Octopus Cosy + Solar: Maximising Cheap Heating Slots
Octopus Cosy is a heat-pump-focused tariff with three cheap windows per day (4–7am, 1–4pm, and 10pm–midnight) at around 14.5p/kWh, and a peak window 4–7pm at around 51.7p/kWh. Combined with solar PV and battery storage, Cosy works best for households with a heat pump that can pre-heat during cheap windows and ride out the 4–7pm peak from the battery and solar surplus. Typical savings vs flat tariff: 20–35% on a heat-pump home with 5kWp solar + 16kWh battery.
Octopus Cosy was designed for heat-pump households — the tariff structure assumes you can run electricity-intensive heating during three specific time windows when wholesale prices are typically lower. For homes without flexible heating, Cosy isn’t the right tariff. For homes with a heat pump, solar and battery storage, it’s often the sweet spot.
The Cosy tariff structure
| Time window | Rate (p/kWh, indicative) |
|---|---|
| 4am–7am | ~14.5p |
| 7am–1pm | ~33p |
| 1pm–4pm | ~14.5p |
| 4pm–7pm | ~51.7p (peak) |
| 7pm–10pm | ~33p |
| 10pm–midnight | ~14.5p |
| Midnight–4am | ~33p |
Rates change. Check Octopus’s live Cosy page for current pricing.
Why heat-pump households benefit
A heat pump pulls 2,500–4,000 kWh/year on a typical UK home, with seasonal peaks in January/February. If you run the pump uniformly across the day, you’ll hit the 4–7pm peak window at 51.7p/kWh repeatedly. If you run it preferentially in the 14.5p windows (4–7am, 1–4pm, 10pm–midnight), the cost per kWh of heat falls by roughly 70%.
The trick is two things: a heat pump that can pre-heat (running harder in cheap windows to build thermal mass in the home), and battery storage to cover the actual 4–7pm peak when you’d otherwise be paying 51.7p/kWh.
How Spectrum’s default setup handles Cosy
- Battery charges from solar during the day — whenever generation exceeds household load
- Battery tops up from grid in the 1–4pm cheap window — ensures full charge before 4pm peak
- Heat pump runs hard in 4–7am cheap window — pre-heats the home before peak
- Heat pump runs in 1–4pm cheap window — tops up daytime heating need
- Battery discharges during 4–7pm peak — covers household and heat-pump load
- Heat pump and battery run minimally during 7–10pm shoulder — remaining cheap-window opportunities used overnight
This sequencing is what Predbat automates. Without Predbat, you’d need to set static schedules for heat pump and battery and hope they hold — clunky and rarely optimal.
Heat pump + solar + battery + Cosy
Spectrum designs the solar and battery, configures Predbat for Cosy, and coordinates with your heat-pump installer for the cleanest setup.
Request a QuoteCosy vs Agile for heat-pump homes
Both tariffs work for heat-pump households. The difference:
- Cosy: Predictable three-window structure. Easier to set heat-pump schedules. Lower software/automation complexity.
- Agile: Variable half-hourly. Higher potential savings (deeper cheap windows on windy nights). Needs Predbat to capture the upside.
For households with sophisticated Home Assistant + Predbat setups, Agile typically beats Cosy by 5–15% net of automation cost. For households who want a more ‘set and forget’ tariff, Cosy delivers ~80% of Agile’s savings with significantly less complexity.
Cosy without a heat pump
Cosy isn’t the right tariff for homes without flexible heating. The peak rate is high (~51.7p/kWh in 4–7pm) and you have to actively avoid it. Without a heat pump that can pre-heat or a battery that can cover the peak, you’d be paying ~51.7p/kWh during exactly the window most households consume heaviest. Net result: Cosy costs you money.
If you don’t have a heat pump, look at Octopus Agile (with battery + Predbat) or Octopus Flexible (flat) instead.
Battery sizing for Cosy
Sized to cover 4–7pm peak window plus heat-pump load during that window. For a typical UK heat-pump home pulling 3kW continuous during cold-evening peak (heat pump + lighting + cooking + appliances), three hours at 3kW is 9kWh. Add headroom — 13–16kWh battery is the sweet spot.
Larger heat-pump homes (5-bed, well-insulated old stock retrofitted) often need 16–24kWh.
Solar sizing for Cosy
Solar sizing for Cosy follows standard residential rules — don’t over-size for the heat pump on the assumption that summer surplus ‘balances’ winter draw. They don’t. Summer surplus exports at 5–15p/kWh SEG. Winter draw imports at 14.5–51.7p/kWh Cosy. The arbitrage is far better captured by battery sizing than by oversized solar.
5–7kWp on a typical roof is usually right for the solar portion of a Cosy household.
FAQs
Is Octopus Cosy worth it without a heat pump?
Generally no. The ~51.7p/kWh 4–7pm peak rate is high enough that households without flexible heating end up paying more on Cosy than on a flat tariff like Octopus Flexible. Cosy’s structure is built around heat-pump pre-heating during cheap windows — if you can’t do that, look at Agile or Flexible instead.
Can I have Octopus Cosy and a battery?
Yes — battery storage materially improves Cosy savings by covering the 4–7pm peak window. Charge in the 1–4pm or 10pm–midnight cheap windows, discharge during peak. Spectrum’s standard Cosy setup is Solis hybrid + Fogstar 16.1kWh + Predbat tuned for the three-window structure.
Does Cosy work with electric vehicles?
Cosy and EV charging coexist but EV-heavy households often do better on Octopus Intelligent Go (dedicated EV tariff). Cosy’s cheap windows are shorter than Intelligent Go’s, so an EV that needs 8 hours to charge can’t comfortably fit in any single Cosy window. We’d model both tariffs for households with EVs and heat pumps.
Can I switch between Cosy and Agile?
Yes — through your Octopus account. Switches typically take 1–2 days. Worth experimenting if you’re unsure which suits better. Many heat-pump households start on Cosy for simplicity, then move to Agile once they’ve installed Home Assistant and Predbat.
What heat pump works best with Cosy?
Any heat pump that supports time-of-use scheduling — most modern UK heat pumps do. The key is being able to pre-heat the home during cheap windows and then ride out the peak with stored thermal mass. Spectrum doesn’t install heat pumps directly but coordinates with specialist heat-pump installers on solar + heat-pump packages.
Are Cosy rates fixed or variable?
Cosy rates are time-banded but fixed for the contract period (typically 12 months). Within a day, you have three cheap windows, one peak, and three shoulder periods. Octopus periodically updates the rate levels but doesn’t move them half-hourly the way Agile does.
Does Cosy include free hot water heating?
No. Cosy is an electricity tariff. Hot water heated electrically (immersion heater or heat pump) consumes Cosy-priced kWh. The simple optimisation is to schedule the immersion or heat pump’s hot-water cycle to run inside a 13p Cosy window, not during peak.
Related reading
- Solar PV with heat pump: should you install both?
- Octopus Agile + Solar: 2026 guide to maximum savings
- What is Predbat? Solar battery optimisation explained
For the full Spectrum service overview see domestic solar overview.
Speak to Spectrum Energy Systems
MCS NIC200223. Heat pump + solar + battery + Cosy is the cleanest setup we install for low-carbon heating. We’ll quote and configure end-to-end.
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