saves a year, Paid back in, Profit or Warranty to sort; Best buy stays the most profitable
Battery, How many	HomeWizard, 1	The battery the rest of the page shows; clicking a ranking row picks it too
Price of the first one	From batteries.csv	Overrides that battery's price for this visit
Years you'll use it	15	The lifetime for profit after N years and for what counts as paid back
What-if presets and sliders	Realistic	Grid use (−30% to +50%), solar left over (−30% to +30%), grid price change per year (−5% to +8%), battery ageing per year (0–5%) and real-world shortfall (0–20%), for the year-by-year projection only
Monthly payment, Welcome discount	€65, €150	The supplier bill with and without the battery

Defaults in the code

Constant	Where	Value
DEFAULT_SCENARIO	src/main.ts	eneco_1y, a scenario in tariffs-2027.csv
DEFAULT_BATTERY	src/main.ts	homewizard, an id in batteries.csv
RANKING_ROWS	src/main.ts	5 rows shown before "Show all"
HEAVY_KG	src/main.ts	40 kg, from which the weight label turns amber
WHAT_IF_PRESETS	src/main.ts	The four what-if presets and their slider values
DAYS	src/bookmarklet/runtime.ts	365 days fetched by the bookmarklet, up to yesterday
KEEP_DAYS	src/dataset.ts	365 days kept in the browser; older days are dropped
KEY	src/dataset.ts	home-battery-payback:dataset, the localStorage key

Adding a supplier

1. Create src/providers/<id>/info.ts with the supplier's id, name, origin (where the bookmarklet runs, for example https://mijn.greenchoice.nl) and loginUrl.
2. Create src/providers/<id>/bookmarklet.ts that calls runBookmarklet from src/bookmarklet/runtime.ts with a fetchHours(range, onProgress) that returns [timestamp, importKwh, exportKwh] rows for that supplier. Keep it polite: a few requests at a time, as the supplier's own site would make. src/providers/greenchoice/ is the reference.
3. Add it to PROVIDERS in src/providers/index.ts, importing the bookmarklet with the ?bookmarklet suffix. plugins/bookmarklet.ts bundles and minifies it into the link.
4. Add parsing tests with a trimmed real response, like tests/import.test.ts does for Greenchoice.

Layout

Path	What it is
index.html	Page markup
src/main.ts	Wires the controls and renders every block
src/simulate.ts	CSV parsing, battery simulation and cost comparison (no DOM)
src/charts.ts	Dependency-free SVG charts and tooltips
src/import.ts	Loading data (sample, upload, bookmarklet handoff, local storage) and which screen shows
src/router.ts	Paths, Back and Forward, old hash links
src/start.ts · src/start.css	The start screen
src/dataset.ts	Stored dataset: merge, clean and save
src/bookmarklet/	Shared bookmarklet runtime and the message protocol with the page
src/providers/	One folder per supplier, plus the dev-only demo
plugins/bookmarklet.ts	Vite plugin that turns ?bookmarklet imports into a minified script string
plugins/site-url.ts	Vite plugin that fills %SITE_URL% in index.html for the share links, from the SITE_URL environment variable or else the Vercel production domain
public/data/	Battery specs and prices, 2027 tariff presets, the sample household
scripts/generate-sample.ts	Generates the sample household
scripts/og/	Sources of the share image and home-screen icon
docs/screenshot.png	The screenshot in this README
vercel.json	Sends every page path to index.html
tests/	Vitest tests

Where the data comes from

Every figure was collected by hand from the sources below; nothing is fetched automatically. Prices and tariffs go stale within months, so each file records when it was checked. The page shows the oldest battery price_checked under the ranking and each tariff's checked date under the price picker.

Data	Source	Checked
Battery prices	Tweakers Pricewatch, manufacturers' and retailers' list prices (price_source)	price_checked: all 27 Oct 2026
Meter prices and HomeWizard P1 support	Manufacturers' and retailers' stores (meter_source)	Oct 2026
Efficiency, standby and usable storage	Tweakers lab tests for the Marstek, Anker Max and EcoFlow AC 5000; a year of home use for the HomeWizard; one review each for the Zendure, Anker E2700 Pro and Sessy; assumed for the EcoFlow AC Pro (*_source, review_url)	Reviews from 2025 and 2026
Size and weight	Spec sheets, manuals and retailer listings (size_source)	Aug 2024 to Oct 2026
2027 tariffs	Suppliers' own sites, feed-in costs via Keuze.nl (source_url)	checked: 27 Oct 2026
Sample household	Made up by scripts/generate-sample.ts	—

To refresh a figure, change it in the CSV and update its date column.

Data formats

• Tariffs (public/data/tariffs-2027.csv): one row per supplier contract: scenario,supplier,contract,import_eur_kwh,feedin_eur_kwh,feedin_cost_eur_kwh,net_export_eur_kwh,source_url,checked,note. Grid prices include VAT and energy tax; net_export_eur_kwh is what a kWh sent back is worth after feed-in costs, the figure the calculation uses. feedin_eur_kwh, feedin_cost_eur_kwh, source_url and checked can be empty, as in the stress what-if row. The picker groups rows by supplier and shows note, the source and checked under it.
• Batteries (public/data/batteries.csv): one row per battery. Add a row to compare another one.
  • Name: id (unique; DEFAULT_BATTERY refers to it), brand, model.
  • Specs: unit_kwh, usable_share, charge_kw, discharge_kw, max_power_kw, standby_w, rated_cycles, warranty_years.
  • Efficiency curve: rte_200, rte_500, rte_800, rte_max, the round-trip efficiency at 200 W, 500 W, 800 W and full power, as Tweakers measures it. Leave the low-power columns empty when only an 800 W figure is known; they're then estimated from the average drop Tweakers found.
  • Combining units: power_scales (1 if each extra unit adds power, 0 if it only adds storage) and max_units.
  • Costs: first_unit_eur, extra_unit_eur, install_eur, and price_checked (YYYY-MM-DD, when the price and meter price were last checked).
  • Meter: meter (what the battery needs to read the grid meter), meter_eur (0 when included), homewizard_p1 (1 if a HomeWizard P1 meter can stand in for it) and meter_source.
  • Size: size_cm (one unit's outside size, for example 40 × 30 × 20, in the order the source gives it), weight_kg and size_source.
  • Provenance: capacity_source, and (, , or ; can also be , a year measured at home, used at every power), and (who measured it, for ), , , .

License

MIT © Lars van Braam. The figures in public/data/ stay credited to the sources named in their source columns; see Where the data comes from.