Useful next calculations
When to use this calculator
- Before choosing between saving, investing or changing a contribution.
- When you want to compare cautious, base and optimistic return assumptions.
- When you need a projection before making a longer-term decision.
- When you want to see whether starting earlier or contributing more changes the outcome more.
A worked example
These are the example figures the calculator opens with.
| Input | Value |
|---|---|
| System Size (kWp) | 4 |
| Installation Cost ($) | $7,000 |
| Peak Sun Hours Per Year | 900 |
| Electricity Price ($/kWh) | $0.24 |
For those figures the calculator returns:
- Annual generation: 3,600.00kWh
- Annual saving: $864.00
- Payback: 8.10 years
- 10yr return: $1,640.00
How to read your results
Annual Saving
The recurring yearly benefit of the change or upgrade under the assumptions you entered.
Payback
How long it takes to recover the upfront cost through the ongoing saving or income you entered.
10yr Return
A projected outcome under the growth rate you entered. Real-world returns fluctuate, so also check a more conservative assumption.
Method & assumptions
This calculator estimates the financial return from a domestic solar panel installation by modelling three income streams: electricity you generate and use directly (avoiding grid purchases at your unit rate), electricity you export under the Smart Export Guarantee (paid at your entered SEG rate), and any applicable government incentives. It assumes a panel output figure based on the system size you enter, adjusted for typical UK irradiation levels for a south-facing roof. East- or west-facing installations produce roughly 15–20% less annually. The calculator does not account for panel degradation over time, which averages around 0.5% per year for quality panels, nor does it model battery storage independently. Installation costs vary considerably by region and installer, so obtaining at least three quotes is advisable before using this tool for budgeting purposes.
Common mistakes
- Using an assumption that is not supported by a current local quote, bill, statement or official source.
- Treating a generic estimate as a lender, provider, payroll or tax authority decision.
- Mixing monthly and annual inputs without converting them consistently.
- Testing only one scenario instead of checking how a cautious assumption changes the result.
What to do next
- Run a second scenario with a cautious assumption so you can see the downside clearly.
- Compare the result with the related calculators below before making a decision.
- Check current local rules, eligibility and provider terms before applying or committing money.
- Keep a record of the assumptions so you can update the estimate when a quote, bill or pay figure changes.