Introduction
Heat pump homes often choose between a Cosy-style heat pump tariff and Octopus Flux. Cosy offers several cheap import windows through the day. Flux prices import and export by time of day, with a high peak export rate that rewards battery discharge into the evening peak.
Six historic simulations for the same UK site in 2025 compare the two tariffs at 5 kWh, 10 kWh, and 15 kWh of storage. Flux produces a lower annual bill once the battery is large enough to arbitrage peak export. At 5 kWh the annual bills are close, Cosy is cheaper in winter, and Cosy shows less month-to-month bill swing. Cosy also needs less aggressive battery control.
What's not considered in this case study is additional setpoints that can be applied to the heat pump, raising the setpoint temperature during off-peak periods cause the heat pump to work harder in these cheaper periods and provide additional savings on energy bills with the heat-pump tariff.
These reports were modelled using our solar calculator: open the free Solar Butter solar calculator, which is free to use with no sign up required.
Methods
Study design
Six historic simulations were run in Solar Butter for one site across 2025. Site, solar, inverter, heat pump, and household load are fixed. Only the tariff and battery capacity change. Battery setpoints use auto-optimise in every run.
| Parameter | Value |
|---|---|
| Site location | UK (51.67, −1.92), 2025 historic year |
| Solar PV | East/west arrays, 7 × 445 W each side (~6.2 kW total) |
| Inverter | Fox ESS H1-5.0-E (5 kW) |
| Household load | 3,500 kWh/year (non-heat-pump) |
| Heat pump | 5 kW rated; design house 21 °C; design flow 45 °C; weather-compensated model |
| Heat pump electricity | 3,152 kWh space heating + 689 kWh hot water (~7,341 kWh total site load with house load) |
| Solar generation | 5,322 kWh/year in every run |
| Battery control | Auto-optimise setpoints |
| Tariff | Import rates | Export rates | Standing charge |
|---|---|---|---|
| Octopus Flux (example) | 15p / 26p / 36p per kWh | 5p / 10p / 29p per kWh | 48p/day |
| Cosy-style heat pump tariff (example) | 15p / 28p / 45p per kWh | 12p per kWh | 60p/day |
Rates are illustrative examples for modelling. Real Flux and Cosy products, unit rates, and availability change over time and by region.
| # | Scenario | Report |
|---|---|---|
| 1 | Flux, 5 kWh battery Octopus Flux example with a 5 kWh pack and auto-optimise setpoints. | Download PDF |
| 2 | Flux, 10 kWh battery Same Flux rates with 10 kWh storage. | Download PDF |
| 3 | Flux, 15 kWh battery Same Flux rates with 15 kWh storage. | Download PDF |
| 4 | Cosy, 5 kWh battery Cosy-style heat pump tariff with a 5 kWh pack and auto-optimise setpoints. | Download PDF |
| 5 | Cosy, 10 kWh battery Same Cosy-style rates with 10 kWh storage. | Download PDF |
| 6 | Cosy, 15 kWh battery Same Cosy-style rates with 15 kWh storage. | Download PDF |
Results
Table 4 summarises the annual bill and main energy-cost outputs. Table 5 separates winter (December–February) from summer (June–August) using the monthly net electricity costs in each report.
| Scenario | New bill | Grid import | Grid export | Avg import | Avg export | Solar & battery benefit |
|---|---|---|---|---|---|---|
| Flux, 5 kWh | £805 | 5,130 kWh / £1,145 | 2,996 kWh / £514 | 22p/kWh | 17p/kWh | £1,268 |
| Flux, 10 kWh | £551 | 5,762 kWh / £1,211 | 3,580 kWh / £835 | 21p/kWh | 23p/kWh | £1,522 |
| Flux, 15 kWh | £322 | 6,858 kWh / £1,366 | 4,607 kWh / £1,218 | 20p/kWh | 26p/kWh | £1,751 |
| Cosy, 5 kWh | £848 | 4,321 kWh / £889 | 2,168 kWh / £260 | 21p/kWh | 12p/kWh | £1,353 |
| Cosy, 10 kWh | £708 | 3,849 kWh / £690 | 1,680 kWh / £202 | 18p/kWh | 12p/kWh | £1,494 |
| Cosy, 15 kWh | £681 | 3,639 kWh / £638 | 1,467 kWh / £176 | 18p/kWh | 12p/kWh | £1,520 |
| Scenario | Winter (Dec, Jan, Feb) | Summer (Jun, Jul, Aug) | Winter − summer | Winter self-consumption |
|---|---|---|---|---|
| Flux, 5 kWh | £526 | −£145 | £671 | 13.1% |
| Flux, 10 kWh | £470 | −£214 | £684 | 4.7% |
| Flux, 15 kWh | £417 | −£274 | £691 | −13.9% |
| Cosy, 5 kWh | £479 | −£115 | £594 | 12.5% |
| Cosy, 10 kWh | £414 | −£121 | £535 | 12.9% |
| Cosy, 15 kWh | £401 | −£121 | £522 | 13.4% |
Negative summer totals mean export credit exceeded import and standing cost for those months. Negative winter self-consumption on Flux at 15 kWh means the model exported more than it used from PV and battery for that season while chasing peak export value.
Discussion
Battery size changes which tariff looks better
At 5 kWh, the annual bills are close: £805 on Flux versus £848 on Cosy. Cosy’s modelled solar and battery benefit is higher at that size (£1,353 versus £1,268). Most of the £43 annual gap is the higher Cosy standing charge (£219/year versus £174/year on Flux).
As battery capacity grows, Flux pulls ahead on the annual bill because more energy can be discharged into the 29p peak export window. From 5 kWh to 15 kWh, the Flux bill falls by £483, while Cosy falls by only £167. Cosy is therefore more favourable when storage is small; Flux needs a larger pack to use its export structure well.
Flux often costs more in winter
Heat demand is highest in winter, when solar is low. Peak export then has less surplus to sell, so Flux’s main advantage weakens. In every battery size in Table 5, winter (December–February) costs more on Flux than on Cosy: £526 versus £479 at 5 kWh, £470 versus £414 at 10 kWh, and £417 versus £401 at 15 kWh.
Summer is the opposite. Flux summers show deep credits as the battery supports high-value export. Cosy summers are milder because export stays at a flat 12p/kWh.
Bill variability and control effort
The winter–summer gap is larger on Flux (~£670–£690) than on Cosy (~£520–£590). Households that prefer a steadier bill through the year may prefer Cosy even when Flux wins on the annual total.
Flux also asks more of battery control: charge and discharge are steered toward peak export and cheap import bands. Cosy is mainly about filling the battery in off-peak Cosy windows and covering later house and heat pump load. That usually means less control interference with day-to-day operation.
Extra Cosy savings from heat pump setpoints
These simulations keep the same heat pump behaviour on both tariffs. Cosy can also benefit from raising heat pump setpoints during cheap Cosy periods — for example pre-heating the house or hot water cylinder when import is 15p/kWh, then coasting through dearer periods. That strategy is outside the six reports here, but it is a real Cosy advantage that can cut winter bills further without needing a larger battery.
For a large-battery Cosy versus EV-tariff comparison on a different home, see the heat pump tariff versus EV tariff case study.
Limitations
- Tariff rates are example Flux and Cosy-style structures. Products, rates, and eligibility change; treat figures as a worked example, not a quote.
- All runs use auto-optimise battery setpoints. Manual schedules or different inverter limits would change the results.
- Heat pump setpoints are not raised during cheap Cosy windows, so Cosy’s full operating upside is not included. For instance, the heat pump could be scheduled to run harder during cheaper hours, presenting additional savings.
- The model uses hourly weather and load data. Short cloud and load spikes are smoothed, which can understate some battery value.
- Hardware cost, degradation, and inflation are not included. This study compares running costs, not payback.
Conclusion
For this heat pump home with ~6.2 kW east/west PV, a Cosy-style heat pump tariff is more favourable at smaller battery sizes. At 5 kWh the annual bills are close, Cosy delivers a higher modelled solar and battery benefit, and Cosy is cheaper through winter.
Cosy also gives energy bills with less seasonal variability and needs less aggressive control interference. Raising heat pump setpoints in cheap Cosy windows can add further savings that these runs do not capture.
Octopus Flux becomes stronger as battery capacity grows, because peak export arbitrage scales with storage. Choose Flux when you have enough battery to exploit high export rates and can accept larger winter–summer bill swings. Choose Cosy when storage is modest, winter bill control matters, or you want simpler tariff-aligned operation.
It's important to consider the additional control considerations needed with flux to fully maximise gains with the tariff. This involves exporting to generate revenue during the peak export hours, but not completely discharging the battery as to incur steep import charges.
References
- Solar Butter. Historic simulation reports: Octopus Flux and Cosy-style heat pump tariff at 5, 10, and 15 kWh battery capacity, 2025 UK model.
- Octopus Energy. Flux and Cosy tariff product information (rates and availability subject to change). Accessed July 2026.
