A home battery in winter: what to expect
In summer a home battery stores your own solar. In winter there is very little solar to store, and the battery either finds a different job or sits nearly idle. Which of those happens depends almost entirely on your tariff.
How little sun there is
The same roof that produces well in June produces a sixth to an eighth of that in December. Days are short, the sun is low, and cloud is the default. A south-facing 4 kWp array that makes on the order of 3,400–4,000 kWh a year might manage a handful of kWh on a bright December day and well under 1 kWh on a dull one. Against a 13.5 kWh Powerwall, that is a battery you could not fill from the roof in a week.
The house, meanwhile, uses more. Lights are on longer, heating pumps and fans run, and anyone with a heat pump or an electric car sees winter use climb steeply. The gap between what the panels make and what the home needs is widest exactly when the battery has the least to work with.
The battery's job changes
From roughly November to February the battery stops being a store for surplus solar and becomes a way of moving grid energy from one time of day to another. That is a different job, and it only exists if energy costs different amounts at different times. On a tariff with a cheap window the battery fills from the grid at night and runs the house through the day and evening. On a flat tariff there is nothing to move, and the battery mostly waits for spring.
This is why winter is when the choice of tariff matters most. In summer almost any tariff works with solar and a battery. In winter the tariff is the whole game.
The sums on a cheap-window tariff
Take a tariff with a cheap overnight window of a few hours and a higher rate for the rest of the day, such as Octopus Go or Cosy. Say the cheap rate is a quarter of the day rate. Every kWh you store overnight and use during the day costs you the cheap price plus round-trip losses. With losses of around 10 %, a stored kWh costs about 0.25 ÷ 0.9, or 28 % of the day rate. You save the other 72 % on every unit the battery delivers.
How much it delivers is bounded by two things. The first is the battery's capacity: 13.5 kWh a day at most, less the reserve you hold back. The second is how fast it can fill. A Powerwall 2 charges at up to 5 kW, so filling from empty takes under three hours, which fits inside most cheap windows. A house using 14 kWh a day in winter can therefore run most of the day on stored night energy and buy the rest at the day rate.
The evening is the point. On tariffs with a distinct peak, such as Flux or Cosy, the early-evening rate is several times the overnight rate, and that is where the battery earns its keep. Holding enough charge to cover 16:00 to 19:00 comes before anything else. Octopus Go and Intelligent Go with a home battery and Cosy Octopus with a home battery go through each tariff's shape.
When export stops mattering
In summer, export is a real income and worth timing. In winter the surplus mostly disappears. A tariff that pays for battery export, such as Flux, still offers a peak price in the evening, but selling stored grid energy only pays if the peak export price beats the overnight import price by more than the round-trip loss, and even then every kWh you sell is a kWh you cannot use to avoid buying at the peak import price, which is usually higher. Most winter evenings the better plan is to run the house from the battery and export nothing.
Why a flat tariff leaves the battery idle
On a flat tariff every kWh costs the same whenever you buy it, so shifting energy from night to day saves nothing and loses the 10 % round trip. Grid charging never pays. The battery's only fuel is solar, and in December there is barely any. The Tesla app will show the battery cycling a kWh or two on a good day and nothing on a bad one.
That is not a fault. It is what a flat tariff makes of a battery in winter. If you want the battery to do more, the first question is whether a time-of-use tariff would suit your home across the whole year, not just in winter. How much does a home battery actually save in the UK? walks through the arithmetic.
The reserve in storm season
Power cuts are more common in winter, and a battery that is empty at 6 pm is no help in one. The backup reserve is the share held back for exactly that. In summer a high reserve costs you stored solar; in winter, on a cheap-window tariff, it costs very little, because the battery fills every night anyway. Holding 20–30 % through the dark months, and more when a named storm is forecast, is a reasonable trade. Storm Watch, if you turn it on, charges the battery to full ahead of severe-weather alerts on its own, using the grid if it must.
Backup reserve: what percentage to set, and what it costs you covers the trade-off in detail.
Cold, heat pumps and the battery
Cold
A Powerwall is designed for outdoor installation and manages its own temperature. Lithium cells should not be charged below freezing, so on a very cold night the unit warms itself before it will accept charge. Two things follow. Charging can start later and run slower than on a mild night, which matters if your cheap window is short. And some of the energy you buy goes on heating the battery rather than into storage, so the round trip is a little worse in a cold snap. Neither is a fault, and neither is large in a typical UK winter, but a unit outdoors in a hard frost will show both.
Heat pumps
A heat pump changes the scale. A home might draw 20–40 kWh a day from a heat pump in a cold spell, on top of everything else. A 13.5 kWh battery cannot carry that. What it can do is cover the most expensive hours: the early-evening peak and the first cold hours of the morning. Cosy Octopus was designed around this, with cheap windows in the early morning and early afternoon that let the heat pump run hard when energy is cheap and the battery cover the gaps. The rest of the day the heat pump runs on the grid at the day rate.
Two seasons, one set of rules
The practical problem is that the right settings in June are wrong in December. There are two ways to handle it.
- Two rule sets. Write a winter set that charges overnight and holds a high reserve, and a summer set that leaves room for solar. Give each a month condition so they switch themselves.
- Forecast-driven rules or a planner. Rather than trusting the calendar, condition the overnight charge on tomorrow's solar forecast. A rule such as at 23:30, only if solar tomorrow is below 6 kWh, force charge until 05:30 is right in December, right in a wet week in May, and stays out of the way in June.
The shoulder months are where the calendar approach fails. March and October have bright days and grey days back to back, and a fixed rule set gets one of them wrong.
In SunHarvest
Rules can carry a month condition, so a winter set and a summer set can live side by side and take turns. Better, the solar tomorrow signal lets one rule decide each night whether to fill from the grid, and the force charge until action fills the battery flat out inside the cheap window whatever the Powerwall's own tariff says. The AI planner does the same job without rules: it reads the forecast and the next two days of prices and replans every three hours. And the tariff comparison replays a full year of your own history, winter included, so a tariff that looks good in June is judged on December as well.
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.