Methodology Guide

History endpoints are served from Voltstack's own settled archive, not from request-time calls to the upstream. An ingestion worker persists each feed's settled values on the upstream's own publication cadence; one-off backfills seed the depth. The same persisted series feeds both the live headline figure and its history — one source of truth, never two independently maintained pipelines that can disagree.

Honesty clamp: oldestAvailable

A history request never silently returns a shorter series than asked. Every history payload reports oldestAvailable — the earliest real row held, independent of the requested window (per series for commodities, since daily and monthly cadences clamp at different depths). When real coverage starts later than the requested from, the terminal renders "history since X" instead of pretending the full window arrived.

Data age

meta.dataAgeSeconds counts from the moment the underlying persisted data last changed (the worker's ingestion timestamp), not from when the response was computed. It is recomputed on every response — including cache hits — so a cached payload never freezes or hides its own age.

Point-in-time capture

The settled archive keeps one value per delivery interval; a source revision replaces it. Two capture layers sit alongside it. Event-grade series (REMIT urgent market messages, unit outages, redispatch) retain every republished version with its publication timestamp, since 2026-07-14. Since 2026-07-31 an append-only vintage sidecar also records each capture pass of the four forecast feeds (daily weather, forecast risk, the GB NESO forecast, the FR RTE forecast), keyed by the pass's capture timestamp. The as-of query API over these vintages follows in a later revision; no derivation in this guide reads vintages yet.

Day-ahead resolution is not assumed. The single day-ahead coupling moved from hourly (PT60M) to quarter-hourly (PT15M) market time units on 2025-10-01, so a history window can be hourly, quarter-hourly, or genuinely mixed. Resolution is inferred from the actual spacing between consecutive points in each response, and the quality figures follow from it:

• meta.completeness — arrived points ÷ expected points for the window at the inferred resolution, in [0, 1].
• meta.gaps — explicit timestamp ranges where expected points are missing, rather than a silently thinner series.

Long-range price charts and every derived measure in §5 use the daily baseload average: the mean of all delivery-slot prices in the UTC calendar day, whatever the underlying resolution. On the 1-day view, four quarter-hourly prices average into one hourly bar for legibility. A price, like any rate, is honestly summarized by an average over a shorter window — unlike an energy quantity (MWh), which would need summing instead.

Why not implied volatility: no freely redistributable options data exists for European power or gas, so a strike/tenor implied-vol surface cannot be built honestly. Voltstack publishes a realized measure derived from its own settled day-ahead archive instead, and labels it as such.

Why not log returns: EU day-ahead prices go negative routinely — every zone Voltstack tracks has real negative-price hours on record. ln(P_t / P_t−1) is undefined the moment either price is zero or negative, so the equities-style annualized log-return vol is unusable here — not merely fragile.

• Sample standard deviation uses the n−1 denominator — the standard estimator for a finite historical sample.
• Windows are 7, 30 and 90 real trading days of daily averages, not calendar days: if the underlying series has a gap, a window spans a few more calendar days than its name suggests rather than shrinking its sample.
• Units are ccy/MWh — never annualized, never a percent. The figure can be sanity-checked directly against the price series itself (the spread of day-over-day moves).
• A window reads null until enough real changes exist before that display day — rendered as a break in the line, never a fabricated early reading from a too-short sample.
• Archive reads are buffered 90 days before the display window, so the first displayed day carries the same window depth as every later one whenever the real data goes back far enough.

The EU gas-storage "normal range" is computed, not simulated. For each display day, the band is the min / max / average of the same calendar month-day across the five years strictly before that day's own year:

• A display day is never a member of its own comparison band — even when a wide request spans more than one calendar year, each day is compared only against prior years.
• February 29 matches only real February 29 rows. If the trailing five-year window holds no persisted leap day, the band fields read null for that one day — an honest gap, not an approximation borrowed from Feb 28 or Mar 1.
• A display day with no real storage level at all is dropped entirely, never interpolated.

Underground storage follows a repeatable seasonal shape (inject in summer, withdraw in winter), which is what makes a five-year band a meaningful "normal range". LNG send-out does not: it is driven by spot-arbitrage economics — inter-basin price spreads, freight, and terminal slot availability. A min/max/avg band over that series would dress noise up as a norm, so the LNG history endpoint publishes the real trend only.

The merit-order curve is assembled from three measured inputs plus one clearly-labeled modeled layer:

Reference-year rule

One reference year is chosen per country — the most recent calendar year any fuel reports capacity for — and only fuels with a row in that year enter the stack. Taking each fuel's own all-time-latest row independently would resurrect retired fleets (Germany's nuclear exit is the live example: a per-fuel-latest read shows a phantom ~4 GW nuclear block from pre-shutdown years). Excluding a retired fuel entirely is the honest behavior; carrying a stale figure forward is not.

Derates and the AND-gate

Available MW per fuel = installed − the sum of currently active derates (latest version per outage message), floored at zero. Derate collection covers all eight merit-order countries (DE, FR, ES, IT, NL, NO, SE, PL). The AND-gate still applies: a zero derate read only counts as real for a country the collector actually asks about — the stack is marked live only where capacity, derates and demand are all real, and falls back otherwise.

Modeled marginal costs

Fixed short-run assumptions for ordering the stack, carried with a permanent "costs: modeled" label in the terminal regardless of live status. The indicated clearing price is the modeled cost of the marginal fuel where cumulative availability meets demand — an illustration of the stack, not a price forecast. Fuels are eight buckets: ENTSO-E reports one MW figure per production type, so gas cannot be split into CCGT/OCGT cost tiers on real data.

Server-side cache TTLs are matched to each upstream's true publication cadence — polling faster than the TTL returns the cached value with cached: true, and meta.dataAgeSeconds keeps counting from the real ingestion timestamp either way. TTLs per domain are tabulated in the API Specification §7.

Degree days: HDD/CDD are computed at read time from the persisted forecast against a base of 18°C (EU) / 65°F (US) by default. An optional per-request base (°C, clamped 5–30) re-bases both; the base joins the cache key so different bases never share a cached value.

Where a real input does not exist yet, the terminal falls back to simulation — and says so. The Data Sources panel groups every connector by honesty tier (Live · Awaiting Access · Simulated · Enterprise, plus an Available slot for tenant-configured custom REST integrations) so the first question it answers is what is real. A payload is either entirely real or a declared fallback; measured and simulated values are never blended inside one figure.