Octopus Go and Intelligent Go with a home battery
Octopus Go gives you a few cheap hours every night and a higher rate the rest of the day. With a battery big enough to carry the house from one window to the next, you can live almost entirely on the cheap rate. This article works through whether yours is, and what to do with the solar.
The cheap-window mechanic
Go is a two-rate tariff. For a fixed window in the small hours, every kWh you import is charged at a low rate, a fraction of the day rate. Outside the window, everything is charged at the day rate. The window is the same every night, so there is nothing to check each afternoon and nothing that changes with the weather or the wholesale market. That predictability is the whole appeal.
Go and its sibling Intelligent Octopus Go are sold as electric-vehicle tariffs: you need an EV or plug-in hybrid to join, and Octopus's eligibility rules are the ones that count. The car is the load the tariff was designed for. A home battery extends the same idea to the whole house.
Fill overnight, run the house by day
In the window, the battery charges from the grid at the cheap rate. From the end of the window until the next one, the house runs on the battery instead of the grid. Every kWh the battery supplies during the day was bought at the overnight rate rather than the day rate.
On a Powerwall this means Time-Based Control with grid charging permitted and the cheap window entered as its tariff. Or it can be done by rule: raise the backup reserve to 100 % when the window opens and lower it when it closes, which makes the Powerwall top up whatever else it thinks. How Powerwall time-based control works explains both.
Does 13.5 kWh cover the day?
Two questions decide it: can the battery fill in the window, and does what it holds last until the next one.
Filling in the window
A Powerwall charges from the grid at up to about 5 kW. An empty 13.5 kWh battery needs roughly 15 kWh from the grid to fill, allowing for round-trip losses, which is about three hours at full rate. A window of four hours or more fills it from empty comfortably; a battery that starts half full needs half the time. If a car is charging at 7 kW in the same window, the two together draw around 12 kW; whether the house's supply can carry both is a question for your installer.
Lasting until the next window
The battery leaves the window with 13.5 kWh less whatever backup reserve you hold. Everything the house uses until the next window comes from that, plus whatever the panels make.
| Home's daytime use (outside the window) | With no solar | With 6 kWh of solar used directly or stored |
|---|---|---|
| 8 kWh | Covered, with about 5 kWh spare | Covered, battery finishes over half full |
| 12 kWh | Covered, with little spare | Covered easily |
| 16 kWh | About 2.5 kWh bought at the day rate | Covered |
| 24 kWh | About 10 kWh at the day rate | About 4 kWh at the day rate |
Figures assume no reserve and a full battery at the end of the window. A home using around 10–12 kWh a day outside the window lives on the cheap rate all year; a home using twice that does so only with help from solar. For a high-use home, the day-rate kWh are the ones to attack, by moving the biggest loads (immersion heater, dishwasher, tumble dryer) into the window alongside the car.
There is no peak to worry about on Go. Every hour outside the window costs the same, so the only aim is to make the stored energy last until the next window.
What Intelligent Go adds
Intelligent Octopus Go has a longer standard overnight window, plus extra cheap slots at other times when Octopus schedules your car's charging. You tell Octopus when you need the car ready and how much charge you want; Octopus picks the slots, and while the car charges in one, the whole house is on the cheap rate too. The car or the charger must be one Octopus can control.
For a battery, the longer window is the main benefit, and the extra slots are a bonus: when one lands in the afternoon, the battery can top up from the grid too. A battery that reacts to "the cheap rate is on now" rather than "it is 01:00" captures those slots; one with a fixed window on the Powerwall's tariff misses them.
Solar in summer, the window in winter
The seasons ask for opposite behaviour. In winter, solar barely registers and the overnight window is the best energy you will get: fill the battery every night and run on it. In summer, a 4 kWp array can make 20 kWh on a good day and fill the battery for free by early afternoon. Buying a full charge overnight and leaving no room for the solar wastes the cheaper source.
The right overnight charge on a summer night is only what the morning needs before the panels take over, with the rest left empty for the sun. That needs a forecast: before a grey day, buy the full charge; before a sunny day, buy a little. A rule keyed to tomorrow's solar forecast does this without you. Grid charging: when it pays and when it doesn't goes through the arithmetic.
The export side
Go is an import tariff. Export is a separate product, typically Octopus's flat-rate Outgoing, and its terms matter: Outgoing pays for your own solar, not for energy you bought from the grid and stored. Exporting grid-charged energy is against the terms and usually pointless, since the export rate is normally below what you paid overnight.
So the Powerwall's export setting should be Solar only on Go, never Solar + battery. There is one legitimate trick, though. Compare your export rate with your overnight import rate. If export pays more than overnight import costs, then on a sunny day the house should run on the cheap night charge while every solar kWh goes to the grid, because each solar kWh sold earns more than the night kWh replacing it cost. That is why Go often comes out well for a solar home with an EV even though it has no peak rate. Where export pays less than overnight import, store the solar and use it instead.
A typical daily plan as rules
- When the window opens, grid charging on and reserve 100 %, only if tomorrow's solar forecast is below about 10 kWh; on a sunny forecast, reserve 40 % instead.
- When the window closes, reserve back to your everyday figure, say 10 %.
- Export set to Solar only, always.
- On Intelligent Go, when the import price falls to the cheap rate at any time, grid charging on and reserve 100 %; when it rises, put both back.
- If the car is charging, hold the reserve high so the battery does not empty itself into the car.
When Go beats Agile, and when it doesn't
Go wins on predictability and effort: the window and rate are known, and a battery that fills every night does the rest. It also wins in a tight market, when Agile's overnight prices climb while Go's stay put. Agile wins when its cheap half-hours go well below Go's window rate, on windy nights and sunny middays, and when negative-price slots pay you to fill up. Agile rewards a battery steered daily; Go rewards one set up once. A high-use home that cannot run its day on one charge does better on Agile's midday troughs; a modest-use home with a car does well on Go with less to think about.
In SunHarvest
Pick Go or Intelligent Octopus Go as your real import tariff under Tariffs › Your tariff, with Outgoing as the export, and the price signals follow them: a rule on "import price falls below" a threshold fires at the start of the window and, on Intelligent Go, in every extra cheap slot, with a partner rule on "rises above" to close it. The force-charge action does the same job in one line, filling the battery until a time and restoring every setting afterwards. The tariff comparison replays your own history against Go, Intelligent Go and Agile with the battery run the best way for each. See the tariffs guide.
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.