--- title: "Octopus Cosy 6 vs 9 vs 15.5kW Sizing Guide | Greentech" description: "Side-by-side sizing guide for the Octopus Cosy 6, 9 and 15.5kW heat pumps — which one fits a Manchester flat, semi, or detached home, and how Greentech sizes on survey." lang: en-GB json-ld: | [ { "@context": "https://schema.org", "@type": "LocalBusiness", "name": "Greentech Renewables", "image": "https://www.greentechrenewables.co.uk/logo.png", "description": "MCS Certified Solar Panel and Battery Storage Installers in Greater Manchester. Trusted by 2000+ UK homeowners for renewable energy solutions.", "address": { "@type": "PostalAddress", "addressLocality": "Greater Manchester", "addressCountry": "GB" }, "telephone": "0333 577 7010", "email": "admin@greentechrenewables.co.uk", "url": "https://www.greentechrenewables.co.uk", "priceRange": "££", "aggregateRating": { "@type": "AggregateRating", "ratingValue": "4.9", "reviewCount": "105" }, "sameAs": [ "https://www.facebook.com/greentechrenewables", "https://www.instagram.com/greentechrenewables" ], "hasOfferCatalog": { "@type": "OfferCatalog", "name": "Renewable Energy Services", "itemListElement": [ { "@type": "Offer", "itemOffered": { "@type": "Service", "name": "Solar Panel Installation" } }, { "@type": "Offer", "itemOffered": { "@type": "Service", "name": "Battery Storage Systems" } }, { "@type": "Offer", "itemOffered": { "@type": "Service", "name": "Air Source Heat Pumps" } } ] } }, { "@context": "https://schema.org", "@graph": [ { "@type": "Article", "headline": "Octopus Cosy 6 vs 9 vs 15.5kW — Which Size Fits Your Home?", "datePublished": "2026-07-21", "dateModified": "2026-07-21", "author": { "@type": "Organization", "name": "Greentech Renewables" }, "mainEntityOfPage": "https://www.greentechrenewables.co.uk/heat-pumps/octopus-cosy-6-vs-9-vs-15-5kw" }, { "@type": "FAQPage", "mainEntity": [ { "@type": "Question", "name": "Which Octopus Cosy size do I need — 6, 9 or 15.5kW?", "acceptedAnswer": { "@type": "Answer", "text": "Sizing is based on a room-by-room heat loss calculation, not floor area alone. As a rough guide: Cosy 6kW suits flats and small, well-insulated 2–3 bed homes; Cosy 9kW suits most 3–4 bed Manchester semis and detached homes; Cosy 15.5kW suits larger 4–5 bed detached, solid-wall older properties, or homes with high hot-water demand." } }, { "@type": "Question", "name": "Is bigger always better with a heat pump?", "acceptedAnswer": { "@type": "Answer", "text": "No — oversizing a heat pump causes it to short-cycle, which reduces efficiency, increases wear and raises running costs. The right-sized Cosy runs steadily at low output for most of the day, which is where its efficiency (SCOP) is highest." } }, { "@type": "Question", "name": "Will the Cosy 6kW cover hot water for a family of 4?", "acceptedAnswer": { "@type": "Answer", "text": "Yes, in combination with a correctly sized unvented hot water cylinder (typically 210–250 litres) and a scheduled heat-up window on the Cosy Octopus tariff." } }, { "@type": "Question", "name": "How much noise does each Cosy size make?", "acceptedAnswer": { "@type": "Answer", "text": "All Cosy units run at sound levels below UK MCS permitted development limits (42 dB(A) at 1m from a neighbouring window). Larger units are marginally louder at full output but are inaudible from inside the home." } }, { "@type": "Question", "name": "Does the Cosy size affect the BUS grant?", "acceptedAnswer": { "@type": "Answer", "text": "No — the £7,500 (or £9,000 for oil/LPG) Boiler Upgrade Scheme grant is a flat amount regardless of heat pump size, provided the install is MCS-certified." } }, { "@type": "Question", "name": "How does Greentech decide which Cosy to fit?", "acceptedAnswer": { "@type": "Answer", "text": "Every Greentech Cosy install starts with a full MCS heat loss survey. We measure U-values of walls, windows and roof, calculate design heat loss at −3°C outside temperature, and match the smallest Cosy unit that comfortably covers the load. Sizing decisions and calculations are shared with you in writing before you commit." } } ] } ] } ] --- [0333 577 7010 ](tel:03335777010)[admin@greentechrenewables.co.uk ](mailto:admin@greentechrenewables.co.uk) ★ 4.9 on Trustpilot [![Greentech Renewables - MCS Certified Solar Installers](/assets/greentech-logo-DICMm454.jpg)](/) [Home](/) Our Services Solar Energy [ Residential Solar Home solar panel systems ](/solar-panels)[ Commercial Solar Business & industrial solutions ](/commercial-solar)[ Price Estimator Get an instant quote ](/solar-estimator) Energy Storage [ Battery Storage Store excess solar energy ](/battery-storage) Heat Pumps [ Air Source Heat Pumps Efficient home heating ](/heat-pumps)[ Octopus Cosy Installer Accredited Cosy fitting in Manchester ](/octopus-cosy-heat-pump-installer-manchester)[ Boiler Upgrade Scheme £7,500–£9,000 government grant ](/boiler-upgrade-scheme-bus) EV Charging [ Home EV Chargers Charge at home ](/ev-charging)[ Commercial Charging Workplace solutions ](/ev-charging#commercial) Ready to switch to renewable energy? 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You may get free panels [Check Eligibility](/grants/warm-homes-plan-eligibility) [Case Studies](/case-studies) [0% Finance](/finance)[Quick Solar Price](/solar-estimator) Heat Pumps · Octopus Cosy # Octopus Cosy 6 vs 9 vs 15.5kW — Which Size Fits Your Home? A practical sizing guide for Greater Manchester homes — what each Cosy unit is designed for, and how the correct size protects your running costs. ## Cosy 6kW — flats and small 2–3 bed homes Best for well-insulated flats, new-build 2 and small 3-bed homes, and mid-terraces with neighbours either side. Design heat loss of up to c. 5kW at −3°C outside. Compact outdoor unit, quiet at typical operating output, and cheapest of the three to run. ## Cosy 9kW — the Manchester default The default for typical 3–4 bed Manchester semi-detached and detached homes with modern insulation. Design heat loss of c. 5–8kW at −3°C. This is the size we fit most often across Salford, Stockport, Bury, Bolton, Trafford and Oldham. ## Cosy 15.5kW — larger and older properties For larger 4–5 bed detached homes, older solid-wall properties with limited retrofit insulation, or homes with unusually high hot-water demand (large families, two bathrooms both used at peak). Design heat loss of c. 8–14kW at −3°C. ## Why right-sizing matters more than "buy bigger" Heat pumps are most efficient (highest SCOP) when running steadily at part-load. An oversized Cosy short-cycles — turning on and off frequently to hit setpoint — which reduces efficiency, adds wear, and raises running cost. Right-sizing on the Cosy Octopus tariff can be the difference of £150+ per year on running costs. ## How Greentech sizes your Cosy Every install starts with an MCS-standard heat loss survey: U-values measured or estimated for every element, room-by-room heat loss calculated at Manchester design temperature (−3°C), and the smallest Cosy unit that comfortably covers the load specified. You get the sizing calculation in writing before you commit. See related: [Cosy install cost in Manchester](/heat-pumps/octopus-cosy-heat-pump-cost-manchester) · [Octopus Cosy Accredited Installer hub](/octopus-cosy-heat-pump-installer-manchester). ## Frequently Asked Questions ### Which Octopus Cosy size do I need — 6, 9 or 15.5kW? Sizing is based on a room-by-room heat loss calculation, not floor area alone. As a rough guide: Cosy 6kW suits flats and small, well-insulated 2–3 bed homes; Cosy 9kW suits most 3–4 bed Manchester semis and detached homes; Cosy 15.5kW suits larger 4–5 bed detached, solid-wall older properties, or homes with high hot-water demand. ### Is bigger always better with a heat pump? No — oversizing a heat pump causes it to short-cycle, which reduces efficiency, increases wear and raises running costs. The right-sized Cosy runs steadily at low output for most of the day, which is where its efficiency (SCOP) is highest. ### Will the Cosy 6kW cover hot water for a family of 4? Yes, in combination with a correctly sized unvented hot water cylinder (typically 210–250 litres) and a scheduled heat-up window on the Cosy Octopus tariff. ### How much noise does each Cosy size make? All Cosy units run at sound levels below UK MCS permitted development limits (42 dB(A) at 1m from a neighbouring window). Larger units are marginally louder at full output but are inaudible from inside the home. ### Does the Cosy size affect the BUS grant? No — the £7,500 (or £9,000 for oil/LPG) Boiler Upgrade Scheme grant is a flat amount regardless of heat pump size, provided the install is MCS-certified. ### How does Greentech decide which Cosy to fit? Every Greentech Cosy install starts with a full MCS heat loss survey. We measure U-values of walls, windows and roof, calculate design heat loss at −3°C outside temperature, and match the smallest Cosy unit that comfortably covers the load. Sizing decisions and calculations are shared with you in writing before you commit. ## Get Sized on a Free Home Survey Room-by-room heat loss, sized to the smallest Cosy unit that fits your home — with the calculation in writing. 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