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 · Octopus Cosy Accredited Installer hub.
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.
