Update README with fee configuration details (v1.1)

Clarify configuration for provider fees and VAT rates:
negative price provider fee support added in EPrices v1.1

CHANGELOG.md

@@ -1,4 +1,50 @@
-# EPrices v1.0 – Changelog
+# EPrices – Changelog
+
+## v1.1 — 2026-04-06
+
+### Negative price provider fee support
+
+Added a separate provider fee multiplier for negative spot prices, configurable
+via `secrets.yaml`. This correctly models contracts where the provider fee
+structure differs between positive and negative market prices.
+
+**New secret key:**
+
+```yaml
+eprices_neg_prov_fee: "0.30"
+```
+
+**New global:** `neg_prov_fee` (type: `double`)
+
+**Price calculation is now:**
+
+| Market price | Formula |
+|---|---|
+| Positive (`raw_mwh >= 0`) | `(raw / 1000) × (1 + prov_fee) × (1 + vat_rate)` |
+| Negative (`raw_mwh < 0`) | `(raw / 1000) × (1 - neg_prov_fee) × (1 + vat_rate)` |
+
+**Key values for `eprices_neg_prov_fee`:**
+
+| Value | Meaning |
+|---|---|
+| `"0.30"` | Provider keeps 30%, pays you 70% of negative price |
+| `"0.00"` | Provider passes full negative price to you (no fee) |
+
+**Switch point:** raw API price (`raw_mwh`) before any multiplier is applied.
+
+**VAT:** applied symmetrically to both positive and negative prices, consistent
+with net billing models where VAT is calculated on the monthly net sum
+(mathematically equivalent due to VAT being a linear multiplier).
+
+**Changed files:** `eprices.yaml`, `secrets.yaml`
+
+**Changed locations in `eprices.yaml`:**
+- `substitutions:` — added `neg_prov_fee_value`
+- `globals:` — added `neg_prov_fee`
+- `parse_energy_charts_today_script` — `MULT` split into `MULT_POS` / `MULT_NEG`
+- `parse_energy_charts_tomorrow_script` — `MULT` split into `MULT_POS` / `MULT_NEG`
+
+---
 
 ## v1.0 — 2026-04-05
 

README.md

@@ -16,7 +16,8 @@ Assistant automations are required for any core functionality.
 ## Features
 
 - Fetches **15-minute resolution** spot prices for **today** and **tomorrow**
-- Automatically applies your **provider fee** and **VAT rate** to raw €/MWh prices
+- Automatically applies your **provider fee** and **VAT rate** to raw €/MWh prices —
+  with **separate fee configuration for negative spot prices**
 - Exposes **96 per-day price points** plus **24-hour averages** as JSON text sensors
   for use in HA automations and dashboards
 - **NVS persistence** — prices survive device reboots without re-fetching
@@ -28,7 +29,7 @@ Assistant automations are required for any core functionality.
 - Supports **any Energy-Charts bidding zone** (SI, DE-LU, AT, FR, HR, HU and more)
 - Full **diagnostic sensor suite** — NVS status, fetch attempts, API fetch times,
   data loaded times, WiFi signal, human-readable uptime
-- Provider fee and VAT rate configurable via `secrets.yaml` — **no code changes needed**
+- All fee and VAT settings configurable via `secrets.yaml` — **no code changes needed**
 
 ---
 
@@ -46,7 +47,7 @@ Assistant automations are required for any core functionality.
 | `eprices.yaml` | Main ESPHome configuration |
 | `eprices_nvs.h` | NVS helper — save/load price arrays to ESP32 flash |
 | `secrets.yaml` | Your local secrets (not committed to git) |
-| `CHANGELOG.md` | Complete sensor and entity ID reference for v1.0 |
+| `CHANGELOG.md` | Complete sensor and entity ID reference |
 | `VERSION.md` | Version history and release notes |
 | `ENTSO-E-PRICES-MIGRATION.md` | Optional — migration guide from the predecessor project |
 
@@ -71,6 +72,7 @@ eprices_timezone: "Europe/Ljubljana"
 eprices_country_bzn: "SI"
 eprices_prov_fee: "0.12"
 eprices_vat_rate: "0.22"
+eprices_neg_prov_fee: "0.30"
 ```
 
 **Key descriptions:**
@@ -79,8 +81,9 @@ eprices_vat_rate: "0.22"
 |---|---|
 | `eprices_timezone` | Your local timezone — must match a valid [IANA tz name](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) |
 | `eprices_country_bzn` | Energy-Charts bidding zone code — see [supported zones](#supported-bidding-zones) |
-| `eprices_prov_fee` | Provider fee as a decimal multiplier, e.g. `"0.12"` = 12% |
+| `eprices_prov_fee` | Provider fee for **positive** spot prices — decimal multiplier, e.g. `"0.12"` = 12% |
-| `eprices_vat_rate` | VAT rate as a decimal multiplier, e.g. `"0.22"` = 22% |
+| `eprices_vat_rate` | VAT rate — decimal multiplier, e.g. `"0.22"` = 22% |
+| `eprices_neg_prov_fee` | Provider fee kept on **negative** spot prices — decimal multiplier, e.g. `"0.30"` means provider keeps 30%, pays you 70%; use `"0.00"` if provider passes the full negative price to you |
 | `eprices_api_encryption_key` | 32-byte base64 key for HA API encryption — generate with `openssl rand -base64 32` |
 
 ### 3) Flash to your ESP32
@@ -133,11 +136,22 @@ For the full list see the [Energy-Charts API documentation](https://api.energy-c
 ### Price calculation
 
 Raw prices from the API are in **€/MWh**. EPrices converts them to **€/kWh**
-and applies your provider fee and VAT:
+and applies your provider fee and VAT. Positive and negative spot prices use
+separate fee multipliers, configurable independently in `secrets.yaml`:
 
-```
+| Market price | Formula |
-price_eur_kwh = (raw_eur_mwh / 1000) × (1 + prov_fee) × (1 + vat_rate)
+|---|---|
-```
+| Positive (`raw ≥ 0`) | `(raw / 1000) × (1 + prov_fee) × (1 + vat_rate)` |
+| Negative (`raw < 0`) | `(raw / 1000) × (1 - neg_prov_fee) × (1 + vat_rate)` |
+
+The switch happens on the **raw API price** before any multiplier is applied.
+VAT is applied to both cases, consistent with net billing contracts where VAT
+is calculated on the monthly net sum (mathematically equivalent due to VAT
+being a linear multiplier).
+
+**Example with `prov_fee = 0.12`, `neg_prov_fee = 0.30`, `vat_rate = 0.22`:**
+- Spot price `+100 €/MWh` → `(100/1000) × 1.12 × 1.22` = **0.1366 €/kWh** (you pay)
+- Spot price `−50 €/MWh` → `(−50/1000) × 0.70 × 1.22` = **−0.0427 €/kWh** (you receive)
 
 ### Negative prices
 

VERSION.md

@@ -1,5 +1,25 @@
 # EPrices – Version History
 
+## v1.1 — 2026-04-06
+
+### Negative price provider fee
+
+Added separate provider fee support for negative spot prices via a new
+`eprices_neg_prov_fee` secret key. Positive and negative market prices
+now use independent fee multipliers, correctly modelling contracts where
+the provider's fee structure differs between the two cases.
+
+Price calculation:
+- Positive: `(raw / 1000) × (1 + prov_fee) × (1 + vat_rate)`
+- Negative: `(raw / 1000) × (1 - neg_prov_fee) × (1 + vat_rate)`
+
+VAT is applied to both, consistent with net billing where VAT is calculated
+on the monthly net sum (linear equivalence applies).
+
+See `CHANGELOG.md` for full implementation details.
+
+---
+
 ## v1.0 — 2026-04-05
 
 First public release.

eprices.yaml

@@ -1,13 +1,22 @@
 # =============================================================================
-# ESPHome: EPrices v1.0
+# ESPHome: EPrices v1.1
-#
-# ESP32 firmware that fetches day-ahead electricity spot prices from the
-# public Energy-Charts API (https://api.energy-charts.info) and exposes
-# them as Home Assistant sensors. No API token required.
 #
 # Electricity price data provided by Energy-Charts (https://energy-charts.info)
 # under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
 #
+# v1.1 changes:
+# - Separate provider fee for negative spot prices (eprices_neg_prov_fee):
+#   positive prices: (raw / 1000) × (1 + prov_fee)     × (1 + vat_rate)
+#   negative prices: (raw / 1000) × (1 - neg_prov_fee) × (1 + vat_rate)
+#   switch happens on raw API price before any multiplier is applied
+#   VAT applied to both cases (consistent with net billing + end-of-month VAT)
+#   0.30 = provider keeps 30%, pays you 70% of negative price
+#   0.00 = provider passes full negative price to you (no fee)
+# - New substitution: neg_prov_fee_value (!secret eprices_neg_prov_fee)
+# - New global: neg_prov_fee (double)
+# - MULT split into MULT_POS / MULT_NEG in parse_energy_charts_today_script
+#   and parse_energy_charts_tomorrow_script
+#
 # v1.0 release notes:
 # - Fetches 15-min and hourly spot prices for today and tomorrow
 # - Prices converted from raw €/MWh to €/kWh with provider fee and VAT applied
@@ -43,6 +52,7 @@ substitutions:
   country_bzn_value:    !secret eprices_country_bzn
   prov_fee_value:       !secret eprices_prov_fee
   vat_rate_value:       !secret eprices_vat_rate
+  neg_prov_fee_value:   !secret eprices_neg_prov_fee
 
 esphome:
   name: eprices
@@ -497,6 +507,9 @@ globals:
   - id: vat_rate
     type: double
     initial_value: '${vat_rate_value}'
+  - id: neg_prov_fee
+    type: double
+    initial_value: '${neg_prov_fee_value}'
 
   # --- Pre-computed fetch URLs (url: !lambda not supported in scripts) ---
   - id: fetch_url_today
@@ -1261,7 +1274,8 @@ script:
       raw_body: string
     then:
       - lambda: |-
-          const double MULT = (1.0 + id(prov_fee)) * (1.0 + id(vat_rate));
+          const double MULT_POS = (1.0 + id(prov_fee))     * (1.0 + id(vat_rate));
+          const double MULT_NEG = (1.0 - id(neg_prov_fee)) * (1.0 + id(vat_rate));
 
           id(price_values_today).clear();
           id(price_timestamps_today).clear();
@@ -1350,7 +1364,7 @@ script:
             if (end1 == ts_toks[i].c_str() || end2 == prc_toks[i].c_str()) continue;
             if (unix_ts <= 0) continue;
 
-            float eur_kwh = (float)((double)raw_mwh * MULT / 1000.0);
+            float eur_kwh = (float)((double)raw_mwh * (raw_mwh >= 0 ? MULT_POS : MULT_NEG) / 1000.0);
             id(price_timestamps_today).push_back(unix_ts);
             id(price_values_today).push_back(eur_kwh);
           }
@@ -1369,7 +1383,8 @@ script:
       raw_body: string
     then:
       - lambda: |-
-          const double MULT = (1.0 + id(prov_fee)) * (1.0 + id(vat_rate));
+          const double MULT_POS = (1.0 + id(prov_fee))     * (1.0 + id(vat_rate));
+          const double MULT_NEG = (1.0 - id(neg_prov_fee)) * (1.0 + id(vat_rate));
 
           id(price_values_tomorrow).clear();
           id(price_timestamps_tomorrow).clear();
@@ -1437,7 +1452,7 @@ script:
             if (end1 == ts_toks[i].c_str() || end2 == prc_toks[i].c_str()) continue;
             if (unix_ts <= 0) continue;
 
-            float eur_kwh = (float)((double)raw_mwh * MULT / 1000.0);
+            float eur_kwh = (float)((double)raw_mwh * (raw_mwh >= 0 ? MULT_POS : MULT_NEG) / 1000.0);
             id(price_timestamps_tomorrow).push_back(unix_ts);
             id(price_values_tomorrow).push_back(eur_kwh);
           }

secrets.yaml

@@ -0,0 +1,107 @@
+# =============================================================================
+# EPrices – Template secrets file
+# Copy this file to your ESPHome config directory as "secrets.yaml"
+# and fill in your own values.
+#
+# Notes on value formats:
+#  - All values are strings in secrets.yaml.
+#  - Numeric values (prov_fee, vat_rate) must use a DECIMAL DOT (e.g. 0.12),
+#    not a decimal comma. They are loaded as substitutions and passed directly
+#    into C++ double globals, so the dot is mandatory.
+# =============================================================================
+
+# --- WiFi credentials (shared with other ESPHome devices - remove if already present) ---
+wifi_ssid: "your_wifi_ssid"
+wifi_password: "your_wifi_password"
+
+# --- EPrices fallback AP (shown when WiFi is unavailable) ---
+eprices_fallback_ap_ssid: "EPrices-Fallback"
+eprices_fallback_ap_password: "your_fallback_ap_password"
+
+# --- ESPHome API encryption key (generate with: esphome generate-api-key) ---
+eprices_api_encryption_key: "your_base64_api_encryption_key_here"
+
+# --- Timezone string (IANA format, must match your location) ---
+# Examples: "Europe/Ljubljana", "Europe/Berlin", "Europe/London"
+eprices_timezone: "Europe/Ljubljana"
+
+# --- Bidding zone for Energy-Charts API ---
+# This determines which country's spot prices are fetched.
+# Examples: "SI" (Slovenia), "DE-LU" (Germany/Luxembourg), "AT" (Austria),
+#           "FR" (France), "IT-North" (Italy North), "HR" (Croatia)
+# Full list: https://api.energy-charts.info/
+eprices_country_bzn: "SI"
+
+# --- Provider fee multiplier for POSITIVE spot prices ---
+# Added on top of the raw spot price to account for your energy provider's fee.
+# Value is a DECIMAL MULTIPLIER, not a percentage.
+# Example: 0.12 means 12% provider fee added to the spot price.
+# Use decimal DOT, not comma (correct: 0.12 | wrong: 0,12).
+eprices_prov_fee: "0.12"
+
+# --- Provider fee multiplier for NEGATIVE spot prices ---
+# When the market spot price is negative, the provider pays you a share of
+# the negative price. This value is the fraction the provider keeps.
+# Example: 0.30 means provider keeps 30%, pays you 70% of the negative price.
+# Example: 0.00 means provider passes the full negative price to you (no fee).
+# Use decimal DOT, not comma (correct: 0.30 | wrong: 0,30).
+eprices_neg_prov_fee: "0.30"
+
+# --- VAT rate multiplier ---
+# Value added to the price to account for Value Added Tax in your country.
+# Value is a DECIMAL MULTIPLIER, not a percentage.
+# Example: 0.22 means 22% VAT (Slovenia). Germany is 0.19, Austria is 0.20.
+# Use decimal DOT, not comma (correct: 0.22 | wrong: 0,22).
+eprices_vat_rate: "0.22"
+
+# ==============================================================================
+# --- ADDITIONAL INFO: Bidding zone list for Energy-Charts API on 2026-04-04 ---
+# (Check https://api.energy-charts.info/ for potential future changes)
+#
+# AT - Austria (click to show all available bidding zones)
+# BE - Belgium
+# BG - Bulgaria
+# CH - Switzerland
+# CZ - Czech Republic
+# DE-LU - Germany, Luxembourg
+# DE-AT-LU - Germany, Austria, Luxembourg
+# DK1 - Denmark 1
+# DK2 - Denmark 2
+# EE - Estionia
+# ES - Spain
+# FI - Finland
+# FR - France
+# GR - Greece
+# HR - Croatia
+# HU - Hungary
+# IT-Calabria - Italy Calabria
+# IT-Centre-North - Italy Centre North
+# IT-Centre-South - Italy Centre South
+# IT-North - Italy North
+# IT-SACOAC - Italy Sardinia Corsica AC
+# IT-SACODC - Italy Sardinia Corsica DC
+# IT-Sardinia - Italy Sardinia
+# IT-Sicily - Italy Sicily
+# IT-South - Italy South
+# LT - Lithuania
+# LV - Latvia
+# ME - Montenegro
+# NL - Netherlands
+# NO1 - Norway 1
+# NO2 - Norway 2
+# NO2NSL - Norway North Sea Link
+# NO3 - Norway 3
+# NO4 - Norway 4
+# NO5 - Norway 5
+# PL - Poland
+# PT - Portugal
+# RO - Romania
+# RS - Serbia
+# SE1 - Sweden 1
+# SE2 - Sweden 2
+# SE3 - Sweden 3
+# SE4 - Sweden 4
+# SI - Slovenia
+# SK - Slovakia
+# ==============================================================================
+