Model 2026.08 · reviewed 26 August 2026
The method is part of the result.
BackMyDecision is an early decision-support model, not a roof survey or financial recommendation. This page makes the calculation inspectable so a visitor can see what the model knows, assumes and leaves out.
1. Estimated generation
System size is multiplied by an indicative annual regional yield, then adjusted for roof direction and reported shade. The four broad UK regions are a quick proxy; they do not replace a coordinate-level solar-resource calculation.
2. Estimated use at home
Daytime-use answers set an illustrative self-consumption rate. Including a battery raises that rate, capped at 76%. The model limits electricity used at home to the estimated annual household demand.
3. Annual benefit
Electricity used at home is valued at the selected import rate. Remaining generation is valued at the selected export rate. The result excludes standing charges because solar does not normally remove them.
4. Simple payback
Total installed cost is divided by estimated annual benefit. The indicator reflects this simple payback only; it is not a probability, confidence score, investment return or guarantee.
Quote claim extraction
PDF and text quotes are read locally in the browser. Pattern matching looks for labelled figures such as system capacity, installed price, annual generation, annual saving, energy tariffs and payback. Extracted figures are shown for confirmation before calculation; the tool does not treat a detected number as independently verified.
Current defaults and sources
- Quick-check import rate: 26.11p/kWh, aligned with Ofgem's average direct-debit electricity unit rate for 1 July–30 September 2026.
- Quick-check export rate: 15p/kWh, an editable illustration rather than a promise of an available tariff.
- Regional generation factors are broad modelling defaults. Quote-check users should compare them with a site-specific estimate such as PVGIS and an installer survey.
- Result range: ±16% around the baseline to communicate sensitivity. This is not a statistical confidence interval.
Not included
- Roof pitch, exact coordinates and local obstructions
- Panel degradation, downtime and maintenance
- Finance interest, opportunity cost and tax
- Time-of-use tariffs and hourly consumption
- Future tariff or household-behaviour changes
What to verify before buying
- A roof-specific generation forecast
- All-in installed price and warranties
- Panel, inverter and battery specifications
- Export-tariff eligibility
- Structural, planning and electrical requirements
Version history
2026.08 — Added regional yield, editable tariffs, quote-cost and system-size inputs, explicit result ranges, timestamped receipts, calendar reminders and actual-outcome comparison.