Solar calculators

They use conservative South East assumptions — 950 kWh per kWp per year (PVGIS baseline for southern-England south-facing 35° pitch), a 30p/kWh import price and a 15p/kWh SEG export rate. Real-world results vary ±15% based on roof orientation, pitch, shading and your specific import/export tariff. A vetted installer will confirm figures after a roof survey.

Size to cover 80-110% of yearly consumption. A 3-bed South East home using ~3,600 kWh fits a 4 kWp array on a south-facing roof; a 4-bed with an EV or heat pump typically needs 6-8 kWp. Oversizing beyond 110% pushes more generation into low-value SEG export and rarely pays back the extra hardware.

Yes — the battery ROI calculator estimates the extra self-consumption savings from storing daytime surplus for evening use. On a typical 4-6 kWp South East system, a 10 kWh battery lifts self-consumption from 35% to 75-85% and adds £400-£700 a year in savings on top of the panels alone.

Marketing figures typically assume 100% self-consumption, best-case regional yields (often northern-Europe averages, not conservative UK), and pre-inflation prices. Our defaults use 35% self-consumption without a battery, 950 kWh/kWp/year, and today's real import and export prices. It's less flattering but far closer to what most South East homeowners actually experience.

30p/kWh import (roughly the Ofgem price-cap unit rate at the time these calculators were tuned) and 15p/kWh export (mid-range across popular SEG tariffs like Octopus Outgoing Fixed and E.ON Next Export Exclusive). If you're on a time-of-use tariff like Octopus Flux, your real-world savings will typically be 20-30% higher than these estimates.

The single-year figures do not. Modern Tier-1 panels degrade at roughly 0.4-0.5% per year, so 25-year lifetime generation is around 92% of year-one output. That means a 10-year payback estimate becomes roughly a 10.5-year payback once degradation is modelled — a small effect that we deliberately keep out of the headline number to avoid false precision.

They're tuned for the South East (roughly a line from Portsmouth to Colchester and everything below). Scotland and the far north of England typically yield 5-15% less per kWp; the East Midlands and East Anglia yield 2-5% less; the Isle of Wight and inland Sussex yield 3-6% more. Adjust the annual generation figure accordingly if you're modelling outside our footprint.

No — an approximate annual kWh figure is more useful than a precise bill amount because unit rates change. If you don't know your yearly usage, most bills show it in a summary panel, or you can request the last 12 months of half-hourly data free from your supplier.