Cosy Octopus with a home battery
Cosy Octopus spreads its cheap hours across the day instead of bunching them overnight. That changes how a battery should be run. By the end you will know how the windows fit together, why the afternoon one is awkward in summer, and when Cosy beats Go or Flux.
Who Cosy is for
Cosy was designed around heat pumps. A heat pump runs for long stretches, especially in winter, and can't wait for a single overnight window the way a car can. So the tariff offers cheap electricity several times a day, timed to warm the house before people get up, top it up in the afternoon and settle it before bed. Between those windows the price sits at a day rate, and there is a dearer peak in the early evening when the grid is busiest.
Batteries came second, but they fit well. A battery can take a cheap window whenever it appears and can cover the peak from storage. You don't need a heat pump to join. Eligibility and the exact times change between tariff generations, so check the current terms with Octopus before you switch.
The shape of a Cosy day
The day has three cheap windows: a few hours before breakfast, a couple of hours in the early afternoon and a shorter one late in the evening. The early-evening peak, roughly the hours when people come home and cook, is the dearest band, typically several times the cheap price. Everything else is the day rate.
| Part of the day | Price band | What the battery does |
|---|---|---|
| Early morning window | Cheap | Fills from the grid |
| Late morning | Day rate | Runs the house, with solar if there is any |
| Early afternoon window | Cheap | Tops up, unless solar already has |
| Early evening peak | Dearest | Runs the house; nothing imported |
| Late evening window | Cheap | Refills for the night |
| Overnight | Day rate | Runs the house on the late-evening fill |
The surprise for anyone used to Go is the last row. Overnight is not cheap on Cosy. The late-evening window is what carries you to morning.
Three fills a day instead of one
On an overnight tariff the battery fills once and has to carry the house for the best part of twenty hours. A home that uses 20 kWh a day can't do that with a 13.5 kWh battery, and buys the rest at the day rate.
On Cosy the battery only has to bridge the gap between windows, and each gap is around six hours. A third of the day's use fits in a Powerwall with room to spare. That is why a battery that is too small for a high-use home on Go can be enough on Cosy.
The charge rate now matters as much as the capacity. A Powerwall charges at about 5 kW, so a three-hour window moves roughly 15 kWh, enough to fill it from empty. A two-hour window moves about 10 kWh. In practice you rarely start a window empty, so each fill is a top-up rather than a full charge.
There is a cost. Three fills a day cycle more energy through the battery, which means more round-trip loss and more wear. See where the missing ten percent goes and whether daily cycling wears a battery out. Cosy's cheap price is also cheap relative to its own day rate; an overnight-only tariff's window may be cheaper still.
Summer and winter
The afternoon window and your solar
In summer the early-afternoon window lands exactly when the panels are producing most. If the morning sun has already filled the battery, the window is worth nothing to it. Worse, if a rule filled the battery from the grid before breakfast, it bought energy that the roof would have supplied for free two hours later, and then had nowhere to put the solar.
The fix is to make the morning fill conditional on the forecast. On a day that will fill the battery anyway, skip the morning window, let solar do the work and treat the afternoon window as a safety net for a forecast that turns out too optimistic. On a grey day, fill in the morning as normal. In winter the question doesn't arise: the afternoon window is a real fill, because the panels are producing a sixth to an eighth of what they do in June.
Winter is Cosy's season
In winter almost every kWh comes from the grid, and Cosy lets you buy most of them cheaply as long as the battery keeps pace. A heat-pump home benefits twice: the heat pump runs straight from the grid in the windows and from the battery between them. In summer the panels do most of the work and the tariff's structure matters less. If you compare tariffs on summer data alone, Cosy looks worse than it is over a year.
Covering the evening peak
The peak is where a battery earns its keep on Cosy. The aim is to enter it as full as possible and import nothing until it ends. A few things help:
- Keep the backup reserve low through the peak. A high reserve stops the battery discharging, which is the opposite of what you want.
- Time-Based Control with the Cosy shape on the Powerwall will hold energy back for the peak on its own, because it sees the price step coming. See how time-based control works.
- A typical home uses 3–5 kWh in the peak hours, well within the battery. Power is the limit, not energy: an oven, a hob and a kettle together can pass a Powerwall 2's 5 kW, and the grid tops up the difference at the peak price. It is a small cost, and it explains the odd bit of import in the app.
After the peak, the late-evening window refills for the night.
A rule set that follows the windows
Because the windows are the same every day, a small set of clock rules does the job:
- At the start of each cheap window: grid charging on, reserve 100 %.
- At the end of each cheap window: grid charging off, reserve back to your normal figure.
- On the morning rule, add a condition: only if solar forecast for today is below what the battery needs. That is the summer skip.
- Before the peak, nothing. With the reserve down, the battery discharges on its own.
Alternatively, put the real Cosy prices on the Powerwall as its tariff and let Time-Based Control follow them. Cosy repeats daily, so it suits the Powerwall's own tariff far better than Agile does. You still want the forecast condition for the morning fill.
Cosy, Go and Flux for a battery home
| Cosy | Go | Flux | |
|---|---|---|---|
| Cheap windows | Three, spread through the day | One, overnight | One, overnight |
| Evening peak | Dear to import | Day rate | Dear to import, premium to export |
| Needs | A smart meter | An EV | Solar and a battery, import and export with Octopus |
| Battery must carry | About six hours at a time | The whole day | The day, then sell the rest |
| Suits | High-use and heat-pump homes, smaller batteries | EV owners with moderate daytime use | Solar homes that can export at the peak |
For a home with a battery and no EV, the choice is usually Cosy or Flux. Flux pays you to empty the battery into the evening peak and is the stronger tariff when you have plenty of solar to sell. Cosy wins when the home uses more than one overnight fill can cover, when there is little solar or when an export limit makes selling at the peak hard. Go's single window usually beats Cosy's cheapest price, but only an EV owner can join, and only a battery that carries the whole day gets the full benefit; see Go and Intelligent Go with a battery.
In SunHarvest
Choose Cosy from the Octopus feed under Tariffs › Your tariff and your region's prices sync within seconds, so the money figures on Home and Energy are valued at the real bands. The rules above map directly onto the rule editor: a "clock reaches" trigger for each window edge, with "solar remaining today" as a condition on the morning fill. Tariff comparison, part of Pro, replays your own metered history against Cosy, Go, Flux and the rest with the battery run the best way for each, which answers the winter-versus-summer question with your own numbers.
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.