How We Calculate Solar Savings
This page provides a detailed, step-by-step explanation of how our solar savings calculator works. For a higher-level overview, see our methodology page.
Step 1: Determine system size
We recommend a system size based on your property type. These recommendations are informed by industry averages from MCS data:
| Bedrooms | System size | Panels |
|---|---|---|
| 1 bed | 2 kWp | ~5 panels |
| 2 bed | 3 kWp | ~8 panels |
| 3 bed | 4 kWp | ~10 panels |
| 4 bed | 5 kWp | ~13 panels |
| 5 bed | 6 kWp | ~15 panels |
Step 2: Calculate annual generation
We multiply the system size (kWp) by the regional solar yield for your postcode area. The formula is:
For example, a 4kW system in London (yield ~1,020 kWh/kWp/year) generates approximately 4,080 kWh per year.
Step 3: Apply roof direction adjustment
Not all roofs face south. We multiply the generation figure by a direction factor:
South-facing is 100% (baseline). East or west-facing roofs receive 80% of maximum output. North-facing roofs receive approximately 55%.
Step 4: Apply shading adjustment
Shading from trees, buildings, or other obstructions reduces output. We apply a further multiplier:
Step 5: Calculate financial savings
We split the generated electricity into two portions: what you use directly (self-consumption) and what you export to the grid.
Self-consumed savings = Generation × Self-consumption rate × Electricity rate
Export income = Generation × (1 - Self-consumption rate) × SEG rate
Total annual savings = Self-consumed savings + Export income
Without a battery, we assume 50% self-consumption. With a battery, this increases to 80%. The electricity rate used is 24.5p/kWh and the SEG rate is 4.5p/kWh.
Step 6: Estimate payback period
We divide the estimated installation cost by the annual savings to give an approximate payback period:
This is a simplified calculation that does not account for panel degradation over time or potential electricity price changes. In practice, rising electricity prices tend to shorten payback periods.
Step 7: Lifetime savings
We calculate total savings over the 25-year system lifespan, accounting for panel degradation of 0.5% per year. After 25 years, panels still produce approximately 87% of their original output.
Important caveats
- These estimates are indicative. A professional site survey is required for accurate figures specific to your property.
- We use current energy prices. Future price changes will affect actual savings.
- Self-consumption rates vary significantly depending on your daily electricity usage patterns.
- Installation costs vary by property complexity, scaffolding requirements, and regional pricing.
Questions? Get in touch or try the calculator yourself.