The tariff
The tariff decides what each stored kWh is worth, so it matters about as much as the hardware does. For the first six months we were on a plain 12-month fix at 24.5446p. It was already a poor fit for this house and would have got worse once the heat pump arrived. On 14 September we moved to Intelligent Octopus Go. Below is what we're on now, what we left, and the modelling behind the decision.
What we're on now
| Import | Intelligent Octopus Go — 8p/kWh for 7 hours (01:30–08:30), 34.7205p the rest of the day, standing charge 49.4071p/day. Started 14 September 2026, runs to 14 September 2027. Replaced a 24.5446p flat fix we left on 14 September 2026, five months early. E-1R-IOG-SMB-FIX-12M-26-09-02-H |
| Gas | Octopus 12M Fixed — 6.2382p/kWh, standing charge 33.1889p/day. Goes away entirely with the heat pump in December 2026. G-1R-OE-FIX-12M-26-03-03-V2-H |
| Export | Outgoing Octopus — flat 12p/kWh at any time of day. We chose it because the rate is guaranteed. Paying since 22 September 2026, after three and a half months of exporting for nothing. E-1R-OUTGOING-VAR-24-10-26-H |
| VPP | Axle, events only — £1/kWh with a £10/month minimum. It settles through the smart meter, so it was paying for months before the export certification came through. What it's paid us so far → |
We gave our export away for 110 days. The panels exported 2,487 kWh between 4 June 2026 and 21 September 2026 and earned £0 for it, because the export MPAN hadn't been registered. At 12p that was £299. The MPAN came through on 14 September and the tariff started paying on 22 September. The stats show £0 export revenue for every day before that, since we weren't paid anything. If you're having solar fitted, sort the export paperwork out first and worry about optimising later.
VAT on electricity drops from 5% to 0% on 1 October 2026. Every rate on this page falls by about 4.8% without us doing anything. It applies to every tariff, so none of the comparisons change, but the figures above are the last ones that include VAT.
Why the fix stopped working
A flat 24.5446p is fine for a house that just uses electricity. With a battery it makes less sense, and with an EV and a heat pump on the way it makes very little. When every hour costs the same, the battery can't buy cheap and use later, and the heat pump can't be run in cheaper hours. All the new demand would have been paid for at the full rate, which is why we left early and paid the exit fee.
Modelled annual cost, heat pump and car both running
Staying on the fix would have cost £1,101 a year more. The average import price drops from 23.4p to 9.4p, because the battery and the 7-hour cheap window move most of the year's demand off the day rate.
What we moved to: Intelligent Octopus Go
| Night | 8p/kWh, 01:30–08:30 — 7 hours, and it covers the whole house as well as the car. There are also occasional smart dispatch slots outside the window. |
| Day | 34.7205p/kWh. That's expensive, so the aim is to buy as little as possible at this rate. The battery and the heat pump's thermal store are what let us do that. |
| Standing | 49.4071p/day. |
| Timing | Live from 14 September 2026, a few months before the heat pump goes in in December. If we'd waited for the fix to end in March 2027, we'd have spent a whole heating season on the wrong tariff. |
A correction on Agile. An earlier version of this page said Agile came out about £100/yr cheaper than Intelligent Octopus Go. That was a bug in our model. The battery charging logic stopped looking ahead when it reached another half hour at the same price, so in a flat six-hour window the battery never filled up. Once that was fixed and the battery charged fully overnight, a guaranteed 7 hours at 8p beat Agile by £355 a year, and the high day rate mattered much less.
Cosy Octopus is Octopus's heat pump tariff, and it came fourth. Its off-peak rate is around 14.5p against IOG's 8p, and its 16:00–19:00 peak is over 50p, which is when a heat pump works hardest on a cold evening. Much of what Cosy offers is flexibility that a 15 kWh battery already gives you. If you have a battery, do the sums before assuming the heat pump tariff is the right one.
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Paid at £1/kWh for each grid-balancing event, with a £10 monthly minimum. In events-only mode Predbat runs the battery the rest of the time. What it's paid us so far →
The half-hourly price we modelled against
We're not on Agile, but a year of its published half-hourly rates is a good free record of when electricity is expensive. Every comparison above was modelled against it.
A typical day's Agile import price
A battery charges in the pale hours and runs the house through the dark 16:00–19:00 peak. Averaged over the year, the cheapest half hour is 13.21p at 03:30 and the dearest 36.53p at 18:30, with a mean of 18.21p. Intelligent Octopus Go gives us the same cheap overnight hours at a fixed price, so we don't depend on the forecast.
What happened to Flux
The original business case assumed Octopus Flux, the usual battery tariff, with a cheap overnight window and a 30p evening export peak. The plan was to charge cheaply overnight and export from the battery in the 4pm to 7pm peak. In March 2026, months before the panels went up, Flux and Intelligent Flux closed to new customers. Standard Flux has since reopened. Intelligent Flux was still closed as of August 2026.
Lesson one: don't base a payback calculation that runs for years on one tariff staying available. Octopus can withdraw a tariff whenever it likes, so model your second choice as well. We hadn't, and re-planning in a hurry wasn't much fun.
Losing Flux's 30p export peak cost us less than we expected. Instead of exporting at the peak rate, the battery avoids importing at the peak rate, and that's worth about the same. With a heat pump and a car on the way, avoiding import is also the more useful thing for the battery to do, so we're staying off Flux even though it has reopened. Its cheap import window is only about three hours, which isn't enough for a 45 kWh winter day plus a car.
Why we're on flat 12p export
Agile Outgoing pays a half-hourly export price, uncapped, with no exit fees. We ran a year of real rates against our own generation. Flat 12p comes to £488 and Agile Outgoing to £314, an average of only 7.7p. That's £174 a year in favour of flat 12p. Agile only pays more than 12p between 16:00 and 19:00, and this house generates a lot and uses little, so most of our export happens at midday when Agile pays 5–7p. The heat pump will widen the gap. It will use up the spring and autumn surplus, which is when Agile Outgoing pays best, and leave the summer midday surplus, when it pays worst.
What that means for the battery: an Axle VPP event at £1/kWh comes first, then using the energy in the house instead of importing, then exporting at 12p. Predbat is set up with those priorities and plans the day around them. One result surprised us. On Intelligent Octopus Go at 8p, imported power costs less than the 12p we'd get for exporting our own solar, so it makes sense to sell the solar and charge the car overnight. See the car.