Cosy Octopus and Home Batteries: Should Heat Pump Owners Add a Battery in 2026?

Why this question is suddenly worth asking

Two things have changed in 2026 that make a home battery interesting for heat pump owners specifically. First, the Boiler Upgrade Scheme grant is £7,500 for an air or ground source heat pump (and £9,000 for oil and LPG properties following the April 2026 update), so the population of UK homes with a heat pump is growing fast. Second, the April to June 2026 Cosy Octopus rates have settled into a structure where the gap between the cheapest and most expensive hours is large enough to make tariff arbitrage genuinely lucrative.

Cosy Octopus customers without a battery save modestly: Octopus's own published figure is around £219 a year against a gas boiler on a standard tariff, based on actual customer data over the year to March 2026. A battery is what unlocks the rest of the saving sitting in the rate spread.

What Cosy Octopus actually looks like

Cosy Octopus is an import-only tariff. It has three time bands per day, with the same windows seven days a week.

Rates vary by DNO region. Northern Scotland, North Wales and Merseyside, and the South East are typically a little above average, while the East Midlands, West Midlands and Yorkshire tend to be below. The shape of the tariff (cheap, mid, expensive) is the same everywhere.

Why the peak window is the whole story

The 4pm to 7pm peak rate is the single most important number in this tariff. It sits 50% above the day rate, and almost four times above the off-peak rate. That three hour window is also exactly when most UK households are home, cooking dinner, watching TV, and (in winter) running their heat pump hardest as outside temperatures drop.

Without a battery, a Cosy Octopus customer pays roughly 51.7p per kWh for everything used between 4pm and 7pm. With a battery, the household pays nothing at the meter through that window, because the battery is delivering electricity it bought at 14.5p earlier in the day.

A worked example: three-bed home with an air source heat pump

Consider a typical UK three-bed semi with an air source heat pump. Energy Saving Trust figures put annual heat pump electricity use for this kind of home at around 3,000 to 4,500 kWh, on top of about 2,700 kWh of normal household electricity for lights, appliances and cooking. Total annual electricity consumption is therefore in the 6,000 to 7,000 kWh range.

Without a battery, on Cosy Octopus, a sensible weighted estimate puts around 30% of consumption in cosy hours, around 55% on the day rate, and around 15% in the peak window. That is the duty cycle of a heat pump combined with normal evening cooking and TV use.

With a battery sized to cover the peak window plus some daytime use, the picture changes. Assume an 11.5 kWh battery is force-charged in the early-morning cosy hours at 14.5p, and discharged through 4pm to 7pm to cover heat pump and household use during peak. On most days through the heating season that displaces 9 to 11 kWh of peak consumption, plus a further chunk of evening day-rate use.

Numbers are illustrative for a three-bed home in an average DNO region on the April 2026 Cosy Octopus rates. A larger home, a heat pump that runs harder in winter, or a household that does most of its cooking and laundry in the evening will see savings nearer the £800 to £900 end of the range. A small flat with a low-demand heat pump and a regular working pattern will see less.

How to use the three off-peak windows

The three cosy windows are not equally useful for charging. Most batteries are at their lowest state of charge by 10pm, after they have powered the home through peak. The 22:00 to 00:00 window is therefore good for a top-up and for pre-heating the cylinder, but it is short. The 04:00 to 07:00 window is the workhorse: it is three hours, the grid is at its quietest, and there is enough time to bring a depleted battery back to 100%. The 13:00 to 16:00 window is the safety net for the peak window that follows it directly.

• Main charge: 04:00 to 07:00. Bring the battery to full from whatever state it ended the previous day in. This is also a sensible window to run the heat pump's hot water cycle if the cylinder schedule allows.
• Top-up charge: 13:00 to 16:00. Aim to start the peak window at 100%. In summer with low heat demand and some incidental solar self-consumption, this window will rarely be needed.
• Avoid: 19:00 to 22:00. This is the day rate. Avoid running the dishwasher, washing machine and tumble dryer here. Schedule them inside any cosy window instead.
• Avoid: 16:00 to 19:00. Peak. Everything the house uses here should come from the battery, not from the grid.

What the battery does for the heat pump itself

A good heat pump installation runs the compressor little and often, modulated to the heat demand of the building. That means it will run plenty during peak hours unless you actively prevent it. There are two ways to handle that on Cosy Octopus.

The first is to schedule the heat pump's main duty cycle into the cosy windows, particularly the morning 4am to 7am block. This is the approach Octopus's Cosy heat pump controls use by default. The downside is that any heat the building loses through the peak window has to come from somewhere, and on a cold January evening that "somewhere" is almost always the heat pump pulling at the 51.7p rate.

The second, with a battery, is to keep the heat pump running on demand whenever the house actually needs heat, and let the battery decide whether to take that energy from the grid (off-peak) or from stored cheap power (peak). This is more comfortable, less fiddly, and broadly more efficient because the heat pump is not being asked to overheat the building in the early morning to ride out the evening.

What about solar?

If you already have solar, Cosy Octopus is rarely the best fit. Octopus Flux is built specifically for solar plus battery, with a 4pm to 7pm export rate above 30p per kWh that pairs naturally with a battery discharge schedule. Cosy is best for heat pump homes without solar, or with only a small (under 4 kWp) array.

If you are starting from scratch and choosing between solar plus battery and a battery alone, our battery vs solar panels comparison walks through the per-pound payback for each. For most heat pump homes that already have the £7,500 BUS grant in mind, adding a battery first and looking at solar later is the simpler and faster route to bill reduction.

What it means in practice

If you already run a heat pump on Cosy Octopus, a home battery is the single biggest remaining lever you have on your annual electricity bill. The tariff structure has been quietly rebuilt around it: three cheap windows, one expensive window, a 37p per kWh spread that a battery can capture every day of the year. With the price cap forecast to rise again in July 2026, that spread is more likely to widen than to narrow.

If you do not yet have a heat pump but are weighing one up, the right way to think about the BUS grant is in two parts. The £7,500 covers most of the heat pump's upfront cost. A battery installed at the same time, on the same Cosy Octopus tariff, makes the running cost competitive with (and in many homes lower than) a gas boiler. That combination is what makes a heat pump genuinely cheaper to run, not just cleaner.