Solar Panel Savings Calculator UK

How this calculator works

The Solar Panel Savings Calculator turns five inputs — system size, household usage, tariff, Smart Export Guarantee (SEG) rate and install cost — into a single year-by-year cash-flow model out to year 25. At the heart of it is PVGIS, the European Commission Joint Research Centre's open satellite-derived irradiance dataset, which tells us roughly how many kilowatt-hours each kilowatt-peak (kWp) of panels will generate in your part of the UK over a year. The UK ranges from about 850 kWh/kWp in Glasgow up to around 1,035 kWh/kWp on the south coast; we use 950 kWh/kWp as a national default and your postcode to refine it.

Once we know the annual generation, we split it into two buckets: the portion you consume yourself (worth your full import tariff, typically around 27p/kWh at the current Ofgem cap) and the portion you export (worth your SEG rate, usually 8–15p/kWh depending on supplier). We use a self-consumption fraction that scales with household usage — bigger households consume more of their own generation, and a battery lifts that fraction further. Your year-one saving is the sum of the avoided imports plus the export income. Subsequent years apply electricity-inflation compounding so that the undiscounted 25-year figure reflects the real-cash benefit, not a today-money approximation.

Common pitfalls and things people get wrong

• Orientation and tilt. PVGIS defaults assume a south-facing roof at about 35° tilt. East-west splits typically lose around 15% of annual yield; a steep north-facing pitch can lose 30% or more. If your installer is quoting an east-west split, ask for a PVGIS-modelled yield for your specific roof rather than the marketing brochure number.
• Self-consumption. A 4 kWp system on a small household that is out all day will export 60–70% of its generation. Adding a 5 kWh battery typically lifts self-consumption from about 35% to 55–65% — which is a bigger financial swing than most glossy quotes show, because each exported kWh earns roughly half what a self-consumed kWh saves.
• SEG rate variation. The Smart Export Guarantee does not mandate a rate — only that large suppliers must offer one. In 2026 SEG tariffs range from Octopus Outgoing at 15p/kWh down to some incumbents at 5p/kWh. If you are on a 5p SEG, your payback is noticeably longer; always optimise for tariff after install.
• Install-cost drift. UK install prices for a 4 kWp system typically sit between £5,500 and £8,500 in 2026. Quotes at the low end are usually bare-bones (no battery, cheap inverter); quotes above £9,000 often include a battery and a premium inverter. Paying £10,000 for panels alone is rarely competitive.

UK-specific context

VAT on domestic solar installation in the UK remains zero-rated through at least March 2027 for panels and batteries installed as part of the same job, which is the main reason 2025–26 is a good window to install. The SEG is not a feed-in tariff — you are not locked in; you can switch export supplier each year just like your import supplier, and it is normal to be on SEG with one supplier and import with another. Planning is permitted-development for almost all domestic roof-mounted systems unless you are in a conservation area or a listed building.

When this isn't the right answer

This calculator assumes a straightforward roof install on a grid-connected home. It does not model shading (a single chimney can cost 10–20% of annual yield without microinverters or optimisers), inverter replacement at year 10–12 (budget £1,000–£1,500), or the case where you are planning to add a heat pump and EV — both of which dramatically increase the value of self-consumed solar and the economic case for a battery. If your circumstances are unusual (off-grid, export-limited by DNO, flat roof with ballast), treat the payback figure as a ceiling and get a PVGIS-based proposal from an MCS installer.

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