Octopus Flux explained: three windows and a battery
Octopus Flux splits every day into three price bands and applies them to export as well as import. A battery that charges in the cheap band, rests in the middle and sells in the peak turns that structure into income. This article explains each band, how much a battery can actually sell, and what to keep back for the evening.
The three bands
Flux prices every day the same way, whatever the weather:
- Overnight, cheap. A window in the small hours where import is at its lowest rate. Export in this window is also paid at its lowest rate.
- The day rate. Most of the day, a standard rate for import and a standard rate for export.
- The peak, late afternoon to early evening. Typically 16:00–19:00. Import is at its dearest, and export is paid at its highest rate, well above the day rate.
The export peak is what makes Flux different. On most export tariffs a kWh is worth the same whenever it leaves the house. On Flux, a kWh sent out at 17:30 is worth substantially more than the same kWh at 13:00. The only way to move solar from lunchtime to teatime is a battery, which is exactly what the tariff is built for.
What a battery does in each band
Overnight: charge
Grid charging is on and the battery fills at the cheap rate. How full depends on the season and the forecast. On a Powerwall, Time-Based Control with the three bands entered as its tariff does this itself; so does a rule that raises the reserve to 100 % at the start of the window.
Daytime: run the house on solar, store the rest
Through the day the panels run the house and the surplus goes into the battery. The day export rate is less than the peak rate, so every kWh of surplus that fits in the battery is worth more later. Once the battery is full, surplus exports at the day rate.
The peak: sell
At the start of the peak, the battery exports as hard as it can, with the house's own load still served from it. This needs the export setting on Solar + battery, and on a Powerwall it is most reliably done by showing the Powerwall a very high sell price for those hours. At the end of the peak, export goes back to solar only and the battery runs the house through the rest of the evening.
Who can be on Flux
Flux is not a general tariff. It requires solar and a battery, and both your import and your export to be with Octopus. The tariff pays for exported energy regardless of where it came from, so unlike Outgoing, energy bought overnight and sold at the peak is within the terms. The terms, eligibility rules and the exact band times are Octopus's to set and change, so read the current version before switching. UK export tariffs explained puts Flux beside the other export options.
How much can you actually sell?
The limit is usually not the battery. A typical single-phase home is connected under G98, which allows 3.68 kW of export (16 A at 230 V). Over a three-hour peak, that is about 11 kWh at the very most, and only if export runs at the limit for the whole window. A Powerwall 2 can discharge at 5 kW and a Powerwall 3 at 11.5 kW, so the connection is the bottleneck, not the battery. If your network operator has set a different export limit under a G99 approval, use that figure. The 3.68 kW export limit explains where these figures come from.
Two things reduce the 11 kWh further. Any solar still being made during the peak counts towards the limit, so on a bright June evening the panels take some of the 3.68 kW and the battery gets less. And the house is drawing from the battery at the same time: if the home uses 1.5 kWh over the peak, that comes out on top of what is exported.
| Export limit | Three-hour peak, at the limit | Battery needed for peak export plus 1.5 kWh house load |
|---|---|---|
| 3.68 kW (G98) | about 11 kWh | about 12.5 kWh |
| 5 kW (a G99 limit) | 15 kWh | more than one Powerwall holds |
| 2 kW (a restrictive G99 limit) | 6 kWh | about 7.5 kWh |
On a standard connection, one 13.5 kWh Powerwall that reaches 16:00 nearly full can export at the limit for the whole peak with about a kWh left, which is why it fits this tariff so neatly. A second Powerwall adds nothing to what can be sold in the peak; the limit is already reached. It only adds room to store solar for the house.
Keeping enough back for the evening
Selling the battery flat at 19:00 and then buying the rest of the evening at the day rate undoes part of the gain. The right amount to hold back is what the house will use from the end of the peak until the overnight window opens, plus any backup reserve. For a home that uses 4 kWh in that stretch and holds 10 % for cuts, that is about 5.35 kWh, so export should stop at around 40 % rather than at the floor.
The arithmetic is worth doing. If the peak export rate is only modestly above the day import rate, exporting a kWh at the peak and buying one back at 21:00 gains little, and after round-trip losses may gain nothing. Where the gap is large, sell down to what the evening needs and no further. A rule with a condition on battery percentage, or a planner that knows the home's usual evening load, sets the stopping point each day.
Winter
In winter the daytime band has little solar in it, so the pattern simplifies to the overnight window and the peak: fill from the grid at the cheap rate, run the house through the day, and sell whatever the evening can spare into the peak. Whether that sale pays is the gap between the peak export rate and the overnight import rate, less about 10 % for losses. The margin is thinner than on a summer day, when the peak energy was free solar. December generation is typically a sixth to an eighth of June's, so expect the Earned figure to fall with it and the battery to do more shifting and less selling.
Intelligent Flux
Intelligent Octopus Flux is the hands-off version. Octopus controls the battery directly, charging and discharging it on their schedule against the wholesale market, and pays a flat import and export rate through the day in return. You give up control of when the battery charges and sells; in exchange, the optimisation is done for you. It suits people who do not want to run rules or a planner and are content for the battery to serve the grid as well as themselves. Which batteries Octopus supports is their decision; check the current list. On Intelligent Flux, any automation of your own has to stay out of the battery's way, or the two will fight.
When Flux beats Agile
Flux gives a guaranteed peak every day, at a known price, in a known window. Agile and Agile Outgoing give a peak that is usually there but varies in height, plus cheap and occasionally negative import slots that Flux does not have. A solar-and-battery home that can fill the battery by afternoon and sell 8–11 kWh into the peak most days will find Flux's certainty hard to beat, especially in summer. A home with a smaller battery, less solar, or a taste for chasing negative prices may do better on Agile. The honest way to decide is to replay your own year of half-hourly data against both.
In SunHarvest
Flux is the tariff the force-export action was built for: a rule such as "when the clock reaches 16:00, force export until 19:00, only if battery % is above 60" shows the Powerwall a huge sell price for the window, blocks grid charging so it cannot buy to sell, and restores the real tariff and every changed setting at 19:00. Pick Flux for both import and export under Tariffs › Your tariff and the Earned tile on Home shows what each peak actually made at real prices. The tariff comparison replays your history against Flux and Intelligent Flux alongside the rest. The rules guide has the action.
General information, not financial or electrical advice. Tariff terms, prices, warranties and connection rules change and differ by supplier, region and installation; check the current documents for yours. SunHarvest is not affiliated with Tesla or Octopus Energy.