Solar Panel Savings Calculator
How it works
The calculator combines roof size, orientation, shading and your annual electricity use to estimate generation, self-consumption (the bit you use directly), and export (sold via the Smart Export Guarantee). Generation is calculated using regional irradiance data; tariff savings use 2025 price cap rates.
Worked example
4kWp south-facing London roof, no shading, household using 4,000 kWh/year: generates ~3,700 kWh, uses ~1,500 kWh directly (saves £400), exports 2,200 kWh (earns £330 at Octopus 15p SEG) — total £730/year. With a 5kWh battery: ~£1,100/year, payback ~7 years.
Who should use this
- •Homeowners with south, east or west facing roofs
- •High daytime electricity users (EVs, heat pumps, working from home)
- •People planning to stay 5+ years
Common mistakes
- ×Going for the cheapest panels — quality inverter matters more
- ×Skipping the battery if you're often out during the day
- ×Forgetting MCS certification — required for SEG payments and warranties
- ×Believing 'free solar' deals — you sign over the export income for 25 years
Frequently asked questions
Is there a government grant?▾
No direct grant for owner-occupiers since FiTs ended in 2019. ECO4 occasionally funds solar for low-income homes; 0% VAT on installs runs until 2027.
What's a good SEG tariff?▾
Octopus Outgoing Fixed (15p) and Intelligent Octopus Flux (up to 35p peak) lead the market. Most others pay 5–7p.
Do they work in winter?▾
Yes, but at ~25% of summer output. Annual figures already account for this.