Calculation methodology

How we calculate your savings.

Every figure on the site is derived from public Irish retail tariffs, SEAI consumption averages, and published emission factors. Wholesale data (ENTSO-E) is shown on the live-data widgets but is not used in any savings calculation.

1. Electricity retail tariffs we model against

All household savings on the site are calculated against typical Irish retail tariffs in 2026. These are the rates a household actually pays after standing charges, taxes and supplier margins (i.e. what would appear on a bill from Electric Ireland, Energia, SSE Airtricity, Bord Gáis or Pinergy).

Variable	Rate	What it represents
BLENDED	€0.34 / kWh	Single-rate domestic tariff, blended with standing charge effect. Range across suppliers in 2026: €0.32–€0.36.
OFFPEAK	€0.17 / kWh	Day/Night NightSaver night-rate window (typically 23:00–08:00). Range: €0.14–€0.18.
PEAK	€0.45 / kWh	Smart / Time-of-Use peak rate, typically 17:00–19:00. Range: €0.40–€0.48.
CEG	€0.24 / kWh	Clean Export Guarantee, the regulated minimum supplier pays for surplus PV exported to the grid.

References: supplier price publications (Electric Ireland, Energia, SSE Airtricity, Bord Gáis Energy, Pinergy); CRU Clean Export Guarantee guidance.

2. Where the rates are used in code

The retail rates are defined once in src/lib/constants.ts as PRICING and re-used in:

src/lib/calc.ts — the full Energy Score calculation engine
src/components/HeroSavings.tsx — the hero quick-estimate widget

ENTSO-E day-ahead wholesale prices are fetched in functions/api/prices.ts and only used to render the wholesale price curve on /live-data and the homepage live tile. They are not multiplied into any savings figure.

3. Annual usage estimate

From the visitor's monthly bill we derive annual kWh:

annualSpend = monthlyBill × 12
annualKwh   = annualSpend / blendedRate

blendedRate adjusts to €0.31 if the user selects Day/Night and €0.30 if smart Time-of-Use, reflecting their tariff's average c/kWh.

4. Per-device savings formulas

Battery load-shifting

batterySavings = usableKwh × 0.9 × (PEAK − OFFPEAK) × cyclesPerYear

0.9 = round-trip efficiency. cyclesPerYear scales 250–450 with home load (small home cycles less). Typical Powerwall 2 (13.5 kWh) → ~€1,000–€1,400/yr at the (€0.45 − €0.17) spread.

Heat pump shifting

heatPumpSavings = annualKwh × 0.5 × 0.6 × (BLENDED − OFFPEAK)

0.5 = heating share of electricity (heat-pump home). 0.6 = portion shiftable to off-peak via pre-heating. SEAI typical heat demand by BER is in BER_HEATING_KWH.

EV smart charging

evKwh        = min(6,000, 2,500 + 0.3 × annualKwh)
smartCharging = evKwh × 0.8 × (PEAK − OFFPEAK)

0.8 = portion of charging sessions shiftable to overnight off-peak. Tesla Model 3 baseline ~3,800 kWh/yr (16,000 km/yr × 240 Wh/km).

Smart hot water

dailyKwh     = 4 + min(6, annualKwh / 1500)
hotWaterSav  = dailyKwh × 365 × (BLENDED − OFFPEAK)

Daily immersion electricity scales 4–10 kWh with household size.

Solar self-consumption + export

solarGen     = solarKwp × 900           # SEAI Irish PV yield, kWh/kWp/yr
solarSelf    = solarGen × 0.3 × (BLENDED − CEG×0.7)
exportReward = solarGen × 0.25 × CEG

900 kWh/kWp/yr is SEAI's Irish PV yield assumption. 30% of generation is redirected to the battery for self-consumption when one is present; 25% is exported on a typical install.

5. Carbon footprint

electricityCO2 = annualKwh × gridFactor
heatingCO2     = heatingKwh × heatingFactor
evCO2          = evKwh × gridFactor

SEAI 2026 emission factors used directly:

Grid electricity (annual avg): 224.1 gCO₂/kWh
Natural gas: 204.7 gCO₂/kWh
Kerosene (heating oil): 257.2 gCO₂/kWh
Coal: 341.3 gCO₂/kWh
Peat: 391.5 gCO₂/kWh
Heat pump effective (after COP 3.5): ~64 gCO₂/kWh of heat delivered

"Time-shifted carbon saving" uses the difference between annual-average grid carbon (224.1) and a typical off-peak intensity (100 gCO₂/kWh, EirGrid wind-dominated overnight average), applied to 30% of household kWh.

6. ETS2 cost projection

Heating fuel kWh estimated from BER + heating type (table BER_HEATING_KWH), converted to tonnes CO₂ via the SEAI factor, then:

ets2_2027 = annualHeatingTonnes × €45  # legislated initial price cap
ets2_2030 = annualHeatingTonnes × €65  # mid-range projection

7. Data feeds (display only)

ENTSO-E Transparency Platform — SEM day-ahead wholesale prices, used for the wholesale curve on /live-data and the homepage live tile.
EirGrid Smart Grid Dashboard — real-time demand, wind generation, CO₂ intensity.
Eurostat nrg_pc_204 — EU retail household electricity prices for the EU comparison chart.

None of these feeds is multiplied into a household savings figure. They power the live-data displays only.

References

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