Leaving the fix five months early for Intelligent Octopus Go

Since March we’d been on a plain 12-month fix at 24.5446p, with a 44.35p standing charge and an exit fee of £50 per fuel, and it had five months left to run. Today we left it for Intelligent Octopus Go: 8p/kWh overnight, 34.7205p by day and 49.41p standing, fixed for twelve months to September 2027.

Why we didn’t wait for March

On a flat rate a battery can only do half its job. Every stored kilowatt-hour is worth exactly what it cost, so there’s nothing to arbitrage. The battery still moves solar from the day into the evening, which is useful, but it can’t buy cheap and use it later when power is expensive.

That was about to get much worse. The Škoda Enyaq arrived in July and has been charging off a granny lead at the full day rate, and the heat pump arrives in December. Both are big new loads that want the same cheap hours, and waiting for March would have put a whole heating season on the wrong tariff to save a £50 exit fee.

The numbers

Modelled over a year with the heat pump and the car both running, on our own consumption profile:

Tariff	Modelled net cost
Intelligent Octopus Go	£591
Octopus Go (5h)	£775
Agile import	£946
Cosy Octopus	£1,123
Staying on the fix	£1,692

Staying on the fix would have cost us £1,101 a year more. The blended import price drops from 23.4p to about 9.4p, because the battery and a long cheap window between them shift nearly all the year’s demand off the day rate. The full modelling, including why Cosy (the heat pump tariff) comes fourth, is on the tariff page.

The standing charge does go up, from 44.35p to 49.41p a day, which is £18.47 a year before you’ve used a single unit. It’s small next to £1,101, and the comparison above already includes it.

We nearly got this wrong

An earlier version of our model said Agile beat IOG by about £100 a year. That was a bug in our own code. The battery’s charging logic stopped looking ahead as soon as it found another equally cheap half hour, so inside a flat overnight window it never filled up. Agile’s uneven prices hid the bug and a flat window showed it up. Fixing it reversed the result.

We would have switched to the wrong tariff because of our own arithmetic. If you’re modelling this yourself, check that your simulated battery is actually reaching 100% overnight before you trust the answer.

Our cheap window isn’t the advertised one

Intelligent Octopus Go is sold as 23:30–05:30, six hours. Ours is different. Reading the half-hourly rates off our own meter, the 8p period runs from 01:30 to 08:30. That’s seven hours, starting two hours later than advertised and finishing three hours later.

We’re happy with an extra hour, and the timing suits us, since 08:30 is after the morning peak and the house can run on cheap power well into the morning. But the tariff you get is whatever your meter is actually running, so check yours before building automations around it. Predbat is now scheduling the car, the battery and, from December, the heat pump against that window.

One other consequence: at 8p, imported electricity is now cheaper than our own solar is worth. That changes what we should do with surplus solar, which became a practical question when export went live eight days later.