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Big Roof Britain Solar Payback Index Standing Charge Drag

Solar Panels in TR21 (Isles of Scilly)

See if solar pays off for you in Isles of Scilly — price, savings and payback in under a minute (official UK data).

Solar Calculator

Change the settings below to see how costs and savings work for your situation.

System Size

Roof Direction

Battery Storage

Export Value

Your Numbers

Break-even
11.4 years
After this, it's pure profit
Upfront Cost
£10,173
What you pay to get started
Annual Benefit
£892
What you save every year
Profit Over 25 Years
£12,115
After paying itself off, you're this much ahead

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How we calculate this

Solar generation: Based on typical south-facing roof at 35° angle. We account for weather, dirt, and system losses (14%).

How much you'll use directly:

  • Without battery: You'll use about 40% of what you generate. The rest gets sold back to the grid.
  • With 5 kWh battery: You'll use about 70% (battery stores extra for evening use).
  • With 10 kWh battery: You'll use about 75% (more storage means less exported).
  • Panel-only payback: Bigger systems don’t shorten payback—cost and savings rise together per kW.
  • Battery & payback: Batteries raise self-use and cut bills, but added hardware cost often lengthens break-even at typical UK rates.
  • When a battery can still make sense:
    • Evening-heavy usage patterns
    • Time-of-use tariffs / cheap night rates
    • Low export rate vs. high unit rate
    • Heat pump or EV charging synergy
    • Optional backup/resilience (system-dependent)
    • Lower evening-hour CO₂ vs. grid

Costs: Battery prices are typically £700 per kWh. Export tariffs vary by supplier (usually 5–15p per kWh).

Grid connection: Systems up to about 3.7 kW typically need just a notification (G98). Larger systems may need permission (G99).

These are typical values based on average usage patterns. Your actual results will vary based on your roof, shading, and how you use electricity.

Your area in context

In Southern Western, the current unit rate is 26.82p/kWh with a 54.03p/day standing charge. Your postcode’s south-facing sunshine yield is 1066 kWh/kW — 53 above the regional median (1013).

Read the full story

A typical roof here lands around 5.0 kW, with most eligible homes comfortably fitting within 3–6 kW. Each kilowatt of panels produces roughly 1066 kWh per year on a south-facing roof. We value self-used electricity at 26.8p and exported surplus at a baseline 10p via SEG. Use the calculator above for your exact tariff.

Unit rate
26.8p/kWh
Standing charge
54.0p/day
Sunshine yield (south)
1066 kWh/kW
Export price (baseline)
10p/kWh

Browse the scenarios below. Each battery size has its own expand — open the one that matches your plan (or just explore). For each system size (2–8 kW) we show annual output, how much you use directly, what gets exported, annual benefit, upfront cost, break-even, and a 20-year net. Negative numbers are possible — that’s the honest trade-off when adding more battery than your usage profile justifies.

No battery 5 kW example → Annual benefit: £892, Upfront: £10,173, Break-even: 11.4 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2132 kWh 853 kWh 1279 kWh £229 £128 £357 £4,069 11.4 yrs £3,063
3 kW 3198 kWh 1279 kWh 1919 kWh £343 £192 £535 £6,104 11.4 yrs £4,595
4 kW 4264 kWh 1705 kWh 2558 kWh £457 £256 £713 £8,138 11.4 yrs £6,126
5 kW 5330 kWh 2132 kWh 3198 kWh £572 £320 £892 £10,173 11.4 yrs £7,658
6 kW 6395 kWh 2558 kWh 3837 kWh £686 £384 £1,070 £12,207 11.4 yrs £9,189
7 kW 7461 kWh 2985 kWh 4477 kWh £800 £448 £1,248 £14,242 11.4 yrs £10,721
8 kW 8527 kWh 3411 kWh 5116 kWh £915 £512 £1,426 £16,276 11.4 yrs £12,253

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

3 kWh battery 5 kW example → Annual benefit: £1,071, Upfront: £12,273, Break-even: 11.5 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2132 kWh 1279 kWh 853 kWh £343 £85 £428 £6,169 14.4 yrs £2,397
3 kW 3198 kWh 1919 kWh 1279 kWh £515 £128 £642 £8,204 12.8 yrs £4,646
4 kW 4264 kWh 2558 kWh 1705 kWh £686 £171 £857 £10,238 12.0 yrs £6,895
5 kW 5330 kWh 3198 kWh 2132 kWh £858 £213 £1,071 £12,273 11.5 yrs £9,144
6 kW 6395 kWh 3837 kWh 2558 kWh £1,029 £256 £1,285 £14,307 11.1 yrs £11,392
7 kW 7461 kWh 4477 kWh 2985 kWh £1,201 £298 £1,499 £16,342 10.9 yrs £13,641
8 kW 8527 kWh 5116 kWh 3411 kWh £1,372 £341 £1,713 £18,376 10.7 yrs £15,890

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

5 kWh battery 5 kW example → Annual benefit: £1,160, Upfront: £13,673, Break-even: 11.8 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2132 kWh 1492 kWh 640 kWh £400 £64 £464 £7,569 16.3 yrs £1,715
3 kW 3198 kWh 2238 kWh 959 kWh £600 £96 £696 £9,604 13.8 yrs £4,322
4 kW 4264 kWh 2985 kWh 1279 kWh £800 £128 £928 £11,638 12.5 yrs £6,929
5 kW 5330 kWh 3731 kWh 1599 kWh £1,001 £160 £1,160 £13,673 11.8 yrs £9,536
6 kW 6395 kWh 4477 kWh 1919 kWh £1,201 £192 £1,393 £15,707 11.3 yrs £12,144
7 kW 7461 kWh 5223 kWh 2238 kWh £1,401 £224 £1,625 £17,742 10.9 yrs £14,751
8 kW 8527 kWh 5969 kWh 2558 kWh £1,601 £256 £1,857 £19,776 10.7 yrs £17,358

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

7.5 kWh battery 5 kW example → Annual benefit: £1,187, Upfront: £15,423, Break-even: 13.0 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2132 kWh 1556 kWh 576 kWh £417 £58 £475 £9,319 19.6 yrs £180
3 kW 3198 kWh 2334 kWh 863 kWh £626 £86 £712 £11,354 15.9 yrs £2,895
4 kW 4264 kWh 3112 kWh 1151 kWh £835 £115 £950 £13,388 14.1 yrs £5,609
5 kW 5330 kWh 3891 kWh 1439 kWh £1,043 £144 £1,187 £15,423 13.0 yrs £8,324
6 kW 6395 kWh 4669 kWh 1727 kWh £1,252 £173 £1,425 £17,457 12.3 yrs £11,039
7 kW 7461 kWh 5447 kWh 2015 kWh £1,461 £201 £1,662 £19,492 11.7 yrs £13,754
8 kW 8527 kWh 6225 kWh 2302 kWh £1,670 £230 £1,900 £21,526 11.3 yrs £16,469

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

10 kWh battery 5 kW example → Annual benefit: £1,205, Upfront: £17,173, Break-even: 14.2 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2132 kWh 1599 kWh 533 kWh £429 £53 £482 £11,069 23.0 yrs £-1,427
3 kW 3198 kWh 2398 kWh 799 kWh £643 £80 £723 £13,104 18.1 yrs £1,360
4 kW 4264 kWh 3198 kWh 1066 kWh £858 £107 £964 £15,138 15.7 yrs £4,146
5 kW 5330 kWh 3997 kWh 1332 kWh £1,072 £133 £1,205 £17,173 14.2 yrs £6,933
6 kW 6395 kWh 4797 kWh 1599 kWh £1,286 £160 £1,446 £19,207 13.3 yrs £9,719
7 kW 7461 kWh 5596 kWh 1865 kWh £1,501 £187 £1,687 £21,242 12.6 yrs £12,506
8 kW 8527 kWh 6395 kWh 2132 kWh £1,715 £213 £1,928 £23,276 12.1 yrs £15,292

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

13.5 kWh battery 5 kW example → Annual benefit: £1,223, Upfront: £19,623, Break-even: 16.0 yrs
System Annual output Self-use Exported Bill saving Export income Annual benefit Upfront cost Break-even 20-year net
2 kW 2132 kWh 1641 kWh 490 kWh £440 £49 £489 £13,519 27.6 yrs £-3,733
3 kW 3198 kWh 2462 kWh 735 kWh £660 £74 £734 £15,554 21.2 yrs £-875
4 kW 4264 kWh 3283 kWh 981 kWh £880 £98 £979 £17,588 18.0 yrs £1,983
5 kW 5330 kWh 4104 kWh 1226 kWh £1,101 £123 £1,223 £19,623 16.0 yrs £4,841
6 kW 6395 kWh 4924 kWh 1471 kWh £1,321 £147 £1,468 £21,657 14.8 yrs £7,700
7 kW 7461 kWh 5745 kWh 1716 kWh £1,541 £172 £1,712 £23,692 13.8 yrs £10,558
8 kW 8527 kWh 6566 kWh 1961 kWh £1,761 £196 £1,957 £25,726 13.1 yrs £13,416

Assumptions: south-facing, sensible tilt; self-use shares from site rules; export at 10p baseline. Costs: panels £/kW = 2,035, battery £/kWh = 700.

Figures are illustrative and use the same method as the calculator above. For exact results, adjust your roof direction, tariff, and usage in the calculator.

Insights for TR21

Figures below are tailored to TR21 — click through for full league tables.

Big-Roof Britain Rank #1214 of 2182
4.9 kWp (UK avg 5.1 kWp)
~10 panels (≈430 W)
Solar Payback Index Rank #172 of 2997
9.9 years (UK avg 11.3 years)
~£1,026 /yr • £10,173 upfront
Standing-Charge Drag Rank #1248 of 2997
19.2% (UK avg 21.7%) drag on savings
£197 fixed • ~£1,026 /yr value

Your Area at a Glance

Your Electricity Rate
26.82p/kWh
Daily Fixed Charge
54.03p/day
Local Sunshine
1066kWh/kW
A 1 kW system generates ~1066 kWh per year. A typical home uses ~2,700 kWh per year.
Typical System Size
5.0kW
For houses & bungalows in this area

Local Property Data

Property Types

Houses: 46% • Flats: 31% • Bungalows: 17% Based on 679 properties

Coverage

Isles of Scilly (84%)|Cornwall (16%)

Areas Covered

St Mary's, Penzance East

Data Sources & Assumptions

Solar yield: PVGIS (EU Joint Research Centre), January 2025 data

Electricity rates: Ofgem price cap, Q4 2025

Installation costs: MCS certified installer data, March 2025

Property data: EPC open data (Open Government Licence), aggregated

All estimates are typical values. Actual results vary based on your specific situation, roof characteristics, and usage patterns.

Compare nearby areas

Nearby in Southern Western

TR22 (Isles of Scilly)
≈3.3 km away
Local sunshine: ~1,099 kWh/kWp · +3% vs TR21
TR24 (Isles of Scilly)
≈5.0 km away
Local sunshine: ~1,096 kWh/kWp · +3% vs TR21
TR23 (Isles of Scilly)
≈5.3 km away
Local sunshine: ~1,101 kWh/kWp · +3% vs TR21
TR25 (Isles of Scilly)
≈5.6 km away
Local sunshine: ~1,092 kWh/kWp · +2% vs TR21
TR19 (Cornwall)
≈50.8 km away
Local sunshine: ~1,083 kWh/kWp · +2% vs TR21
TR18 (Cornwall)
≈59.6 km away
Local sunshine: ~1,066 kWh/kWp · +0% vs TR21

Similar sunshine across the UK

DT2 (Dorset)
~1,066 kWh/kWp · +0% vs TR21
SS3 (Southend-on-Sea)
~1,066 kWh/kWp · +0% vs TR21
BN8 (Lewes)
~1,067 kWh/kWp · +0% vs TR21