Solar Panel Savings Calculator (UK)

How to use this calculator

Enter five numbers and the tool returns year-1 savings, average monthly cash flow, 10-year savings, and the full 25-year lifetime savings:

1. Annual generation (kWh) — what your system produces in year 1. MCS-certified installers must provide an SAP-based estimate. UK Solar Wizard and the Energy Saving Trust solar calculator both use the SAP method. A 4 kWp South-facing system in London produces about 3,800-4,000 kWh/year; in Edinburgh, roughly 3,200-3,500 kWh/year.
2. Electricity rate (£/kWh) — your unit rate from your latest bill. Ofgem default tariff cap was £0.27/kWh in Q2 2026. Octopus Tracker and Octopus Agile customers see lower averages (£0.20-£0.24); fixed deals are typically £0.25-£0.30.
3. SEG export rate (£/kWh) — what your supplier pays you for exported kWh. Octopus Outgoing Fixed: £0.15. EDF Export Variable: £0.12. Tesla Energy Plan: £0.115. British Gas Flex: £0.067. Default if you’re on a passive supplier: £0.05-£0.075.
4. Self-consumption (%) — fraction of generation used at the moment of production. Without battery: 30-40% in a typical UK home. With a 5-10 kWh battery: 60-80%. Heat-pump and EV households can hit 50-60% even without a battery.
5. Annual rate escalation (%) — Ofgem and DESNZ trend data supports 3-5%. Use 4% as the realistic middle.

How the calculation works

UK savings split between self-consumed energy (avoiding the import rate) and exported energy (paid at SEG):

year_n_self_kWh   = annual_kWh × (1 - 0.005)^(n-1) × self_pct
year_n_export_kWh = annual_kWh × (1 - 0.005)^(n-1) × (1 - self_pct)
year_n_savings    = year_n_self_kWh × import_rate × (1 + escalation)^(n-1)
                  + year_n_export_kWh × seg_rate
lifetime_savings  = sum of year_n_savings from y = 1 to y = 25

SEG rates are typically held flat or only modestly indexed in supplier tariffs, so we hold export rate constant. Import rate escalates annually.

Worked example for a typical Manchester semi with 4 kWp South-facing:

• Generation: 3,800 kWh/year
• Import rate: £0.27/kWh
• SEG rate: £0.15 (Octopus Outgoing Fixed)
• Self-consumption: 35%
• Escalation: 4%
• Year 1 savings: (3,800 × 0.35 × £0.27) + (3,800 × 0.65 × £0.15) = £359 + £371 = £730
• Year 25 savings: roughly £1,750 (rate escalation × degradation)
• Lifetime total: roughly £26,000

Lifetime savings by UK region (2026 reference)

Based on Energy Saving Trust and MCS data, 25-year lifetime savings for a typical 4 kWp South-facing system at 35% self-consumption and Octopus Outgoing Fixed (£0.15 SEG):

What changes lifetime savings in the UK

Increases savings

• Battery storage — lifts self-consumption from 35% to 75%, adding £150-£300/year for a typical home, especially if paired with Octopus Agile or Cosy Octopus tariffs.
• High SEG rates — Octopus Outgoing Fixed (£0.15) versus a passive supplier (£0.05) means roughly £100-£200/year more on a 4 kWp system.
• Heat pump or EV — daytime electrification raises baseline consumption, lifting self-consumption to 50-60% without battery.
• Time-of-use tariffs — Octopus Agile, Tide, and Cosy let you shift cheap-grid imports overnight while keeping solar exports priced at the SEG rate.

Decreases savings

• East/West roof orientation — typically 10-15% less generation than South-facing; flat shed roofs lose 5-8% versus optimal pitch.
• Shading from chimneys, dormers, neighbouring trees — even partial shade on one panel can drop string output 20-30% under older string-inverter setups.
• Older panels and degradation — Tier-3 panels can degrade 0.7-0.9%/year vs the 0.5% assumed here.
• Inverter replacement — 12-15 year service life means most owners replace once over the 25-year horizon (£800-£1,600 typically).

Savings vs. payback vs. ROI

• Lifetime savings answers “how much money saved over 25 years?”
• Payback period answers “when do I break even on the £6,500-£8,000 install cost?”
• ROI / IRR answers “what annualised return does this match versus a Cash ISA or pension?”

Run all three. Pair this calculator with our UK solar payback calculator and solar ROI calculator.

Pair this with the payback calculator, ROI calculator, and system cost calculator

Savings gives the lifetime cash; payback gives the break-even year; ROI gives the annualised return; system cost gives the gross outlay. Run all four before signing an MCS quote.

Sources

• Ofgem default tariff cap — current and historical residential rates.
• Energy Saving Trust solar advice — generation estimates and savings benchmarks.
• MCS Installation Database — certified installer search and standards.
• Smart Export Guarantee — Ofgem — SEG rules and supplier tariff list.
• DESNZ Energy Trends — wholesale and retail electricity price forecasts.
• Solar Energy UK — industry market data and consumer code.