A little house.
A lighter bill.

Published references. An openly explained model.

Every card’s “Evidence & conditions” panel identifies its numerical input, source, geography and starting condition. “Observed household median” means a metered-data study. “Published model estimate” means an indicative saving calculated by Energy Saving Trust. Neither is a guarantee for an individual home.

The reference bill

The DESNZ NEED 2026 report gives separate 2024 England/Wales medians of 10,000 kWh gas and 2,500 kWh grid electricity. We combine those medians to create a reference budget; they are not one observed household or a survey of this drawn house.

We price that budget using Ofgem’s GB-average Direct Debit snapshot for 1 October–31 December 2026: gas 7.97p/kWh and 29.68p/day standing charge; electricity 26.32p/kWh and 54.83p/day. The stated rates include the VAT treatment for that quarter. Holding them constant for 365 days gives about £1,763. This is comparison arithmetic, not a tariff quote, the headline price-cap amount or a forecast of next year’s rates. London rates can differ.

Single measures and combinations

NEED Figure 3.1 supplies the cavity-wall, loft, replacement-boiler and solar percentages. These are population medians for 2023 installations and 2024 consumption, with flats excluded; they incorporate household behaviour. They do not specify one identical building or installation quality. In particular, the loft number does not mean a completely bare loft was insulated, and the replacement-boiler number does not mean a G-rated boiler was replaced.

Other cards use Energy Saving Trust’s published GB annual pound estimates, with their stated eligibility. We retain those source pound amounts; we do not infer original kWh or reprice them as if their source tariff matched Ofgem’s snapshot. For the reference scenario we allocate a gas-heated measure to the gas budget and LED savings to electricity. This currency-to-budget transfer is our assumption, not a result published by the source.

The combined total is not a measured or validated package result. For each fuel we calculate remaining budget = starting budget × product(1 − each reference saving ÷ starting fuel budget). Percentage references are used directly as fractions. Standing charges remain unchanged. This order-independent residual rule avoids simply adding all headline savings, but does not establish physical interactions or prove that a package estimate cannot overstate savings. Double glazing and draught-proofing, for example, can overlap more than this rule represents. Solar changes imported electricity only; we add no export income, gas saving or battery benefit.

Use this as an evidence-linked learning game. It is not SAP/RdSAP, a retrofit design, an uncertainty interval or a personalised energy assessment. Source populations and assumptions differ. Actual results depend on building condition, installation, ventilation, occupancy, tariffs and comfort-taking. The Energy Saving Trust data methodology explains why published savings also vary. Points are game tokens and do not represent installation prices or payback.

Check suitability before real changes

The illustration represents a gas-heated house with opportunities; it does not assert that every home has all twelve. The cylinder measure needs a stored-hot-water cylinder, not a combi boiler. Chimney sealing is only for an unused open chimney, and must be removed before use; never block an active flue or required ventilation. Keep the thermostat comfortable for occupants. Insulation, windows, roof work and boiler replacement require appropriate suitability, moisture, ventilation, installer and permission checks.

Evidence register and open-source code

Download the versioned evidence register: source links, source dates where stated, verified inputs and modelling assumptions, checked 23 October 2026. Review the rate snapshot before using this scenario after 31 December 2026. Sources do not endorse PropBeaver or this game. Contains public sector information licensed under the Open Government Licence v3.0.

The retained heat-loss helper is adapted from Graasp’s Insulation Simulator (Graasp; contributor Thibault Reidy), commit da11213. It no longer drives the bill calculation. PropBeaver created the house drawing and interface and replaced the first edition’s arbitrary bill factors with this source-anchored model. The standalone game remains AGPL-3.0-only. Download its complete source, original helper and tests. The existing site and brand assets are separate.

Scenario version 2 · 23 October 2026 · No account needed. The game does not save or transmit your choices. Source links open the publisher’s website only when you choose them.