सारांश
Solar panel payback period is the time it takes for the cumulative savings from a solar PV installation to equal the upfront installation cost. After payback, the system generates free electricity for the remainder of its 25+ year lifespan.
Savings come from two sources: avoided electricity purchases (self-consumption) and income from exporting surplus electricity to the grid via the Smart Export Guarantee (SEG).
यह कैसे काम करता है
A solar PV system generates electricity from sunlight. The electricity you use directly offsets your grid purchases at the full retail rate. Any surplus is exported to the grid, earning a smaller SEG tariff.
Key factors that determine payback:
- System size (kWp) - larger systems generate more electricity but cost more
- Installation cost - the upfront investment to recover
- Electricity rate - the higher the grid rate, the more you save per kWh self-consumed
- Self-consumption ratio - the proportion of generated electricity used on-site (typically 30–50% without battery, 70–90% with)
- SEG export tariff - income per kWh exported (ranges from 1p to 25p/kWh depending on supplier)
- Panel degradation - output decreases ~0.5% per year (NREL median)
- Electricity price inflation - rising grid prices increase future savings
सूत्र
Where
Each year, generation decreases by the degradation rate and the electricity rate increases by the assumed inflation rate:
- Year N generation = base generation × (1 − degradation)^(N−1)
- Year N electricity rate = base rate × (1 + inflation)^(N−1)
The payback year is the point where cumulative savings exceed the total installation cost. ROI is calculated as (total net savings ÷ total cost) × 100%.
Generation estimate
Annual generation depends on system size and location:
| Region | Typical kWh per kWp per year |
|---|---|
| South England (e.g., Brighton) | ~1,050–1,130 |
| Midlands | ~900–1,000 |
| North England | ~850–950 |
| Scotland (e.g., Inverness) | ~830–870 |
| UK average | ~950 |
Source: MCS Standard Estimation Method, Energy Saving Trust.
Smart Export Guarantee (SEG) rates
The SEG requires licensed electricity suppliers with 150,000+ customers to offer an export tariff. Rates vary widely:
| Tariff type | Typical rate | Notes |
|---|---|---|
| Basic fixed SEG | 3–8p/kWh | Available to all; no special requirements |
| Premium flat rate | 12–16p/kWh | Often requires being a customer of the supplier |
| Time-of-use / agile | 15–25p/kWh | Requires battery + smart meter; rate varies by time of day |
Average household SEG earnings in 2024–25: 13p/kWh (Ofgem data).
हल किया गया उदाहरण
4 kWp system, no battery, UK average
Year 1 generation
= 3,800 kWh
Self-consumed (50%)
= £465.50 saved
Exported (50%)
= £142.50 earned
Year 1 total savings
= £608.00
Result
With 3% annual electricity inflation: payback in ~9–10 years on a £6,500 installation. 25-year net savings: ~£9,000–£10,000.
इनपुट की व्याख्या
- System size (kWp) - the peak power rating of the solar array. A typical UK home installs 3–6 kWp. The average in 2024 was 4.6 kWp (MCS data).
- Installation cost - total upfront cost including panels, inverter, mounting, and labour. Currently 0% VAT (until March 2027). UK median for 4kW: ~£7,500 (DESNZ 2024/25); market range £5,500–£8,000.
- Electricity rate - your current grid electricity cost in pence per kWh. The Ofgem Q1 2026 price cap is 27.69p/kWh.
- Export tariff (SEG) - the rate your energy supplier pays for exported electricity. Ranges from 1p to 25p/kWh.
- Self-consumption % - the proportion of solar generation used on-site. Typically 30–50% without battery, 70–90% with.
- Battery storage - optional addition that increases self-consumption by storing daytime surplus for evening use.
- Panel degradation - the annual reduction in panel output. Industry standard is 0.5%/year (NREL).
- Electricity price inflation - assumed annual increase in electricity prices. Historical UK long-term average is ~3–4%.
आउटपुट की व्याख्या
- Payback period - years until cumulative savings equal installation cost. Interpolated for fractional years.
- 25-year net savings - total cumulative savings minus installation cost over the system lifespan.
- Year 1 savings - first-year savings (useful as a baseline before inflation effects).
- Return on investment (ROI) - net savings as a percentage of total cost.
- CO2 saved - total carbon emissions avoided, using DEFRA 2024 UK grid factor (0.22535 kg CO2/kWh including transmission and distribution losses).
धारणाएं और सीमाएं
- The calculator assumes a fixed self-consumption ratio throughout the system life. In reality, this varies seasonally and with household behaviour.
- SEG export tariff is assumed fixed over 25 years. In practice, SEG rates may change.
- The calculator does not model battery degradation (batteries lose capacity over time, typically 1–3% per year).
- Maintenance costs (inverter replacement at ~10–15 years, typically £500–£1,000) are not included.
- Generation estimates assume an unshaded, well-oriented roof. Real-world output depends on orientation, pitch, shading, and weather.
- The calculator uses UK-average figures. Actual performance varies significantly by location (see generation table above).
- CO2 savings use a static grid carbon factor. The real factor is declining as the UK grid decarbonises.
सत्यापन
| Test case | Input | Expected year 1 savings | Source |
|---|---|---|---|
| Default 4kWp, no battery | 4kWp, £6,500, 950 kWh/kWp, 24.5p, 7.5p SEG, 50% self-consumption | £608.00 | Manual calculation |
| 100% self-consumption | 4kWp, 24.5p rate, 100% self-consumption | £931.00 (3,800 × 24.5p) | Manual calculation |
| 0% self-consumption (all exported) | 4kWp, 7.5p SEG, 0% self-consumption | £285.00 (3,800 × 7.5p) | Manual calculation |
| With battery (80% self-consumption) | 4kWp, 24.5p rate, 7.5p SEG, 80% self-consumption | £788.80 (£745.36 + £57.00) | Manual calculation |