Plug-in Solar ROI: Calculating Your Return on Investment

Payback period tells you when your system breaks even. But return on investment (ROI) tells you how much profit you'll actually make over the system's lifetime. This is the number that matters for comparing solar to alternative investments—stocks, bonds, savings accounts, or even doing nothing.

This article walks you through calculating ROI for a 15, 20, and 25-year horizon, accounts for realistic inflation, and compares plug-in solar to other investments so you can see where it stands.

Simple ROI: The Bare Numbers

ROI formula:

ROI (%) = (Total Savings - System Cost) ÷ System Cost × 100

Example: 800W system, London, standard tariff

ROI interpretation:

• 15-year ROI of 380% means you make £3.80 for every £1 invested. Good, but not spectacular.
• 25-year ROI of 700% means you make £7 for every £1 invested. Excellent.

The key insight: Solar ROI improves dramatically over time because there's no ongoing cost. Year 1–3 you're paying back the system; years 4–25 are pure profit.

Real ROI: Accounting for Electricity Price Inflation

The simple calculation above assumes electricity stays at 30p/kWh forever. In reality, prices rise 3–5% annually (historical average).

Realistic scenario: 4% annual tariff increase

Key difference: With inflation, payback is still 3 years (payback period doesn't change much, because savings grow proportionally with costs). But cumulative ROI is dramatically better.

25-year ROI with 4% inflation: 1,404% (vs 700% without inflation). You make £14 for every £1 invested instead of £7.

This is why solar is a long-term investment: inflation is your friend. Every year electricity gets more expensive, your solar's value grows.

Conservative vs Optimistic Scenarios

Conservative (3% inflation, 4-year payback)

• System cost: £800
• Annual saving year 1: £256
• Inflation: 3% annually
• 25-year total saving: ~£9,500
• 25-year profit: £8,700
• ROI: 1,088%

Realistic (4% inflation, 3-year payback)

• System cost: £800
• Annual saving year 1: £256
• Inflation: 4% annually
• 25-year total saving: ~£11,200
• 25-year profit: £10,400
• ROI: 1,300%

Optimistic (5% inflation, Time-of-Use tariff, 2.8-year payback)

• System cost: £800
• Annual saving year 1: £300
• Inflation: 5% annually
• 25-year total saving: ~£13,500
• 25-year profit: £12,700
• ROI: 1,588%

ROI by System Size

Payback period is constant across system sizes (all ~3.1 years at £1/W), so ROI is also similar:

ROI is identical because cost scales with generation. Larger systems don't have better ROI; they have larger absolute profit. Choose size based on budget and space, not ROI.

Comparing Solar to Other Investments

How does solar's 1,300% 25-year ROI compare to alternatives?

Savings Account (4% Gross Interest)

• Investment: £800
• 25-year return (gross): £2,170
• Tax (20% basic rate): -£434
• Net gain: £1,370
• Net ROI: 171%

Solar (1,300%) beats a savings account (171%) by 7.6×.

Premium Bonds (0% Guaranteed Return)

• Investment: £800
• 25-year return: £0 (expected)
• Net gain: £0
• ROI: 0%

Solar (1,300%) beats premium bonds (0%) infinitely.

FTSE 100 Index (5.5% Average Annual Return)

• Investment: £800
• 25-year return (pre-tax): ~£4,100
• Tax (20% dividend tax): -£820
• Net gain: £3,280
• Net ROI: 410%

Solar (1,300%) beats a balanced stock index (410%) by 3.2×. (Note: stock returns vary; 5.5% is historical average but not guaranteed.)

Cash ISA (4.5% Tax-Free)

• Investment: £800
• 25-year return: £2,500
• Tax: £0
• Net gain: £2,500
• Net ROI: 312%

Solar (1,300%) beats a Cash ISA (312%) by 4.2×.

Buy-to-Let Property (5% Rental Yield + 3% Appreciation)

• Investment (deposit): £8,000 (assuming 10% down on £80k property)
• 25-year net return (after tax, maintenance, voids): ~£15,000–£20,000
• Net ROI: 188–250%

Solar (1,300%) beats property (200–250%) by 5–6× on an inflation-adjusted basis. (Property assumes capital gains; actual returns vary widely by location.)

The Key Advantage: No Ongoing Costs

Unlike other investments, solar has zero ongoing costs:

• No annual fees (stocks: 0.5–1.5% per year)
• No maintenance budget (property: 1–2% per year)
• No property tax (council tax applies, but not to solar)
• No reinvestment needed (stocks need rebalancing; property needs upgrades)

This is why ROI compounds so nicely over time. Year 1 you make £256; year 10 you make £379 (more, due to inflation). The system itself never wears out or becomes obsolete (panels last 30+ years).

Break-Even Analysis: When Solar Wins

Over 15 years:

• Solar: £3,840 saving, £3,040 profit
• Savings account: £1,620 after-tax profit
• Solar wins by £1,420

Over 25 years:

• Solar: £11,200 saving, £10,400 profit
• Savings account: £1,736 after-tax profit
• Solar wins by £8,664

The longer you hold, the bigger solar's advantage. This is because solar's cost is upfront; savings grow with inflation. Savings accounts pay fixed interest (no growth). After 15 years, solar is ahead. After 25 years, it's vastly ahead.

ROI by Region

Solar's ROI varies slightly by region (due to generation differences), but the pattern is universal:

Even Scotland's ROI (1,000%) is exceptional compared to any traditional investment.

What About System Degradation?

Solar panels degrade ~0.5% per year. After 25 years, output is ~87.5% of original.

Impact on ROI:

• Without degradation: Year 25 saving is £786
• With 0.5% annual degradation: Year 25 saving is ~£689
• Difference: ~£97/year less, or ~£2,425 less over 25 years
• Revised 25-year ROI: 1,303% (down from 1,404%, a 1% reduction)

Degradation is negligible for ROI calculations. It's real, but it barely dents long-term returns.

Sensitivity: How Changes Affect ROI

If System Cost Rises to £1,000 (£1.25/W)

• 25-year saving: £11,200 (unchanged)
• Profit: £10,200
• ROI: 1,020% (down from 1,300%, still excellent)

If You're on a Time-of-Use Tariff (£300/year saving instead of £256)

• 25-year saving (4% inflation): ~£13,000
• Profit: £12,200
• ROI: 1,525% (up from 1,300%, exceptional)

If Tariff Inflation is Only 2% (Unlikely, but Conservative)

• 25-year saving: ~£8,500
• Profit: £7,700
• ROI: 963% (down from 1,300%, still excellent)

Conclusion: Solar's ROI is robust to most assumptions. Even in worst-case scenarios (high cost, low tariff growth), it beats traditional investments.

Should You Discount Future Savings?

Some investment analyses use Net Present Value (NPV), which discounts future cash flows to today's value. This accounts for the time-value of money (a pound today is worth more than a pound in 25 years).

NPV approach:

• £256 saved in year 1 is worth £256 today
• £256 saved in year 25 is worth only ~£60 today (discounting at 4%)
• Total NPV of 25-year saving: ~£4,200

Using NPV, solar's ROI is lower (~4.2 years payback instead of 3, due to the time-value discount). But most homeowners don't use NPV for home investments—they use simple payback and cumulative savings.

For plug-in solar, simple ROI is appropriate because:

1. You're not a professional investor; you're a homeowner optimizing your own costs
2. Utility costs are real, inflation-adjusted expenses you'd pay regardless
3. Solar is a long-term asset you plan to keep 15+ years

Don't overcomplicate it. Use simple payback (3.1 years) and 25-year profit (£10,400) as your decision metrics.

Key Takeaways

• 25-year ROI: 1,300% – you make £13 for every £1 invested (realistic scenario with 4% inflation)
• Payback period: 3.1 years – unchanged by inflation (savings and costs rise together)
• Beats every traditional investment – 7.6× better than savings accounts, 3.2× better than stock index, 4.2× better than ISAs
• No ongoing costs – unlike stocks (fees), property (maintenance), or bonds (tax)
• Robust to assumptions – even pessimistic scenarios (high cost, low inflation) exceed 1,000% ROI
• Longer timeline = bigger advantage – 15-year ROI (655%) is good; 25-year ROI (1,300%) is exceptional

Next Steps

1. Run the calculator – see your exact annual saving for your postcode (/calculator)
2. Calculate your ROI – multiply annual saving by 25 years, subtract system cost
3. Account for inflation – multiply year-1 saving by (1.04^25) to rough 25-year total
4. Compare to your alternatives – savings account, stocks, or doing nothing
5. Make your decision – if ROI is >500% over 25 years and payback <5 years, it's worth doing

Ready to see your exact ROI? Run the calculator with your postcode, then multiply annual saving by 25 to estimate 25-year profit.