Octopus Agile with a home battery
A battery on a half-hourly tariff lets you buy energy when it is cheap and use it when it is dear. Whether that beats a simpler tariff depends on your usage, your battery and how much of the work you automate.
How Agile pricing works
Octopus Agile sets a different import price for every half-hour of the day, based on the wholesale electricity market. A few features matter for battery owners:
- Tomorrow's prices arrive the afternoon before. Each afternoon Octopus publishes the prices from 23:00 that night to 23:00 the next day, usually around 16:00 but sometimes later in the evening. So you normally know somewhere between about 7 and 31 hours ahead, never further.
- The shape repeats, the numbers don't. Overnight and the middle of sunny, windy days tend to be cheapest. The 16:00–19:00 evening, every day of the week, is usually the dearest. How deep the troughs go and how high the peak climbs changes from day to day and season to season.
- Prices can go negative. When the grid has more renewable power than it needs, some half-hours drop below zero and you are paid to import.
- Prices are capped at a maximum set in the tariff terms, which limits the worst half-hour. Check the current cap and terms with Octopus.
What a battery does with it
Without a battery, you benefit from Agile only by moving flexible loads, like the washing machine, dishwasher or car, into cheap slots. The evening peak, when most homes use the most, is hard to avoid.
A battery changes that. It can fill up in the cheapest slots and run the house through the peak, so the expensive half-hours are served from storage. A Powerwall's usable capacity, around 13.5 kWh, covers most of a typical home's evening, and often the night as well.
The daily job looks like this:
- When tomorrow's prices land, find the cheapest slots overnight and during the day.
- Check the solar forecast. A sunny day will fill the battery for free, so buy less overnight.
- Charge in the cheapest slots, only as much as the day needs.
- Hold the charge for the peak, then let the battery run the house through it.
- On negative-price slots, charge and run heavy loads from the grid; you are being paid to.
Doing this by hand every day gets old fast. That is exactly the job rules and the AI planner exist for.
A worked example
Illustrative numbers only, not current prices. Plug in your own from your bill and the Octopus website.
Say a winter weekday has an average overnight price of 15p/kWh and an evening average of 35p/kWh. Your home uses 9 kWh between 16:00 and midnight.
- Without a battery, the evening costs about 9 × 35p = £3.15.
- With a battery, you buy the 9 kWh overnight. Batteries lose some energy charging and discharging, so allow about 10 %, which means roughly 10 kWh bought: 10 × 15p = £1.50.
- That is about £1.65 a day on that day.
Summer days look different. The panels often fill the battery for free, and the saving comes from not buying anything at all. Across a year, the result depends heavily on your usage, your solar and how deep Agile's price gaps are that year.
The risks
- Price spikes. In a tight market, even overnight prices can be high, and a variable tariff passes that straight to you. The cap limits the worst half-hour, not the average.
- Getting it wrong costs money. Charging from the grid before a sunny day, or running empty before a peak, can wipe out a week's gains. The forecast matters as much as the prices.
- It needs attention or automation. Agile rewards acting on each day's prices. If you won't do that, or automate it, a tariff with a fixed cheap window may earn you more for less effort.
Agile versus a fixed overnight window
Tariffs such as Octopus Go or Intelligent Octopus Go offer the same cheap window every night, with a standard rate for the rest of the day. Both are for electric-vehicle owners: you need an EV or plug-in hybrid to join. With a battery big enough to carry the house from the end of that window to the start of the next, you can avoid the day rate almost entirely.
| Half-hourly (Agile) | Fixed window (Go-style) | |
|---|---|---|
| Predictability | Low: the price changes daily | High: the same window every night |
| Best case | Very cheap or negative slots | A known low overnight rate |
| Worst case | A run of expensive days | Buying at the day rate if the battery runs out |
| Effort | Daily decisions, best automated | Set it up once |
| Suits | People who automate, and flexible homes | EV owners who want low effort |
Many battery owners compare the two using a year of their own half-hourly usage. SunHarvest records exactly that data, and you can enter either tariff to see the money figures.
Setting it up in SunHarvest
- In Prices › Your tariff, choose the Octopus feed, pick Agile and your region. Prices sync in seconds, and tomorrow's arrive each afternoon.
- Add your location and roof arrays so the solar forecast is available (guide).
- Start with one rule, such as when the import price falls below 10p, grid charging on and reserve 100 %, paired with one that turns grid charging off again. Or describe your goal and let the AI planner write the rules.
- Watch the money tiles on Home for a couple of weeks, then adjust.
General information, not financial advice. Tariff terms, prices, caps and availability change; check Octopus Energy's current terms before switching. SunHarvest is not affiliated with Octopus Energy.