SEG Net Export Savings Calculator (UK)

How the calculator works

The SEG Net Export Savings Calculator estimates your total annual benefit from a domestic PV system under the UK Smart Export Guarantee, separating the two distinct value streams that SEG creates:

• Avoided imports — solar kWh consumed by your home directly, displacing imports priced at your retail electricity rate (Ofgem cap April 2026: 27.03p/kWh single-rate).
• SEG export payments — solar kWh exported to the grid, paid at your chosen SEG supplier’s rate (4–16.5p depending on tariff).

Plug in seven numbers and the calculator returns annual production, annual imports, annual exports, your gross bill before solar, your net bill after solar, total annual savings, and percentage bill reduction.

1. System size (kWp) — DC nameplate. MCS marketplace data shows the average UK domestic install in 2025 was 4.2 kWp. Use the how many solar panels do I need calculator for sizing.
2. Peak sun hours/day — UK PVGIS averages: London 2.7, Manchester 2.5, Edinburgh 2.4, Cardiff 2.7, Belfast 2.4, Plymouth 2.9.
3. Annual usage (kWh) — Ofgem TDCV 2026 medium consumption is 2,900 kWh for electricity-only households; high consumption 4,300 kWh.
4. Import rate (p/kWh) — Ofgem cap April 2026 is 27.03p single-rate; Economy 7 day-rate 30p / night-rate 14p. Fixed-tariff customers may have lower rates.
5. SEG export rate (p/kWh) — your chosen tariff: 8p market average, 15p Octopus Outgoing, 16.5p E.ON Next, 4–5p OVO/British Gas legacy.
6. Self-consumption (%) — 50% typical without battery, 75–85% with battery. Higher for daytime-occupied homes.
7. Standing charge (annual) — Ofgem cap April 2026 single-rate standing charge averages 60p/day = £219/year. This is non-bypassable.

How the math works

annual_kWh_produced  = system_kWp × peak_sun_hours × 365 × 0.77
self_consumed        = min(annual_use, annual_prod × self_pct/100)
imports_kWh          = max(0, annual_use - self_consumed)
exports_kWh          = max(0, annual_prod - self_consumed)
gross_bill           = annual_use × import_rate + standing_charge
import_cost          = imports_kWh × import_rate
seg_payment          = exports_kWh × seg_rate
net_bill             = max(standing_charge, import_cost - seg_payment + standing_charge)
annual_savings       = gross_bill - net_bill

The 0.77 performance ratio reflects MCS-typical residential losses: inverter (3%), DC/AC cable (2%), soiling (3%), thermal derating in summer UK conditions (4–5%), and module mismatch (1–2%). MCS MIS 3002 uses 0.75–0.80 in its standard yield estimation methodology.

Worked example: 4 kWp south-facing in Reading

• System: 4 kWp DC, 2.7 PSH (Reading PVGIS), Ofgem import 27.03p
• Annual production: 4 × 2.7 × 365 × 0.77 = 3,036 kWh/yr
• Annual usage: 2,900 kWh, SEG Octopus Outgoing 15p
• Self-consumption 50% → 1,518 kWh × 27.03p = £410.40 avoided imports
• Imports: 2,900 − 1,518 = 1,382 kWh × 27.03p = £373.55
• Exports: 3,036 − 1,518 = 1,518 kWh × 15p = £227.70
• Standing charge: £219
• Gross bill: 2,900 × 27.03p + £219 = £1,002.87
• Net bill: max(£219, £373.55 − £227.70 + £219) = £364.85
• Annual savings: £638 — 64% bill reduction

Worked example: same system with 5 kWh GivEnergy battery

• Self-consumption rises to 80% → 2,320 kWh × 27.03p = £627.10 avoided
• Imports: 2,900 − 2,320 = 580 kWh × 27.03p = £156.77
• Exports: 3,036 − 2,320 = 716 kWh × 15p = £107.40
• Net bill: max(£219, £156.77 − £107.40 + £219) = £268.40
• Annual savings: £734 — 73% bill reduction

Battery adds about £96/year savings under Octopus Outgoing 15p. Under a lower SEG rate like OVO 4p the battery delta widens substantially because the import-export spread is bigger.

SEG provider comparison (Q1 2026)

Sources: Ofgem SEG annual report 2025, Solar Energy UK tariff tracker, individual supplier websites verified Q1 2026.

You can switch SEG providers independently of your import supplier — Ofgem confirmed in the April 2024 SEG decision that multi-supplier configurations are allowed and growing in uptake. Octopus Outgoing Fixed 15p is currently the highest universally-available rate (E.ON Next 16.5p requires being an E.ON import customer).

SEG vs. closed FIT — what changed

The Feed-in Tariff closed to new applicants on 31 March 2019. Existing FIT customers (installed pre-April 2019) continue to receive their original generation-tariff rate (typically 4–11p depending on installation year) plus 5.4p export deemed at 50% of production. SEG replaced this for new installations from January 2020 onwards.

Key differences:

• FIT paid for generation (every kWh produced) — SEG pays only for export.
• FIT locked the rate for 20 years — SEG rates are contractual with your supplier, often 12-month fixed.
• FIT required MCS install + MCS deemed export — SEG requires MCS + smart meter (MMR or SMETS2) for half-hourly export measurement.
• FIT had a generation tariff far above retail — SEG rates are well below retail.

If you have a closed FIT, do not switch to SEG; the FIT generation payment alone is usually 2–3× the SEG export rate per kWh produced.

VAT, planning, and MCS in 2026

The Energy-Saving Materials VAT zero-rate (originally to March 2027) was extended in the November 2025 Autumn Statement to March 2029 for residential solar PV, battery storage, and heat pumps. This continues to remove £700–£1,500 from a typical 4 kWp install. MCS certification remains mandatory for SEG eligibility. Planning permission is generally not required under Permitted Development for roof-mounted systems unless in a conservation area or on a listed building. Battery storage was confirmed in 2022 to fall under Permitted Development for installations under 28 m³.

Octopus Intelligent Flux — beyond plain SEG

Octopus Energy now offers Intelligent Octopus Flux, a four-rate import/export tariff designed for battery storage owners. Import rates: 17p off-peak (00:00–05:00), 27p day, 35p evening peak (16:00–19:00). Export rates: 25p super-peak (16:00–19:00), 15p day, 5p overnight. This is closer to time-of-use net metering than plain SEG, and for battery-equipped homes can lift annual benefit by 20–35% over flat Octopus Outgoing 15p. The calculator can approximate Flux economics by entering a blended SEG rate of 17–20p and a higher self-consumption percentage (battery-shifted exports show up as higher self-consumption in the model).

Sources

• Ofgem, Smart Export Guarantee Annual Report 2025; Default Tariff Cap Decision April 2026.
• Solar Energy UK, SEG Tariff Tracker Q1 2026.
• MCS, MCS Installation Database and MIS 3002 Solar PV Standard v4.1.
• Energy Saving Trust, Solar PV Savings Calculator and 2025 domestic PV economics report.
• HMRC, Domestic microgeneration income tax exemption guidance HS370.
• HM Treasury, Autumn Statement 2025 (VAT zero-rate extension to 2029).