--- title: "How a Solar ROI Calculator Actually Works | South East Solar Network" description: "Most online solar calculators are sales tools. Here's the maths they use — and where they fudge it." lang: en json-ld: | [ { "@context": "https://schema.org", "@type": "Organization", "name": "South East Solar Network", "url": "https://southeastsolarnetwork.co.uk", "areaServed": "South East England", "telephone": "+443335671183", "logo": "https://southeastsolarnetwork.co.uk/favicon.ico", "image": "https://storage.googleapis.com/gpt-engineer-file-uploads/attachments/og-images/1696c2df-452b-4380-b418-fd3f1c9a128a" }, { "@context": "https://schema.org", "@type": "BreadcrumbList", "itemListElement": [ { "@type": "ListItem", "position": 1, "name": "Home", "item": "https://southeastsolarnetwork.co.uk/" }, { "@type": "ListItem", "position": 2, "name": "Guides", "item": "https://southeastsolarnetwork.co.uk/guides" } ] }, { "@context": "https://schema.org", "@type": "FAQPage", "mainEntity": [ { "@type": "Question", "name": "Who writes these solar guides?", "acceptedAnswer": { "@type": "Answer", "text": "Every guide is written or reviewed by a member of our in-house team with hands-on install experience, and re-checked quarterly against the latest MCS, Ofgem and DESNZ figures. We link primary sources (PVGIS, MCS database, Ofgem SEG tariff table, HMRC capital-allowance guidance) directly so you can verify claims yourself." } }, { "@type": "Question", "name": "Are the price and payback figures up to date for 2026?", "acceptedAnswer": { "@type": "Answer", "text": "Yes. Cost figures are re-benchmarked every quarter against real quotes issued across our vetted South East installer network. Payback figures assume 30p/kWh import, 15p/kWh SEG export and conservative 950 kWh/kWp/year South East yield — matching the assumptions our calculators use." } }, { "@type": "Question", "name": "Do you accept sponsored content or paid product placements?", "acceptedAnswer": { "@type": "Answer", "text": "No. We do not accept sponsored content, paid rankings or manufacturer affiliate commissions. Brand recommendations reflect real-world install and warranty-claim experience across our partner base — not commercial arrangements." } }, { "@type": "Question", "name": "Which guide should I read first?", "acceptedAnswer": { "@type": "Answer", "text": "Start with 'Are solar panels worth it in the UK?' for the honest economic case, then 'Solar panel costs UK' to sanity-check any quote you receive. If you're weighing a battery, read the battery costs guide next; if you have an EV, jump to the EV charging and solar guide." } }, { "@type": "Question", "name": "Do the guides cover batteries and EVs, not just panels?", "acceptedAnswer": { "@type": "Answer", "text": "Yes — we treat batteries, hybrid inverters, EV chargers and time-of-use tariffs as first-class topics. Solar-only makes less financial sense every year as time-of-use tariffs mature, and our guides reflect that." } }, { "@type": "Question", "name": "Can I ask a question the guides don't cover?", "acceptedAnswer": { "@type": "Answer", "text": "Yes. Call 01903 000 000 or use the contact form and a specialist will answer directly — we frequently turn new questions into new guide sections, so you're helping the next reader too." } } ] }, { "@context": "https://schema.org", "@type": "Article", "headline": "How a Solar ROI Calculator Actually Works", "description": "Most online solar calculators are sales tools. Here's the maths they use — and where they fudge it.", "mainEntityOfPage": "https://southeastsolarnetwork.co.uk/guides/solar-roi-calculator-explained", "image": [ "https://southeastsolarnetwork.co.uk/og-guides.jpg" ], "datePublished": "2025-01-15", "dateModified": "2026-08-21", "author": { "@type": "Organization", "name": "South East Solar Network", "url": "https://southeastsolarnetwork.co.uk" }, "publisher": { "@type": "Organization", "name": "South East Solar Network", "logo": { "@type": "ImageObject", "url": "https://southeastsolarnetwork.co.uk/favicon.ico" } } }, { "@context": "https://schema.org", "@type": "BreadcrumbList", "itemListElement": [ { "@type": "ListItem", "position": 1, "name": "Home", "item": "/" }, { "@type": "ListItem", "position": 2, "name": "Guides", "item": "/guides" }, { "@type": "ListItem", "position": 3, "name": "How a Solar ROI Calculator Actually Works", "item": "/guides/solar-roi-calculator-explained" } ] } ] --- MCS-accredited · Vetted local installers · Free no-obligation quotes [0333 567 1183](tel:+443335671183) [South East Solar Network ](/) [Services](/services)[Areas](/areas)[Guides](/guides)[Calculators](/calculators)[About](/about)[Contact](/contact) / [0333 567 1183](tel:+443335671183)[Free Quote](/contact) 1. [Home](/)/ 2. [Guides](/guides)/ 3. How a Solar ROI Calculator Actually Works # How a Solar ROI Calculator Actually Works Most online solar calculators are sales tools. Here's the maths they use — and where they fudge it. MCS Certified Installers RECC Consumer Code 0% VAT on Solar Up to 25-Year Warranty Finance From 0% APR Rated 4.9 / 5 on Reviews Jump to a section in this guide ## The inputs that matter Roof aspect and pitch, annual irradiance for your postcode, shading factor (the most-fudged input), system size in kWp, expected self-consumption %, current import price, current export rate, and battery capacity if specified. ## Where calculators mislead Many ignore shading, assume optimistic self-consumption (60%+ without a battery is rare), and project import prices upward indefinitely. Our quotes use conservative real-world assumptions — and we'll show you the spreadsheet. ## The base formula in plain English Annual generation (kWh) = system size (kWp) × site-specific yield (kWh/kWp/year, derived from PVGIS). Annual savings (£) = (self-consumed kWh × import price) + (exported kWh × export rate). Payback (years) = installed cost (£) ÷ annual savings (£). Every reputable calculator runs that same formula; what differs is the realism of the inputs. A calculator that quotes you 4-year payback is either using inflated price assumptions or unrealistic self-consumption rates. ## How we model self-consumption honestly Without a battery: 25-35% for a household out at work weekdays, 35-50% for a household with someone home daytime, 40-55% if you also have an EV charged at home during daylight hours. With a battery: 70-90% depending on capacity match and tariff. We use 80% as a default conservative number; many homes do better in practice once they tune their settings over the first three months. ## What irradiance number should I use? PVGIS (European Commission) is the industry-standard data source. For the South East, typical numbers are 1,000-1,100 kWh/kWp/year on south-facing pitched roofs at 30-40°. East/west aspects lose 12-18%. North-facing pitches lose 25-32%. Pitch above 50° or below 15° loses a further 3-7%. ## Should I model electricity price inflation? We don't, by default. Conservative payback at today's prices is the cleanest number to compare quotes against. If you want to see the picture with inflation, we'll run scenarios at 0%, 3% and 5% — but we won't use those scenarios to justify a different quote. ## Sanity checks on any quote Annual generation should be roughly 950-1,100 × system size in kWp. Annual saving should be roughly £150-£200 per kWp on solar-only, £220-£320 per kWp with a well-sized battery. Payback should fall in 7-12 years. Anything noticeably outside those ranges is worth questioning before signing. MCS-accredited Vetted installers RECC · TrustMark ## Get a free quote based on your guide Share a few details and a trusted MCS-accredited installer in your area will be in touch shortly. Full namePhoneEmail (optional)Postcode Notes (optional) Your privacy matters: We will only pass your details to **1 vetted solar firm** who we feel is the best match for your needs — never multiple companies. Get my free quote By submitting this form you agree to our [Terms of Use](/terms) and [Privacy Policy](/privacy). Take the next step ## Services covered in this guide [ Solar Panel Installation MCS-certified solar PV installs across the South East. ](/services/solar-panel-installation)[ Solar Finance 0% APR options on qualifying installs. ](/services/solar-finance)[ Solar Battery Storage Tesla, GivEnergy, Fox ESS and more — fully fitted, fully certified. ](/services/solar-battery-storage)[ Solar Panel Maintenance Annual health checks that protect your investment. ](/services/solar-panel-maintenance) Find a local installer ## Areas we cover in the South East [ Solar installers in Kent Maidstone · Canterbury · Ashford ](/solar-installers-kent)[ Solar installers in Surrey Guildford · Woking · Cobham ](/solar-installers-surrey)[ Solar installers in Hampshire Southampton · Portsmouth · Winchester ](/solar-installers-hampshire)[ Solar installers in West Sussex Worthing · Crawley · Horsham ](/solar-installers-west-sussex)[ Solar installers in East Sussex Brighton · Hove · Eastbourne ](/solar-installers-east-sussex)[ Solar installers in Berkshire Reading · Bracknell · Slough ](/solar-installers-berkshire) Explore the topic ## Keep exploring — calculators, services and local installers Descriptive links across our calculators, guides, services and local installer areas — pick the next step that fits where you are. Calculate Run the numbers - [Solar savings & payback calculator Model annual bill savings and payback years ](/calculators) - [What size solar system do I need? Right-size your array to your usage ](/calculators) - [Battery storage ROI calculator See the extra savings a battery unlocks ](/calculators) - [EV home charging cost calculator Compare charging on solar vs the grid ](/calculators) Services covered Get this fitted - [Solar Panel Installation MCS-certified solar PV installs across the South East. ](/services/solar-panel-installation) - [Solar Finance 0% APR options on qualifying installs. ](/services/solar-finance) - [Solar Battery Storage Tesla, GivEnergy, Fox ESS and more — fully fitted, fully certified. ](/services/solar-battery-storage) - [Solar Panel Maintenance Annual health checks that protect your investment. ](/services/solar-panel-maintenance) Keep reading More UK solar guides - [Are Solar Panels Worth It in the UK in 2026? An honest, numbers-led look at solar economics, payback periods and RO ](/guides/are-solar-panels-worth-it-uk) - [Solar Panel Costs UK 2026: Real Prices, No Sales Spin What you should actually expect to pay for a residential solar system ](/guides/solar-panel-costs-uk) - [Solar Battery Costs UK 2026: Brands, Capacities, Payback What batteries cost in 2026, which brands lead, and how to size storag ](/guides/solar-battery-costs-uk) - [Solar Payback Period UK: 7-10 Years on Most Homes How to model solar payback honestly — and what drives it up or down. ](/guides/solar-payback-period-uk) Find installers Local installers we vet - [Solar installers in Kent Maidstone · Canterbury · Ashford ](/solar-installers-kent) - [Solar installers in Surrey Guildford · Woking · Cobham ](/solar-installers-surrey) - [Solar installers in Hampshire Southampton · Portsmouth · Winchester ](/solar-installers-hampshire) - [Solar installers in West Sussex Worthing · Crawley · Horsham ](/solar-installers-west-sussex) ### More guides [ Are Solar Panels Worth It in the UK in 2026? Read ](/guides/are-solar-panels-worth-it-uk)[ Solar Panel Costs UK 2026: Real Prices, No Sales Spin Read ](/guides/solar-panel-costs-uk)[ Solar Battery Costs UK 2026: Brands, Capacities, Payback Read ](/guides/solar-battery-costs-uk) [All guides](/guides) South East Solar Network MCS-accredited residential solar installers across the South East of England. Vetted, insured, RECC-protected. [0333 567 1183](tel:+443335671183) [quotes@southeastsolarnetwork.co.uk](mailto:quotes@southeastsolarnetwork.co.uk) South East Solar Network Courtwick Ln, Wick Littlehampton BN17 7TL Counties - [Kent](/solar-installers-kent) - [Surrey](/solar-installers-surrey) - [Hampshire](/solar-installers-hampshire) - [West Sussex](/solar-installers-west-sussex) - [East Sussex](/solar-installers-east-sussex) - [Berkshire](/solar-installers-berkshire) - [Buckinghamshire](/solar-installers-buckinghamshire) - [Oxfordshire](/solar-installers-oxfordshire) - [Isle of Wight](/solar-installers-isle-of-wight) - [Essex](/solar-installers-essex) - [All areas →](/areas) Services - [Solar Installation](/services/solar-panel-installation) - [Battery Storage](/services/solar-battery-storage) - [Maintenance](/services/solar-panel-maintenance) - [Repairs](/services/solar-panel-repairs) - [Commercial](/services/commercial-solar) - [EV Chargers](/services/ev-charger-installation) - [Inverters](/services/solar-inverters) - [Cleaning](/services/solar-panel-cleaning) Popular guides - [Are Solar Panels Worth It in the UK in 2026?](/guides/are-solar-panels-worth-it-uk) - [Solar Panel Costs UK 2026: Real Prices, No Sales Spin](/guides/solar-panel-costs-uk) - [Solar Battery Costs UK 2026: Brands, Capacities, Payback](/guides/solar-battery-costs-uk) - [Solar Payback Period UK: 7-10 Years on Most Homes](/guides/solar-payback-period-uk) - [Best Solar Batteries UK 2026: Honest Brand Comparison](/guides/best-solar-batteries-uk-2026) - [Solar Grants UK 2026: ECO4, 0% VAT, Smart Export Guarantee](/guides/solar-grants-uk-2026) - [All guides →](/guides) Company - [About](/about) - [Contact](/contact) - [Guides](/guides) - [Calculators](/calculators) - [Privacy](/privacy) - [Terms](/terms) © 2026 South East Solar Network. All rights reserved. MCS-accredited installer network · RECC consumer code · FCA-regulated finance partners [Call Now](tel:+443335671183) [Free Quote](#quote)