Introduction
Heat pump homes often choose between a Cosy-style heat pump tariff, Octopus Flux, Octopus Agile, and a simple fixed unit rate. 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. Agile uses half-hourly wholesale-linked import and export rates that change every day. A fixed tariff charges the same import and export rates around the clock, so a battery can only raise self-consumption.
Fourteen historic simulations for a comparable UK site in 2025 compare Flux, Cosy, and Agile at 5, 10, and 15 kWh of storage, plus Agile and a fixed tariff with no battery and the same three pack sizes. Flux produces the lowest annual bill at every battery size modelled. A fixed tariff is the dearest option at every size, including no battery. Agile with no battery (£1,136) already beats a fixed tariff with a 5 kWh pack (£1,174). At 15 kWh Agile just beats Cosy (£659 versus £681) but stays far behind Flux (£322). Cosy remains cheaper in winter and 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
Fourteen historic simulations were run in Solar Butter across 2025. Inverter, heat pump, household load, and PV specification are the same in every run. Only the tariff and battery capacity change, including two runs with no battery. Battery setpoints use auto-optimise wherever a pack is present. Flux and Cosy share one site; Agile and the fixed tariff use a nearby Gloucester location with the same hardware (see Table 1). No-battery baselines are available for Agile and the fixed tariff only.
| Parameter | Value |
|---|---|
| Site location | Flux and Cosy: UK (51.67, −1.92). Agile and fixed: Gloucester (51.89, −2.19). 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 | Flux and Cosy: 3,152 kWh space heating + 689 kWh hot water (~7,341 kWh total site load with house load). Agile and fixed: 3,216 kWh + 707 kWh (~7,423 kWh total) |
| Solar generation | Flux and Cosy: 5,322 kWh/year. Agile and fixed: 5,408 kWh/year |
| Battery control | Auto-optimise setpoints where a battery is present |
| 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 |
| Octopus Agile (2025 historic) | Half-hourly wholesale-linked import | Half-hourly wholesale-linked export | 60p/day |
| Fixed tariff (example) | 28p per kWh | 12p per kWh | 60p/day |
Flux, Cosy, and fixed rates are illustrative examples for modelling. Agile uses 2025 historic half-hourly rates. Real 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 |
| 7 | Agile, 5 kWh battery Octopus Agile historic rates with a 5 kWh pack and auto-optimise setpoints. | Download PDF |
| 8 | Agile, 10 kWh battery Same Agile historic rates with 10 kWh storage. | Download PDF |
| 9 | Agile, 15 kWh battery Same Agile historic rates with 15 kWh storage. | Download PDF |
| 10 | Agile, no battery Octopus Agile historic rates with solar only and no storage. | Download PDF |
| 11 | Fixed, no battery Flat 28p import / 12p export with solar only and no storage. | Download PDF |
| 12 | Fixed, 5 kWh battery Same fixed rates with a 5 kWh pack and auto-optimise setpoints. | Download PDF |
| 13 | Fixed, 10 kWh battery Same fixed rates with 10 kWh storage. | Download PDF |
| 14 | Fixed, 15 kWh battery Same fixed 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 |
| Agile, 5 kWh | £860 | 4,721 kWh / £877 | 2,464 kWh / £235 | 19p/kWh | 10p/kWh | £940 |
| Agile, 10 kWh | £735 | 4,921 kWh / £844 | 2,594 kWh / £328 | 17p/kWh | 13p/kWh | £1,065 |
| Agile, 15 kWh | £659 | 5,255 kWh / £846 | 2,852 kWh / £406 | 16p/kWh | 14p/kWh | £1,141 |
| Agile, no battery | £1,136 | 5,580 kWh / £1,214 | 3,565 kWh / £297 | 22p/kWh | 8p/kWh | £665 |
| Fixed, no battery | £1,354 | 5,580 kWh / £1,562 | 3,565 kWh / £428 | 28p/kWh | 12p/kWh | £944 |
| Fixed, 5 kWh | £1,174 | 4,421 kWh / £1,238 | 2,358 kWh / £283 | 28p/kWh | 12p/kWh | £1,124 |
| Fixed, 10 kWh | £1,088 | 3,870 kWh / £1,084 | 1,784 kWh / £214 | 28p/kWh | 12p/kWh | £1,209 |
| Fixed, 15 kWh | £1,060 | 3,684 kWh / £1,032 | 1,591 kWh / £191 | 28p/kWh | 12p/kWh | £1,238 |
| 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% |
| Agile, 5 kWh | £507 | −£91 | £598 | 12.7% |
| Agile, 10 kWh | £470 | −£121 | £591 | 12.8% |
| Agile, 15 kWh | £453 | −£140 | £593 | 10.0% |
| Agile, no battery | £590 | −£41 | £631 | 11.4% |
| Fixed, no battery | £657 | −£49 | £706 | 11.4% |
| Fixed, 5 kWh | £643 | −£113 | £756 | 14.7% |
| Fixed, 10 kWh | £640 | −£129 | £769 | 15.4% |
| Fixed, 15 kWh | £640 | −£129 | £769 | 15.4% |
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, Flux is cheapest at £805, then Cosy at £848, then Agile at £860. A fixed tariff at the same size costs £1,174. Cosy’s modelled solar and battery benefit is highest at that size among the time-of-use tariffs (£1,353 versus £1,268 on Flux and £940 on Agile). Most of the £43 Flux–Cosy gap is the higher Cosy standing charge (£219/year versus £174/year on Flux). Agile and the fixed tariff use the same 60p/day standing charge as Cosy.
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, Agile by £201, Cosy by £167, and the fixed tariff by only £114. Agile only overtakes Cosy at 15 kWh (£659 versus £681) because extra export value (£406 versus Cosy’s £176) outweighs extra import. Flux’s peak export still dwarfs that: average export on Flux is 17–26p/kWh versus 10–14p/kWh on Agile. The fixed tariff cannot arbitrage at all: import stays at 28p and export at 12p, so extra storage only raises self-consumption. Cosy is therefore more favourable when storage is small; Flux needs a larger pack to use its export structure well; Agile sits between them; a fixed tariff is the dearest option at every size here.
A battery is worth more on a time-of-use tariff
Without a battery, Agile costs £1,136 and the fixed tariff costs £1,354. The same solar generation and grid flows (5,580 kWh import, 3,565 kWh export) cost £218 more on the fixed rate because import is 28p/kWh versus an Agile average of 22p/kWh. Adding a 5 kWh pack then saves £276 on Agile and £180 on the fixed tariff. Going from 5 kWh to 15 kWh saves a further £201 on Agile and only £114 on the fixed tariff.
That gap matters for kit choices. Agile with no battery (£1,136) already beats a 5 kWh pack on a fixed tariff (£1,174). Flux with 5 kWh (£805) undercuts a 15 kWh pack on a fixed tariff (£1,060) by £255. A time-of-use tariff with a modest battery beats a larger battery on a flat rate in this model.
Larger batteries are only worthwhile on a dynamic tariff. Stepping from 10 kWh to 15 kWh saves £229 on Flux and £76 on Agile, but only £27 on Cosy and £28 on a fixed tariff. Extra storage pays for itself when cheap import and high-value export windows can be filled; on a flat rate the extra kWh mostly sit unused in winter and add little annual saving.
Agile prices are volatile from year to year
These Agile results use 2025 historic half-hourly rates. Unlike Flux, Cosy, or a fixed tariff, Agile unit rates are not published as a stable schedule: they track wholesale prices and can look very different in another year. A calmer wholesale year would shrink the cheap-import and high-export spreads that a battery exploits; a more extreme year could widen them. Treat the Agile bills here as one year’s outcome, not a rate you can lock in.
That year-to-year volatility is additional risk on top of the usual tariff-product changes. Households that want a more predictable running cost may prefer Flux, Cosy, or a fixed structure even when 2025 Agile looks competitive at 15 kWh. A larger battery sized for Agile arbitrage could under-earn if the next year’s price shape is flatter.
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. Cosy is the cheapest winter tariff at every battery size in Table 5: £479 versus Flux £526, Agile £507, and fixed £643 at 5 kWh; £414 versus Flux and Agile both at £470 and fixed £640 at 10 kWh; and £401 versus Flux £417, Agile £453, and fixed £640 at 15 kWh. Without a battery, winter is £590 on Agile and £657 on the fixed tariff. The fixed winter bill barely moves with extra storage because there is little surplus solar to store in December–February.
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. Agile summers sit between them at 15 kWh (−£140) and match Cosy at 10 kWh (−£121). The fixed tariff also credits in summer (−£113 to −£129 with a battery) from 12p export, but winter remains expensive enough that the annual bill stays high.
Bill variability and control effort
The winter–summer gap is largest on the fixed tariff with a battery (~£756–£769) and on Flux (~£670–£690). Agile’s gap (~£590–£631) is closer to Cosy (~£520–£590). Households that prefer a steadier bill through the year may prefer Cosy even when Flux wins on the annual total.
Flux and Agile both ask more of battery control than Cosy or a fixed tariff. Flux steers charge and discharge toward peak export and cheap import bands. Agile needs half-hourly optimisation against wholesale-linked rates that change every day. Cosy is mainly about filling the battery in off-peak Cosy windows and covering later house and heat pump load. On a fixed tariff the battery only stores surplus solar for later use, so control is simpler still, but that simplicity comes with the highest bills in this study.
Extra Cosy savings from heat pump setpoints
These simulations keep the same heat pump behaviour on all 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 fourteen 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, Cosy-style, and fixed structures plus 2025 historic Agile half-hourly rates. Agile wholesale-linked prices can change sharply from year to year; treat the Agile bills as one year’s outcome, not a locked-in rate.
- Agile and fixed-tariff runs use a nearby Gloucester site with the same hardware spec, yielding slightly higher solar (5,408 kWh versus 5,322 kWh) and site load (7,423 kWh versus 7,341 kWh). That difference does not change the ranking in this study.
- No-battery baselines are included for Agile and the fixed tariff only, not for Flux or Cosy.
- Battery runs use auto-optimise 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 Flux and Cosy annual bills are close, Cosy delivers a higher modelled solar and battery benefit, and Cosy is cheaper through winter. A fixed tariff is the dearest option at every size, including no battery.
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. Larger packs are only worthwhile on a dynamic tariff: the step from 10 kWh to 15 kWh barely moves Cosy or a fixed bill, while Flux drops by £229. 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. Agile is not the annual winner at any battery size in this study; it only edges Cosy at 15 kWh (£659 versus £681) and stays far behind Flux (£322). A 5 kWh pack on Flux still beats a 15 kWh pack on a fixed tariff. Agile rates can also change sharply from year to year, so treat that ranking as extra risk rather than a locked-in saving. Agile can still be useful where the system is battery-only, with no solar, and where a Cosy tariff is not available: half-hourly cheap import remains the main way to charge without Cosy’s off-peak windows or Flux’s peak export.
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. Agile needs similar half-hourly optimisation against rates that change every day.
References
- Solar Butter. Historic simulation reports: Octopus Flux, Cosy-style heat pump tariff, Octopus Agile, and a fixed tariff at no battery, 5, 10, and 15 kWh battery capacity, 2025 UK model.
- Octopus Energy. Flux, Cosy, and Agile tariff product information (rates and availability subject to change). Accessed August 2026.
