A heat pump changes the battery sums. It adds thousands of kWh a year, much of it in winter mornings and evenings, and it opens up Cosy Octopus, a tariff built around cheap windows on either side of the evening peak. That makes a heat-pump home one of the better candidates for a battery. But the best tariff to pair with it isn’t always Cosy.
The numbers below come from Fortyeight’s model of one heat-pump household in two regions. To see your own figures, run it on your own meter data. Not financial advice — a model, so check real quotes.
Cosy Octopus: the windows and the rule
Octopus’s Cosy page gives three cheap periods “between 04:00 - 07:00, 13:00 - 16:00 and 22:00 - 00:00 every day” and a peak “between 16:00 - 19:00, 50% above the Day rate in your region”. To join, you need a heat pump, electric radiators or an electric boiler, and a smart meter Octopus can connect to.
In London on 25 October 2026, on a 5% VAT basis:
| Cosy period | Times (UK) | London rate |
|---|---|---|
| Cheap | 04:00–07:00, 13:00–16:00, 22:00–00:00 | 14.83p/kWh |
| Peak | 16:00–19:00 | 45.35p/kWh |
| Day | All other times | 30.24p/kWh |
Cosy’s rates barely change between regions: North Scotland’s cheap rate was 14.69p. Agile is different, and that is why the region decides the answer below. The Cosy tariff page tracks the current rates.
There are nine cheap hours a day, split across three windows. Without a battery, the heat pump can heat the house and the hot water in those windows and coast through the peak. With a battery, the home can also run its lights, cooking and the heat pump’s peak-hour demand on electricity bought at about 15p rather than 45p, and refill before the next peak.
When a heat pump uses its electricity
The best large-scale evidence on heat pump timing in British homes is the Renewable Heat Premium Payment (RHPP) field trial. Love and colleagues (Applied Energy, 2017) analysed electricity use for nearly 700 domestic heat pumps, logged every two minutes. Their aggregated profile has two peaks a day, and falls overnight to around 40% of its peak. The after-diversity maximum demand, about 1.7 kW per heat pump, came in the morning, while the national grid’s peak is in the evening.
That shape suits Cosy. The 04:00–07:00 window covers the morning warm-up, the 13:00–16:00 window pre-heats before the evening, and the 22:00–00:00 window covers the night set-back. A battery then deals with what is left in the 16:00–19:00 peak.
Fortyeight’s heat-pump household uses the RHPP-based daily shape from Love et al., scaled through the year with DESNZ degree-day data (DUKES table 1.1.9). It uses 2,700 kWh a year for the home and 3,500 kWh for the heat pump: 6,200 kWh in total, about 17 kWh a day on average. On average it uses about 2.8 kWh between 16:00 and 19:00 and 6.1 kWh between 16:00 and 23:00, roughly twice a typical home, and much more on cold days.
The model: London vs North Scotland
The key column is what the battery adds over the best tariff this home could join without a battery. Octopus Go and Intelligent Octopus Go are not available here: both need an electric car.
London (region C). Flexible Octopus with no battery costs £1,890 a year.
| Setup | Yearly bill | Battery adds over best no-battery tariff | Break-even price over 10 years |
|---|---|---|---|
| Agile, no battery | £1,458 | (best no-battery option) | — |
| Cosy, no battery | £1,902 | — | — |
| Cosy + 5 kWh | £1,423 | £35 | £315 |
| Cosy + 10 kWh | £1,248 | £209 | £1,906 |
| Agile + 5 kWh | £1,160 | £298 | £2,709 |
| Agile + 10 kWh | £1,074 | £384 | £3,495 |
| Agile + 13.5 kWh | £1,044 | £414 | £3,764 |
North Scotland (region P). Flexible Octopus with no battery costs £1,923 a year.
| Setup | Yearly bill | Battery adds over best no-battery tariff | Break-even price over 10 years |
|---|---|---|---|
| Agile, no battery | £1,787 | (best no-battery option) | — |
| Cosy, no battery | £1,935 | — | — |
| Cosy + 5 kWh | £1,461 | £326 | £2,968 |
| Cosy + 10 kWh | £1,288 | £499 | £4,544 |
| Cosy + 13.5 kWh | £1,282 | £505 | £4,594 |
| Agile + 10 kWh | £1,351 | £436 | £3,966 |
| Agile + 13.5 kWh | £1,315 | £472 | £4,296 |
The break-even price is the most the battery could cost and still pay back within 10 years, with its capacity falling 2% a year.
Why the answer flips by region
In London, Agile’s import price averaged 20.7p/kWh over the 12 months to 24 October 2026, so Agile without a battery was £444 a year cheaper than Cosy for this home. Measured against that, Cosy plus a battery looks weak. Agile plus a battery comes first, but most of Agile’s advantage was already there without one.
In North Scotland, Agile averaged 24.9p, and its 16:00–19:00 average was 40.7p, against 35.7p in London. Agile without a battery was still the cheapest base (£1,787 against £1,935 for Cosy), but Cosy’s fixed 15p windows give a battery more to work with: Cosy plus 10 kWh ends the year at £1,288, £63 below Agile plus 10 kWh. A battery on Cosy adds £326 to £505 a year there, the best result of any home in the model without an electric car or solar. See the regional Agile figures for your area, and remember that a past year of Agile prices will not repeat exactly.
Size
On Cosy, the gain levels off at 10 kWh: 13.5 kWh added £6 a year in both regions. On Agile, the extra capacity was worth £30 to £36 a year. The battery sizing guide explains why.
What it means for a quote
Only one battery price is published: the Energy Saving Trust’s figure of around £4,600 for a 5 kWh system. On that basis, the North Scotland home’s 5 kWh battery on Cosy pays back in about 17 years. For 10 kWh, our illustrative £7,600 (an assumption, not a sourced price) gives about 18 years in North Scotland. In London, 10 kWh on Agile doesn’t pay back within 25 years at that price.
Change the numbers to your own quote and the saving the calculator gives you. A battery fails this test if the quote is above the break-even price for your home.
Cycles: Cosy works a battery hard
With three cheap windows, a battery can charge and discharge more than once a day. In the model, the 5 kWh battery on Cosy did about 527 full cycles a year, against about 380 on Agile. That is good for the savings but adds wear, so read the warranty’s limits. Tesla’s UK Powerwall warranty allows unlimited cycles for time-of-use shifting, solar self-consumption and backup, guarantees 80% capacity after 10 years, caps other uses at 37.8 MWh of throughput, and requires an internet connection. Other makers’ warranties may cap total throughput; ask the installer to show you the document. The installer question sheet includes this.
Grants and VAT
The Boiler Upgrade Scheme (England and Wales) pays, per gov.uk:
| Technology | Grant |
|---|---|
| Air source heat pump | £7,500 |
| Ground source heat pump (including water source and shared ground loops) | £7,500 |
| Air-to-air heat pump | £2,500 |
| Biomass boiler | £5,000 |
Air-to-air heat pumps became eligible under SI 2026/390, in force from 28 April 2026. They need an MCS-certified installer and must meet the home’s full space-heating demand. Until March 2027, homes heated by oil or LPG with no mains gas connection can get an extra £1,500 towards an air or ground source heat pump. Hybrid systems and replacements of existing low-carbon heating are excluded. The scheme does not fund batteries.
On VAT, HMRC’s Notice 708/6 zero-rates installed energy-saving materials until 31 March 2027, including permanently fixed air source heat pumps, ground source heat pumps and batteries. From 1 April 2027 the rate is 5%. The VAT deadline guide covers the timing.
Heat pump first, battery second
If you are planning both, the order matters:
- Fit the heat pump and claim the grant. The grant alone is worth more than ten years of the battery gains in this model, and the heat pump is what makes you eligible for Cosy.
- Switch to the better no-battery tariff for your region. In the model that was Agile in London (saving £432 a year against Flexible) and Agile in North Scotland (saving £136), although Cosy with a battery beats Agile with one there.
- Run a winter and download the half-hourly data. The smart meter data guide shows how.
- Price a battery against that data, measuring its saving against your best no-battery bill. Compare the result with the quote.
If you also have an electric car, Intelligent Octopus Go becomes available and changes the picture. The tariff comparison covers that case.
Model assumptions: Octopus prices fetched 25 October 2026, Agile import prices from 25 October 2025 to 24 October 2026, all on a 5% VAT basis (bills from 1 October 2026 to 31 March 2027 carry no VAT, so are about 4.8% lower); 95% usable capacity; 90% round-trip efficiency; 3 kW inverter at 5 kWh and 5 kW above; perfect knowledge of the next 36 hours of use, so results are an upper-middle estimate. The household is synthetic, not measured. Fortyeight is not affiliated with Octopus Energy, Tesla or any installer.