Octopus free electricity sessions: what to do with a free hour
Every so often Octopus gives its customers an hour or two of electricity for nothing. It arrives as a message the day before, it lands at an odd time, and for a home with a battery, a heat pump or a car it is worth far more than the few pounds the unit rate suggests. This article explains how the sessions work, what to do with the hour, what not to do, and how to set a battery up so the day runs itself.
What a free electricity session is
Free Electricity sessions are part of Octoplus, Octopus Energy's loyalty scheme. When the grid has more wind or sun than it needs, usually a windy or bright day with low demand, Octopus announces a session for the next day: a window, typically an hour, sometimes two, during which the electricity you import costs nothing. The standing charge still applies, nothing changes outside the window, and you are billed as normal for the rest of the day.
To take part you need to have joined Octoplus in the app, which is free, and a smart meter sending half-hourly readings, because the free hour is worked out from what the meter recorded in that half hour. The sessions are announced by email and app notification, and they are irregular: a run of them in a windy spell, then nothing for weeks. The terms and the scheme itself change, so treat this as the shape rather than the rules.
The timing is the point. Sessions tend to fall in the middle of the day or the early afternoon, at the weekend or on a bank holiday, when surplus renewable generation is highest. That is not when a battery home normally imports anything, which is exactly why it needs a plan.
What an hour is worth
A free hour saves you the unit rate on everything you can pull in during it. For a home with nothing to shift, that is a kettle and the fridge, worth pennies. For a home that can fill a battery, heat a cylinder, warm the house and charge a car, it is a different matter:
| What you fill | Energy in one hour | Worth at a standard rate of about 24p |
|---|---|---|
| Powerwall 2, charging at 5 kW | about 5 kWh | about £1.20 |
| Powerwall 3, charging faster | up to 11 kWh | about £2.60 |
| Immersion heater, 3 kW | 3 kWh | about 70p |
| Heat pump run hard, 3 kW | 3 kWh of electricity, 9 to 12 kWh of heat | about 70p |
| EV on a 7 kW charger | 7 kWh | about £1.70 |
A home that does all of that pulls 20 kWh in the hour and saves a fiver. That is the honest size of it: a nice bonus, not a change to the economics of the house, and not worth rearranging your life around. It is worth automating, though, because an automated home gets the bonus every time and a manual one gets it when somebody remembers.
What to do with the hour
- Fill the battery from the grid, flat out. Grid charging on, reserve 100 %. On a Powerwall that is a Time-Based Control job: with a cheap window in its tariff it charges; without one it has to be told. SunHarvest's force-charge action does this in one step, by writing a near-free hour over the real tariff on the Powerwall so that Time-Based Control fills as hard as it can, and putting everything back when the hour ends.
- Heat the water. Run the immersion or the heat pump's hot-water cycle in the window. A full cylinder is 3 to 4 kWh you would otherwise buy later.
- Warm the house. A heat pump run a degree or two warmer for the hour puts heat into the fabric that carries into the evening. An air-to-air unit, run at the temperature you actually want, does the same.
- Charge the car. Set the charger to start at the window's start; 7 kWh is 25 miles.
- Run the big appliances. Dishwasher, washing machine, tumble dryer, the oven if a meal fits.
Power is the limit. Everything in that list at once is 20 kW, which no domestic supply allows and which a Powerwall 2 cannot charge through alongside. Pick the order: the battery and the car first, because they hold the most; the water next; the appliances fit around them.
What to do with the rest of the day
This is the part people miss. If the battery is full at 15:00 from a free hour, there is no sense buying a cheap window at 22:00 as the normal routine would. On Cosy Octopus, where the afternoon and late-evening windows normally fill the battery, a free hour at lunchtime means the afternoon fill should be skipped and the evening fill reduced. On Go the overnight fill can be skipped or shortened. In other words, a free-hour day has a different strategy from a normal day, not just one extra step.
The other mistake is exporting what you were given. A battery filled for free and then emptied into the grid at an export rate earns a few pence per kWh and leaves nothing for the evening peak, which costs you the peak rate for the hours it should have covered. Let the free energy run the house.
Setting a battery up to do it alone
Octopus tells you the day before, so the setup can be done in the evening and left. The cleanest way in SunHarvest is a rule group for free-hour days that holds the day's rules, alongside the normal group, which for a Cosy home is the rule set in Cosy Octopus with a battery:
- A rule at the window's start: force charge until the window's end. That fills the battery and reverts the tariff and settings afterwards by itself.
- The heat pump's own schedule and the car charger's timer set to the same hour. SunHarvest reads a linked charger but never starts it, so those two run on their own clocks.
- A rule at the normal tariff's next cheap window that does nothing, or a condition on the normal fill rules that skips them when the battery is already above, say, 80 %.
- A rule at midnight that switches the normal group back on and the free-hour group off, so the next day is ordinary again.
When the announcement arrives, open the Rules tab, press Only this on the free-hour group and set the start time to the window Octopus gave. The following midnight the rule puts things back. A home with a heat pump can get the same effect from the heat pump's own schedule and a battery rule alone.
Things that catch people out
- It is a half-hourly measurement. The free period is the half hours Octopus names; energy drawn a minute early or late is at full price. Start on the hour, not a bit before.
- The standing charge is unchanged. A free day is not a free day.
- Sunny sessions and solar overlap. A session on a bright day comes when your own panels are already filling the battery. Grid charging at the same time is fine, it just means less of the free hour is used; the gain is smaller than the table suggests.
- Octoplus has to be switched on. No membership, no session, however many emails you get about them.
- A missed session costs nothing. There is no penalty, so there is no harm in a rule that fires for a session that was cancelled; the Powerwall simply charges at the day's real price for an hour, which is the one thing worth avoiding. Keep the group off until an announcement arrives.
In SunHarvest
Rule groups are made for days like these. Keep a free-hour group beside your everyday group: a force charge until the window's end, and conditions on the usual fill rules that skip them when the battery is already full. When the announcement arrives, press Only this on the free-hour group and set the time; a rule with the switch-group action puts the everyday group back at midnight. The force-charge action writes a near-free hour over the Powerwall's tariff so Time-Based Control fills flat out, then restores everything. SunHarvest only reads a linked charger and never starts it, so the car stays on the charger's own schedule.
General information, not financial advice. Octoplus and free electricity sessions are Octopus Energy's schemes and their terms change; check the current terms. SunHarvest is not affiliated with Octopus Energy.