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Transparency · Last updated 2026-06-20

Methodology

This page documents every formula, data source, and assumption used in GeraHome's energy scenario calculators. Our principle: every number on every page must be traceable to a real published source. We never fabricate, estimate from memory, or round numbers to look neater.

1. Electricity unit rate

All running costs use the Ofgem electricity price cap unit rate of 26.11p/kWh for the period 1 July to 30 September 2026 (England, Scotland, Wales, Direct Debit payment, inclusive of 5% VAT). This is the statutory maximum unit rate energy suppliers may charge domestic customers on default tariffs. Standing charges are not included in per-appliance cost calculations because they apply regardless of usage.

Cost (pence) = kWh consumed × 26.11p/kWh

Source: Ofgem — Energy price cap unit rates and standing charges. Licence: Open Government Licence v3.0.

2. EV charging cost

We model home charging only (typically a 7.4 kW AC wallbox). Public rapid chargers (DC) are operated by third parties and priced independently of the Ofgem cap — we exclude them to avoid misleading comparisons.

Battery capacities are official usable kWh figures from UK SMMT market reports and manufacturer press packs (not gross capacity, which is larger but not accessible to the driver). WLTP range figures are the official UK homologation values.

kWh per session = battery_kWh × depletion_fraction

Cost per session (p) = kWh per session × 26.11

Weekly cost (p) = cost per session × sessions per week

Battery specs: SMMT / manufacturer press packs — UK EV battery capacity and WLTP range data 2024–2025.

3. Heat pump running cost

Heat pumps do not convert electricity to heat at 1:1 — they move heat from the outside air or ground. The ratio of heat output to electricity input is called the Coefficient of Performance (COP). A COP of 2.7 means 1 kWh of electricity produces 2.7 kWh of heat.

We use the Seasonal COP (SCOP) figures from the DESNZ/BEIS Heat Pump Information Note 2024 — the published range for air-source heat pumps (ASHP) in a UK climate is 2.5–3.5. We use 2.7 as the mid-conservative default for standard ASHP and 3.5 for ground-source (GSHP). The higher the COP, the cheaper the heat pump is to run compared with direct electric heating.

Electricity consumed (kWh/h) = heat output (kW) ÷ COP

Daily electricity (kWh) = kWh/h × hours per day

Daily cost (p) = daily kWh × 26.11

Source: DESNZ/BEIS — Heat Pump Information Note: Seasonal Performance Data 2024. Licence: Open Government Licence v3.0.

4. Electric heating cost

Resistive electric heaters (panel heaters, fan heaters, oil-filled radiators, infrared panels, storage heaters) convert electricity to heat at approximately 100% efficiency — there is no COP multiplier. Electricity consumed equals the rated wattage of the heater.

Wattage figures are typical published ratings from the Energy Saving Trust appliance guide. Where a heater has multiple power settings (e.g. 1 kW / 2 kW), we use the full rated output.

kWh per day = watts ÷ 1000 × hours per day

Daily cost (p) = kWh × 26.11

Weekly cost (p) = daily cost × days per week

Source: Energy Saving Trust — Heating your home guide (typical appliance ratings).

5. Hot tub running cost

Hot tub costs have two components: active use (heater + pump at full power when in use) and standby maintenance (heater at a fraction of rated power to maintain temperature when the cover is on and the tub is not in use).

Maintenance fraction (the share of heater wattage during standby) is set at 10–15% based on Which? and manufacturer data. Warmer ambient temperatures reduce this fraction (we use 5% in summer scenarios). The pump is assumed to be off during maintenance periods.

Active kWh = (heater_W + pump_W) ÷ 1000 × active_hours

Standby kWh = heater_W ÷ 1000 × (24 − active_hours) × maintain_fraction

Daily kWh = active kWh + standby kWh

Weekly cost (p) = daily kWh × days_per_week × 26.11

Sources: Which? — How much does a hot tub cost to run (electricity consumption data); Lay-Z-Spa / Hydropool manufacturer specifications (published product pages).

6. Gaming PC running cost

Gaming PC power draw includes the GPU, CPU, motherboard, RAM, storage drives, cooling fans, and PSU inefficiency. We model the total system power as: GPU TDP + rest of system (CPU TDP + peripherals + PSU overhead, typically 100–230 W depending on build tier).

GPU TDP values are the published Thermal Design Power from Nvidia and AMD product specification pages — the maximum sustained power draw under full load. Real-world gaming draw may be 10–15% lower than TDP, but we use TDP to give a conservative (non-understated) cost figure.

For idle/standby scenarios (overnight downloads), we use a flat 80 W system draw — consistent with published PC idle measurements and BEIS typical PC peripheral load data. For the "2 h gaming + 6 h idle" scenario, we split the session cost accordingly.

total_W = gpu_tdp_W + rest_of_system_W

kWh per session = total_W ÷ 1000 × hours per session (standard)

kWh per session = 80 ÷ 1000 × hours (idle/download)

Cost per session (p) = kWh × 26.11

Weekly cost (p) = cost per session × sessions per week

GPU TDP: Nvidia / AMD / Intel — published TDP specifications for GPU and CPU components.

7. Gera Energy Cost Index (GECI)

The Gera Energy Cost Index (GECI) is a composite score that ranks households' total discretionary electricity spend across the five scenario categories — EV charging, heat pump, electric heating, hot tub, and gaming PC — relative to the current Ofgem cap unit rate.

GECI = (sum of weekly costs across all applicable devices in pence) × 52 ÷ 26.11. A GECI of 1,000 means the household consumes 1,000 kWh per year on discretionary devices alone. This metric is for comparison purposes only and does not include base load (lighting, appliances, cooking).

8. Update cadence and versioning

The Ofgem price cap is reviewed quarterly (January, April, July, October). GeraHome updates all scenario costs each time the cap changes. Current cap period: 1 July to 30 September 2026 (26.11p/kWh). All pages carry a "data as of" label so readers can identify if a newer cap has since been announced.

Device specifications (battery capacities, GPU TDP, heater wattages) are updated annually or when a significant new model launch changes the market landscape. Last data update: 2026-06-20.

9. Primary data sources

Unit rate 26.11p/kWh from Ofgem — Energy price cap unit rates and standing charges (1 July to 30 September 2026). Open Government Licence v3.0. Last updated: 2026-06-20.