Heat pump running costs on Cosy

The Cosy tariff is where heat pump economics really start to work in your favour. Designed specifically for homes with heat pumps, it structures electricity prices around three rate bands that reward you for shifting heating demand into cheaper windows. If you’re new to the topic, our heat pumps and Octopus overview covers the basics.

The three rate bands

Cosy splits the day into off-peak, standard and peak periods:

Off-peak (roughly 14p/kWh): 4:00-7:00am, 1:00-4:00pm and 10:00pm-midnight
Standard (roughly 29p/kWh): most of the remaining hours
Peak (roughly 43p/kWh): 4:00-7:00pm only

The exact rates vary by region and change over time, so check the Octopus app for your current prices. The structure stays the same though: three distinct windows of cheap electricity totalling eight hours per day, and one short expensive window in the late afternoon. Off-peak is roughly half the standard rate, and peak is about 50% above it.

The scheduling strategy

The goal is to push as much of your heat pump’s consumption into off-peak windows as possible. In practice, this means:

Morning window (4-7am): Set your heat pump to ramp up and get the house to your target temperature before you wake up. If you like 20°C during the day, aim for 21-22°C by 7am. The house will gradually cool through the morning, but a degree or two of cooling is barely noticeable.

Afternoon window (1-4pm): This is your midday top-up. The house may have dropped a degree since morning. The heat pump runs again during this cheap window, bringing everything back up to temperature.

Evening window (10pm-midnight): If you’re still up, this window takes the edge off before bed. More importantly, it pre-heats the house so your system doesn’t need to work hard overnight.

The peak window (4-7pm): Turn the heat pump off entirely during this period if you can. Electricity is roughly three times the off-peak price. If you’ve heated the house properly during the afternoon window, most homes will coast through these three hours without dropping below a comfortable temperature.

How thermal mass helps

Thermal mass is your best friend on Cosy. Heavy building materials like brick, concrete and stone absorb heat slowly and release it slowly. If your home has solid walls, a concrete ground floor or stone construction, it holds temperature well between cheap windows.

Lighter constructions, particularly timber-frame homes, lose heat faster. You might notice a bigger temperature swing between windows. It doesn’t mean Cosy won’t work, just that your savings may be slightly lower because you’ll use more electricity at standard rates to maintain comfort.

Underfloor heating pairs especially well with heat pumps on Cosy. The large mass of screed stores heat effectively, keeping rooms warm long after the heat pump switches off. Radiators lose their heat more quickly, though oversized radiators (common in heat pump installations) help.

Running the numbers

Take a typical 3-bed semi-detached house with an 8kW air source heat pump. Annual heating and hot water consumption is roughly 4,000 to 6,000 kWh of electricity.

With disciplined scheduling on Cosy, a realistic consumption split looks like:

50% at off-peak (~14p/kWh)
40% at standard (~29p/kWh)
10% at peak (~43p/kWh)

You can check current Cosy rates for your area on the tariff comparison tool. Using 5,000 kWh as a middle estimate:

2,500 kWh at 14p = £350
2,000 kWh at 29p = £580
500 kWh at 43p = £215
Total: roughly £1,145 per year

That gives an average effective rate of about 23p per kWh. Shift more into off-peak and less into peak, and you’ll bring that down. Some well-optimised households report effective rates closer to 19-20p per kWh. Rates also vary by region, so yours may be higher or lower.

How does that compare to gas?

The same house heated by a gas boiler would use roughly 12,000 to 15,000 kWh of gas per year. At the current standard gas rate of around 6p per kWh, that’s £720 to £900 per year for gas alone, plus you’d still have electricity costs for the boiler pump and controls.

On Cosy, a heat pump comes in at a similar annual cost. The real savings come from eliminating the gas standing charge if you go fully electric, and from the long-term direction of travel. The upfront installation cost is significant, though the BUS grant knocks £7,500 off that. Over time, as electricity continues to green and the government shifts environmental levies from electricity to gas (which has been discussed), the gap should widen in the heat pump’s favour.

Hot water scheduling

Don’t forget the hot water cylinder. If your heat pump heats a cylinder (most do), schedule it during off-peak windows too. A full cylinder at 50-55°C will last most households through the day. Topping it up during the afternoon off-peak window handles evening showers.

Legionella protection (heating the cylinder to 60°C periodically) can also be timed for off-peak. Most heat pump controllers let you set this as a weekly schedule.

Weather compensation and flow temperatures

Getting your flow temperature right has a bigger impact on efficiency than most people realise. A heat pump is most efficient at low flow temperatures. If your system is set to deliver 35°C water to oversized radiators or underfloor heating, the COP will be significantly better than if it’s pushing out 55°C.

Weather compensation adjusts the flow temperature automatically based on outside air temperature. On mild days, the system delivers lower temperature water (more efficient). On cold days, it increases the flow temperature to meet demand. This is standard on most modern heat pump controllers and should be enabled.

Getting these settings right often makes more difference to your bill than optimising your Cosy schedule. If in doubt, ask your installer to review the heating curve settings.