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

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.

Source: Cosy Octopus 12M Fixed (COSY-FIX-12M-26-09-26), Yorkshire (region M), Octopus API, checked 10 October 2026. Rates vary by region, so check the rates for your own region. 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. On the Yorkshire 12M Fixed rates above, 45.19p is 50% above the 30.13p day rate and about three times the 14.77p off-peak rate. That three hour window is also when many households are home, cooking dinner and, in winter, running their heat pump.

Without a battery, a Cosy Octopus customer pays 45.19p per kWh for everything used between 4pm and 7pm. With a battery, the household can cover that window from electricity it bought at 14.77p earlier in the day.

How to use the three off-peak windows

The three cosy windows are not equally useful for charging. A battery is usually at its lowest state of charge after powering the home through the peak. The 22:00 to 00:00 window is good for a top-up, but it is short. The 04:00 to 07:00 window is the workhorse, with enough time to bring a depleted battery back up. 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 full.
• Avoid: 19:00 to 22:00. This is the day rate. Schedule the dishwasher, washing machine and tumble dryer inside a cosy window instead.
• Avoid: 16:00 to 19:00. Peak. Everything the house uses here should come from the battery where it can.

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 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. Octopus covers its Cosy heat pump controls here. The downside is that any heat the building loses through the peak window has to come from somewhere, and on a cold evening that is usually the heat pump pulling at the peak rate.

The second, with a battery, is to keep the heat pump running whenever the house actually needs heat, and let the battery cover the peak window from stored off-peak electricity. This is more comfortable and less fiddly, 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, compare Cosy Octopus with Octopus Flux, which is built for solar plus battery. Cosy suits heat pump homes without solar, or with only a small 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.

What it means in practice

If you already run a heat pump on Cosy Octopus, a home battery storage system lets you capture the gap between the cosy and peak rates every day. Octopus has also announced its own home battery range, the Nook, due on sale next year.

If you do not yet have a heat pump but are weighing one up, check the current Boiler Upgrade Scheme rules for the heat pump, and look at the battery as a separate purchase that lowers its running cost on Cosy Octopus.