Solar Electricity Bill Savings Calculator (UK)

How this calculator works

The Solar Electricity Bill Savings Calculator estimates the immediate monthly bill reduction when an MCS-certified residential PV system replaces grid imports. Side-by-side comparison: today’s bill vs. tomorrow’s bill, in pounds per month, using your real Ofgem-cap or fixed-tariff rate and the Smart Export Guarantee rate from your supplier.

Plug in eight numbers and the tool returns your monthly production (kWh), your new monthly bill, your monthly £ saved, and your annual £ saved:

1. Current monthly bill — what you pay today on average. Your annual bill ÷ 12.
2. Monthly usage (kWh) — Ofgem’s typical domestic consumption is 2,700 kWh/year (medium TDCV), or 225 kWh/month. High-use 4,100 kWh/year homes use 340 kWh/month.
3. System size (kWp) — the DC nameplate of the array. UK average MCS install is 4 kWp (10-12 panels). Use the solar panel estimate calculator to size first.
4. Peak sun hours/day — UK regional average. PVGIS-SARAH3 gives 2.4-2.6 in Scotland/N. England, 2.7-2.9 in Midlands/Wales, 3.0-3.2 in S. England. Pull from re.jrc.ec.europa.eu/pvg_tools/.
5. Electricity rate (£/kWh) — your unit rate from your bill. Default is the April-June 2026 cap of 27.03p.
6. SEG export rate (£/kWh) — your supplier’s export tariff. Octopus Outgoing 15p / EDF 5.6p / OVO 4p / SP 12p / E.ON Next 16.5p / British Gas 6.4-16.5p / Good Energy 5p.
7. Self-consumption (%) — fraction used directly by your home. UK average without battery is 35-45% (later sunset hours help in summer). With a 5 kWh battery: 65-80%.
8. Standing charge (monthly) — the fixed daily charge × 30.4. Default £18 = 60p/day average for 2026.

How the math works

The calculator runs the following sequence:

monthly_kWh_produced = system_kWp × peak_sun_hours × 30.4 × 0.77
self_consumed_kWh    = min(monthly_use_kWh, monthly_kWh_produced × self_pct/100)
exported_kWh         = monthly_kWh_produced - self_consumed_kWh
imported_kWh         = monthly_use_kWh - self_consumed_kWh
import_cost          = imported_kWh × unit_rate
export_credit        = exported_kWh × SEG_rate
new_bill             = max(standing, import_cost - export_credit + standing)
monthly_savings      = current_bill - new_bill

The 0.77 multiplier is the IEC 61724 system performance ratio (inverter, wiring, soiling, temperature, mismatch losses combined). MCS uses the SAP appendix M methodology with a kWh/kWp/yr factor — for a 35° south-facing roof in Cardiff, MCS gives ~975 kWh/kWp/yr; our PSH × 0.77 method gives ~960 kWh/kWp/yr. The two agree to within 2%.

Worked example: 4 kWp system in Birmingham

• System: 4 kWp DC, 2.7 PSH (Birmingham PVGIS-SARAH3 average)
• Monthly production: 4 × 2.7 × 30.4 × 0.77 = 253 kWh/mo
• Use: 270 kWh/mo (medium TDCV)
• Rate: 27.03p/kWh (Ofgem cap Q2 2026)
• 50% self-consumption (no battery, average UK home)
• Self-consumed: min(270, 253 × 0.50) = 127 kWh, offsetting £34/mo
• Exported: 253 - 127 = 126 kWh × 15p (Octopus Outgoing) = £19 credit
• Imported: 270 - 127 = 143 kWh × 27.03p = £39 cost
• Plus £18 standing charge
• New bill: £39 - £19 + £18 = £38/month
• Old bill: 270 × 27.03p + £18 = £91/month
• Monthly savings: £53/month, £634/year (58% reduction)

Add a 5 kWh battery (Givenergy Gen 3, ~£3,500 installed) and self-consumption reaches 75%. New bill drops to £24/month, annual savings £804.

Per-region production and rate table

UK regional sun-hour and rate variation determines your savings. Production from PVGIS-SARAH3 (re.jrc.ec.europa.eu) for a 4 kWp 35° south-facing array; rates from Ofgem default cap April-June 2026 (uniform across GB, Northern Ireland higher under UR cap).

SEG tariff register (Q2 2026)

SEG rates from Ofgem register (ofgem.gov.uk/seg). Update April 2026:

Switching to a higher SEG tariff often requires switching supplier for both import and export. The Ofgem rules permit dual-supplier setups (different supplier for export than import) but only Octopus and a handful of smaller firms accept them.

0% VAT, MCS certification, and grants

The Spring Statement 2022 reduced VAT on residential solar PV from 20% to 0%, in force until 31 March 2027 under HMRC Notice 708/6. To qualify the install must be by a VAT-registered installer for a residential dwelling. After 31 March 2027 the rate reverts to the reduced 5% energy-saving materials rate.

MCS (Microgeneration Certification Scheme) certification is required for any installation under SEG, and most insurance and lender financing requires it. Cost runs £150-£300 per kWp for the certification + design + handover paperwork — already included in MCS-certified installer quotes.

Active grants and loans 2026:

• ECO4 Flex (Great British Insulation Scheme) — solar PV included for fuel-poor households; check with your local authority
• Welsh Government Warm Homes Nest — interest-free loans up to £25,000 for solar + battery in Wales
• Home Energy Scotland Loan — up to £6,000 interest-free, £7,500 cashback (HES portfolio)
• Northern Ireland Sustainable Energy Programme — solar grants up to £1,500 for low-income households

The defunct Feed-in Tariff (closed March 2019) is not coming back. The 2024 Labour manifesto’s “warm homes plan” includes a possible solar grant scheme for 2027 — track at energysavingtrust.org.uk.

Sources

• Ofgem — Default Tariff Cap Q2 2026 announcement, SEG Tariff Register (ofgem.gov.uk)
• Energy Saving Trust — Solar PV cost & savings calculator methodology, 2026 update (energysavingtrust.org.uk)
• MCS — MIS 3002 Solar PV Standard, MCS Installation Database (mcscertified.com)
• Solar Energy UK — Cost & Savings Annual Report 2025
• HMRC — Notice 708/6 (Energy-saving materials and heating equipment)
• PVGIS — JRC SARAH3 satellite-derived irradiance database (re.jrc.ec.europa.eu/pvg_tools/)

For more detail see our 25-year solar panel savings calculator, solar payback period calculator, and solar panel cost calculator.

For questions about this tool: contact@solarcalculatorhq.com