Solar Export Tariff (SEG) Calculator

How the calculator works

The Solar Export Tariff (SEG) Calculator estimates the annual £ value a UK residential solar PV system generates from two streams: self-consumption (offsetting imports at your retail electricity rate) and exports (paid at your Smart Export Guarantee tariff). It uses Ofgem cap defaults and MCS-typical generation factors out of the box.

Plug in six numbers:

1. System size (kWp) — DC nameplate. Average UK domestic install is 4.0 kWp per MCS 2025 installations data. Use the how many solar panels do I need calculator to size for your bill first.
2. Peak sun hours/day — UK average is 2.7 (London), 2.5 (Manchester), 2.4 (Edinburgh), 2.8 (Brighton). MCS uses 0.85 SAP kWh/kWp adjustment which converts to ~2.7 PSH at typical orientation/tilt.
3. Annual usage (kWh) — Ofgem TDCV 2026 medium-use household 2,900 kWh; low-use 1,800 kWh; high-use 4,300 kWh.
4. Retail rate (p/kWh) — Ofgem default tariff cap Apr–Jun 2026 is 27.03p/kWh. Fixed deals from Octopus/British Gas/OVO range 25–29p.
5. SEG export rate (p/kWh) — supplier-specific (see comparison below). Default 8p reflects the mid-market.
6. Self-consumption (%) — 50% typical no-battery, 75–90% with battery.

How the math works

annual_kWh_produced  = system_kWp × peak_sun_hours × 365 × 0.77
self_consumed_kWh    = min(annual_use_kWh, annual_kWh_produced × self_pct/100)
exported_kWh         = annual_kWh_produced - self_consumed_kWh
self_consume_value   = self_consumed_kWh × retail_rate
seg_revenue          = exported_kWh × seg_rate
total_annual_value   = self_consume_value + seg_revenue

The 0.77 multiplier is the IEC 61724 performance ratio — inverter losses, soiling, wire losses, temperature derating. MCS uses 0.85 SAP kWh/kWp for accreditation; we are slightly more conservative to match real-world long-term yield reported by Solar Energy UK’s 2024 fleet study.

Worked example: 4 kWp south-facing array in Reading on Octopus Outgoing

• System: 4 kWp, 2.7 PSH (Reading), 0.77 PR
• Annual production: 4 × 2.7 × 365 × 0.77 = 3,036 kWh/yr (MCS would predict 3,400)
• Annual use: 2,900 kWh, retail rate 27.03p (Ofgem cap Q2 2026)
• Octopus Outgoing Fixed 15p export
• Self-consumption 50% → 1,518 kWh × 27.03p = £410
• Exported: 3,036 − 1,518 = 1,518 kWh × 15p = £228
• Total annual value: £638/yr — blended 21p/kWh

Switch to E.ON Next Export Plus at 16.5p:

• Exported: 1,518 kWh × 16.5p = £250
• Total: £660/yr

Add a 5 kWh battery (Givenergy AC Coupled):

• Self-consumption climbs to 80% → 2,429 kWh × 27.03p = £657
• Exported drops to 607 kWh × 15p = £91
• Total: £748/yr — battery worth £110/yr extra at Octopus Outgoing rates

The battery economics improve when the gap between import (27p) and export (8–16p) is wide. At a 16.5p export rate the gap narrows and the battery has a longer payback. At 4p OVO Smart Export Plus, the battery is a no-brainer.

SEG supplier comparison (Q2 2026)

Sourced from Energy Saving Trust comparison table, supplier tariff sheets, and the Solar Energy UK SEG league table:

Octopus Agile Outgoing is unique — it pays the half-hourly Day Ahead wholesale rate plus a 1p premium. In high-export hours (sunny April Sunday afternoon) wholesale can go negative, leaving you paying to export. In winter evenings (when you’re not producing) it can spike to £1/kWh. Net annual returns average 11–14p/kWh for most UK arrays, occasionally beating Outgoing Fixed for west-facing arrays that export later in the day.

How SEG differs from the legacy FIT

The Feed-in Tariff closed to new installations on 31 March 2019. Existing FIT contracts pay two streams: a generation tariff (paid on every kWh produced regardless of export, indexed to RPI, currently 3–60p depending on installation date and band) and a deemed export tariff (paid on 50% of production assumed exported, also indexed). FIT contracts run 20 or 25 years from installation date.

SEG is fundamentally different. There is no generation payment — you only earn for kWh actually exported via your smart meter half-hourly readings. The trade-off: SEG rates from Octopus and E.ON now exceed late-FIT generation tariffs, and your roof remains MCS-certified for future battery storage or panel upgrade additions.

If you have a legacy FIT contract: do not cancel it. A 2012-vintage FIT often pays 47p+ on the generation tariff regardless of how much you self-consume. SEG cannot compete with that. New 2026 installations get SEG only.

Battery + SEG arbitrage strategy

The most lucrative 2026 setup combines:

1. South-facing PV array at 4–6 kWp (max domestic G98 limit).
2. 5–13 kWh battery charged off-peak from Octopus Agile or Intelligent Octopus Go (3p–7p overnight).
3. Octopus Outgoing Fixed at 15p or Intelligent Octopus Flux for time-of-use export.
4. EV with V2H discharging at peak to grid (Octopus Powerloop pilot live since Q4 2025).

This stack can earn £900–£1,400/yr in combined import savings and export revenue for a 5 kWp + 10 kWh setup. The Energy Saving Trust’s 2025 case study of 47 monitored households running this combo found median net annual benefit of £1,148.

Cite MCS for system performance, Ofgem for import cap, and supplier sheets for export rates. UK SEG tariffs are reviewed quarterly — rerun this calculator at the start of each quarter if you’re shopping for a new export supplier.

Sources

• Ofgem, Default Tariff Cap April–June 2026.
• Energy Saving Trust, Smart Export Guarantee tariff comparison (energysavingtrust.org.uk).
• MCS, Standard Assessment Procedure (SAP) kWh/kWp tables.
• Solar Energy UK, SEG League Table 2026 Q1.
• HMRC BIM40520, Domestic Micro-generation Concession.
• Ofgem Typical Domestic Consumption Values (TDCV) 2026.