Our Methodology
Transparency is central to Home Solar Guide. This page explains how we calculate solar savings estimates, what data we use, and the limitations of our approach. For a step-by-step breakdown of the calculator formulas, see How We Calculate Solar Savings.
Data sources
Our calculations draw on four main sources of publicly available data:
- PVGIS (Photovoltaic Geographical Information System): European Commission tool providing solar irradiance data by location. We use this for regional yield estimates (kWh per kWp per year).
- Ofgem: The UK energy regulator publishes the price cap and typical domestic consumption figures. We use the current electricity unit rate of 24.5p/kWh.
- MCS (Microgeneration Certification Scheme): Industry data on installation costs, system sizes, and installer standards.
- Energy Saving Trust: UK household energy consumption data, self-consumption rates, and general solar performance benchmarks.
Key assumptions
Every solar estimate requires assumptions. We aim to be conservative rather than optimistic. Here are the main values we use:
| Assumption | Value |
|---|---|
| Electricity unit rate | 24.5p/kWh (Ofgem price cap) |
| Smart Export Guarantee rate | 4.5p/kWh (average across suppliers) |
| Self-consumption rate (no battery) | 50% |
| Self-consumption rate (with battery) | 80% |
| Panel degradation | 0.5% per year |
| System lifespan | 25 years |
| VAT on residential solar | 0% (until at least March 2027) |
Regional solar yield
The UK receives varying amounts of sunshine depending on location. We use PVGIS data to assign a solar yield figure (kWh generated per kWp of installed capacity per year) to each region. Southern England typically achieves 1,000–1,070 kWh/kWp, while Scotland averages 880–930 kWh/kWp.
Roof direction and shading adjustments
We apply multipliers to account for non-ideal conditions:
- South: 100% of maximum output
- South-East: 95% of maximum output
- South-West: 95% of maximum output
- East: 80% of maximum output
- West: 80% of maximum output
- North: 55% of maximum output
Shading adjustments:
- No shading: 100% — no trees, buildings, or other obstructions
- Light shading: 90% — some shading for a small part of the day
- Moderate shading: 75% — shading for several hours, especially mornings or afternoons
- Heavy shading: 55% — significant shading for much of the day
Limitations
- Our estimates are indicative, not guaranteed. A professional site survey is needed for an accurate quote.
- We do not account for individual household electricity usage patterns (time-of-use), which affect self-consumption rates.
- Electricity prices and SEG rates change over time. Our figures reflect current rates and do not predict future price movements.
- Roof condition, wiring requirements, and scaffolding complexity can affect actual installation costs.
Updates
We review and update our assumptions at least quarterly, or whenever there is a significant change to energy prices, SEG rates, or installation costs. This page was last reviewed on 2026-03-17.
Questions about our methodology? Get in touch.