Octopus Cosy vs Flux: Which Tariff is Best for Heat Pumps? (Reddit Review)

Understanding the 2026 Smart Tariff Landscape

Octopus Energy has established itself as the dominant force in UK time-of-use (ToU) electricity tariffs, offering a suite of smart tariffs that reward homeowners for shifting their energy consumption away from peak grid hours. For households with heat pumps, solar panels, and battery storage, the choice of tariff can mean the difference between saving £400 a year and saving £1,200 a year. On Reddit's r/SolarUK and r/ukheatpumps communities, the Cosy vs Flux debate is one of the most frequently revisited topics of 2026.

Before diving into the head-to-head comparison, it helps to understand the four main Octopus tariffs that Reddit users discuss:

• Octopus Agile — Half-hourly wholesale pricing. Rates can go negative during high wind or solar generation, but they can also spike dramatically during winter cold snaps. Best for advanced load-shifters with home automation.
• Intelligent Octopus Go — An exceptionally low 7p/kWh overnight rate from 23:30 to 05:30, designed primarily for EV owners. The algorithm automatically schedules EV charging to optimise grid loads.
• Cosy Octopus — A heat-pump-specific tariff with three discounted windows throughout the day, but a severe peak penalty. Designed for homes that can batch their heating into off-peak slots.
• Octopus Flux — A solar and battery tariff with cheap overnight import, high peak import, and a premium export rate during the evening peak. Designed for homes that generate their own electricity and want to sell the surplus.

The critical thing Reddit users emphasise is that you cannot be on Cosy and Flux simultaneously. They are mutually exclusive on a single meter. Your hardware profile — specifically whether you have a heat pump, solar panels, a battery, or all three — determines which tariff will save you the most money. Getting this wrong can be expensive.

Cosy Octopus: The Tariff Built Specifically for UK Heat Pumps

Cosy Octopus was designed from the ground up for homes running air source heat pumps. The tariff structure offers three discounted windows during the day when electricity is significantly cheaper, allowing homeowners to batch their space heating and hot water production into these lower-cost periods.

The three Cosy dip periods are:

• 04:00 - 07:00 — Early morning window, ideal for pre-heating the home before the household wakes up and for producing hot water for morning showers.
• 13:00 - 16:00 — Afternoon window, useful for topping up heat during the day, particularly on cold weekends when the house is occupied.
• 22:00 - 00:00 — Late evening window, perfect for a final heating boost before bed and for overnight hot water preparation.

During these three windows, electricity costs approximately 13p to 14.5p/kWh — roughly half the standard variable rate of 26.11p/kWh under the July 2026 Ofgem price cap. For a heat pump operating at a seasonal performance factor (SPF) of 3.8, this means effective heating costs of around 3.4p to 3.8p per kWh of heat delivered, which is competitive with gas at 7p/kWh when boiler efficiency losses are factored in.

The catch is the peak penalty. Between 16:00 and 19:00, Cosy Octopus charges approximately 51p/kWh — nearly four times the off-peak rate and nearly double the standard variable rate. This is the period when most UK households return from work and start cooking, doing laundry, and turning up the heating. If your heat pump runs during this window, the savings from the three dip periods can be wiped out in a single evening.

Reddit users on r/ukheatpumps have developed sophisticated strategies to avoid the peak penalty. The most common approach is to use the heat pump's weather compensation and scheduling features to pre-heat the thermal mass of the building during the afternoon dip period (13:00-16:00), allowing the house to "coast" through the 16:00-19:00 peak without the heat pump cycling on. Hot water is similarly batched into the morning or late evening windows. Users with well-insulated homes report successfully avoiding the peak window entirely, reducing their effective electricity cost to around 15-16p/kWh blended.

However, the Reddit consensus is clear: Cosy only works well if you have a well-insulated home that can hold heat through the peak window, or if you have a battery that can power the home during those three expensive hours. Without either of those, the peak penalty can make Cosy more expensive than a standard flat-rate tariff.

Octopus Flux: The Secret Weapon for Solar and Battery Owners

Octopus Flux is a fundamentally different beast. Where Cosy is designed to help you consume electricity more cheaply, Flux is designed to help you profit from generating and storing electricity. It is the tariff of choice for Reddit users with solar panels and battery storage who want to maximise their return on investment through tariff arbitrage.

The Flux tariff structure is built around three pricing tiers:

• Overnight import: 14.9p/kWh — A cheap window (typically 02:00-05:00) for charging your battery from the grid when wholesale prices are lowest.
• Daytime import: ~26p/kWh — The standard rate during off-peak daytime hours, roughly in line with the Ofgem price cap.
• Peak import: 34.9p/kWh — The expensive window between 16:00 and 19:00 when grid demand is highest.

The real magic of Flux, however, is the export rate. During the 16:00-19:00 peak, Flux pays 27.1p/kWh for electricity you export back to the grid. This is dramatically higher than the Smart Export Guarantee (SEG) minimum of 15p/kWh and higher than most competing export tariffs. For a solar + battery household, this creates a powerful arbitrage loop: charge the battery overnight at 14.9p, store solar surplus during the day, then discharge the battery to the grid during the peak at 27.1p — pocketing a 12.2p/kWh profit on every unit cycled through the battery.

Reddit users with large solar arrays (6kWp+) and high-capacity batteries (10kWh+) report earning £60-£120 per month in export income during the summer months on Flux. One r/SolarUK user with a 13.5kWh Tesla Powerwall 3 and a 5kWp south-facing array documented earning £840 in export revenue between April and September 2026 alone, effectively making their summer electricity free and generating a surplus that offset their winter bills.

The downside of Flux is that it is less attractive for heat pump owners without solar or batteries. The peak import rate of 34.9p/kWh is significantly higher than the standard variable rate, and there is no discounted daytime heating window like Cosy offers. If your primary load is a heat pump running throughout the day, Flux will likely cost you more than Cosy.

The Reddit Verdict: How to Choose Based on Your Exact Setup

The Reddit consensus on r/SolarUK and r/ukheatpumps has converged on a remarkably clear decision framework. The right tariff depends entirely on your hardware configuration, and the community has distilled it into four archetypal scenarios.

Scenario 1: Heat pump only, no solar, no battery. Choose Cosy. The three dip periods allow you to batch heating and hot water into cheap windows. You must commit to avoiding the 16:00-19:00 peak, which means investing time in programming your heat pump schedule and ensuring your home can thermally coast through the evening. Reddit users estimate savings of £300-£500 per year versus a flat-rate tariff.

Scenario 2: Heat pump + battery, no solar. This is where it gets interesting. Cosy is still the better choice for most users, because the battery can be charged during the cheapest dip period (04:00-07:00 at 13p) and used to power the home through the 16:00-19:00 peak, avoiding the 51p penalty entirely. However, some Reddit users with large batteries (10kWh+) report that Flux can be competitive here too, because the overnight import at 14.9p is close to Cosy's cheapest rate, and you avoid the complexity of managing three separate dip windows.

Scenario 3: Solar + battery, no heat pump. Choose Flux, unambiguously. The premium export rate of 27.1p/kWh during peak is the single most lucrative export mechanism available to UK solar owners in 2026. Reddit users report that the export income alone often exceeds their import costs during summer, and the arbitrage loop (charge cheap, discharge expensive) generates consistent year-round profit.

Scenario 4: Heat pump + solar + battery (the full stack). This is the most debated scenario on Reddit. The consensus leans towards Flux for homes with large solar arrays (5kWp+) and large batteries (10kWh+), because the export arbitrage profits outweigh the slightly higher import costs during the peak. For homes with smaller arrays and batteries, Cosy is safer because the lower off-peak rates provide more predictable savings on heating costs. Several Reddit users recommend running the numbers for your specific consumption profile using Octopus's online tariff comparison tool before committing.

One thing every Reddit thread agrees on: whichever tariff you choose, having a battery transforms the economics. If you are running a heat pump without any battery storage, your ability to exploit time-of-use tariffs is severely limited, and you are leaving significant savings on the table.

Related Community Guides

• Heat Pump Winter Running Costs in the UK: Real Data from Reddit — How tariff choice directly determines your winter heating bills.
• The Best Home Battery Storage in the UK: What Reddit Recommends — Community tier list of battery brands that unlock tariff arbitrage.
• All Reddit Review Guides — Browse the full collection.

This guide synthesises discussions from r/SolarUK, r/ukheatpumps, and Octopus Energy tariff documentation. Tariff rates are accurate as of July 2026 but are subject to change. Always verify current rates on the Octopus Energy website before switching tariffs.