Cosy for heat pumps and electric heating
Cosy is Octopus’s tariff for homes with electric heating. Originally aimed at heat pump owners, it’s now available to households with electric boilers or electric radiators too. The idea is straightforward: give you cheap electricity at the right times to pre-heat your home before the expensive periods. Electric heating systems use a lot of electricity, so the rate you pay per kWh makes a big difference to your annual running costs.
The three rate levels
Cosy splits the day into three price tiers:
Off-peak: The cheapest rate. This is when Cosy wants you to run your heating at full capacity, warming your house and its thermal mass. The off-peak rate is roughly 51% cheaper than the standard day rate in your region.
Standard (day rate): The middle rate, covering the bulk of the day. Similar to or slightly above the Flexible tariff rate.
Peak: The most expensive rate, about 50% above the standard day rate. This is when you should avoid running your heating if possible, relying instead on the stored warmth from the off-peak windows.
Getting your usage pattern right determines whether Cosy saves you money or costs you more.
The time windows
This is where Cosy gets specific. The off-peak, standard and peak windows are:
Off-peak windows (cheapest):
- 04:00 to 07:00 (early morning, pre-heat before the household wakes up)
- 13:00 to 16:00 (afternoon, top up the heat before peak)
- 22:00 to 00:00 (late evening, pre-heat for overnight)
Peak window (most expensive):
- 16:00 to 19:00 (the standard evening peak when grid demand is highest)
Standard covers all remaining hours.
The off-peak windows are strategically placed. The early morning slot lets you heat the house before everyone wakes up. The afternoon slot tops up the warmth before the expensive evening peak. The late evening slot prepares the house for overnight.
How thermal mass makes it work
This tariff only makes sense if you understand thermal mass. A well-insulated house (particularly one with solid walls, concrete floors or underfloor heating) retains heat for hours after the heating system stops running. The house itself acts as a heat battery.
During the off-peak windows, your heat pump runs hard, warming the house and the thermal mass of its structure. Then during the peak window (4pm to 7pm), you turn the heating off or right down. The stored heat keeps the house comfortable for those three hours without the heat pump drawing expensive electricity.
This works better in some houses than others. A well-insulated home with underfloor heating and concrete floors can coast through the peak window with barely any temperature drop. A poorly insulated house with radiators loses heat faster, and you might notice a dip in comfort during peak hours.
Who qualifies
Cosy is available to homes with an air source heat pump, ground source heat pump, hybrid heat pump, electric boiler or electric radiators. You don’t need to have had the system installed by Octopus. If you’re in the process of getting one installed, you can still switch to Cosy.
You also need a smart meter set up for half-hourly readings. A SMETS2 meter works straight away, and some SMETS1 models are compatible too.
Note that storage heater homes are not eligible for Cosy. Octopus offers a separate tariff called Snug for storage heaters, which has its own cheap overnight and afternoon boost periods.
The tariff covers your electricity only. If you still have a gas supply (some hybrid heat pump owners do), that stays on a separate tariff.
Making the most of Cosy
Programme your heating schedule. Most heat pump controllers and electric boiler timers let you set different temperature targets at different times of day. Set higher targets during the off-peak windows and lower targets (or off) during peak.
Use underfloor heating if you have it. Underfloor systems work at lower flow temperatures, which makes your heat pump more efficient, and they heat the thermal mass of the floor slab directly. This combination is ideal for Cosy.
Don’t try to heat a cold house during peak. If the house cools significantly during peak hours because insulation is poor, you’ll be tempted to turn the heating back on at the most expensive rate. This defeats the purpose. If this happens regularly, consider whether your home has enough thermal mass and insulation to make Cosy worthwhile.
Run the hot water cylinder during off-peak too. If your heat pump heats your domestic hot water, schedule it for one of the off-peak windows. This is often a significant electricity draw.
Experiment with the afternoon slot. The 1pm to 4pm off-peak window is sometimes overlooked. Using this period to boost the house temperature before the 4pm peak can make a real difference to comfort during the expensive evening hours.
Different heating systems
Air source heat pumps are the most common type in UK homes. They work well with Cosy. Efficiency (COP) is slightly lower in cold weather, but the cheap off-peak rate more than compensates.
Ground source heat pumps maintain a more consistent efficiency throughout the year because ground temperatures are stable. They pair excellently with Cosy since you can pre-heat with high efficiency during off-peak windows.
Hybrid systems (heat pump plus gas boiler backup) can use the heat pump during off-peak windows and switch to gas during peak if needed. Whether this saves money depends on the relative cost of gas versus peak-rate electricity.
Electric boilers benefit from the same time-shifting approach. Schedule the boiler to heat your water cylinder during off-peak windows rather than on demand throughout the day.
Electric radiators with built-in timers or smart plugs can be programmed to warm rooms during the cheap windows. The key is having enough thermal mass in the room to retain warmth through the peak period.
Savings compared to Flexible
A household with a heat pump using around 50% of its electricity during the off-peak windows, 40% during standard hours and 10% during peak can save roughly 15-25% on their heating electricity costs compared to Flexible. In pounds, that’s typically £150-400 per year, depending on the size of your home and how much heating you need.
The savings are largest for households that successfully avoid peak usage. If you can get through 4pm to 7pm without the heat pump running, those three hours of avoided peak-rate consumption make a meaningful difference across the year.
Conversely, if your house can’t hold heat and you end up running the heat pump during peak hours anyway, Cosy could cost you more than Flexible. The peak rate is designed to be expensive. It’s only good value if you avoid it.
Is it right for you?
Cosy works best for well-insulated homes with good thermal mass, particularly those with underfloor heating. If your house retains heat well and you can programme your heating system to front-load during cheap windows, the savings are genuine.
If your home loses heat quickly or you’re uncomfortable with temperature fluctuations during peak hours, Flexible’s flat rate might suit you better. There’s no penalty for trying, though. Switch to Cosy, run it for a month and compare against what you were paying on Flexible. If it doesn’t work out, switch back.
Use the tariff comparison tool to check current Cosy rates for your area.