Continue your plan
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 realistic UK planning example
Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.
| Input | Value |
|---|---|
| System Size (kWp) | 4.5 kW |
| Installation Cost (£) | £7,500 |
| Peak Sun Hours Per Year | 10 years |
| Electricity Price (£/kWh) | 4.5 kW |
After entering these figures, review annual generation, annual saving and payback together rather than in isolation. Then rerun the tool with one input adjusted.
How to read your results
Annual Generation
The figure produced by this output for the inputs you entered. Change one variable at a time if you want to see what drives it.
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 & assumptionsAuthoritative sources
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
- !Assuming a constant return without checking a more conservative growth rate.
- !Forgetting ongoing contributions, fees or tax wrappers where relevant.
- !Focusing only on the final balance instead of the path required to reach it.
- !Ignoring the drag of charges over a long period.
What to do next
- Test a cautious, expected and optimistic growth rate.
- Compare this result with related savings or retirement tools before committing more money.
- Consider charges and any tax wrapper that applies.
- If the projected balance falls short, increase the contribution until the result meets your goal.
Frequently asked
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