Heat Pumps and the GB Grid: When to Run Yours for Lowest Carbon and Cost
A heat pump is one of the most efficient ways to heat a home. An air source heat pump typically delivers 3–4 units of heat for every 1 unit of electricity consumed — a coefficient of performance (COP) of 3–4. But that electricity still has a carbon cost that varies dramatically depending on when it's drawn from the grid.
Run your heat pump at the wrong time and you're heating your home with high-carbon electricity. Run it at the right time and you're heating with some of the lowest-carbon electricity available in Europe.
Heat Pumps Vs. Gas Boilers: The Carbon Picture
The standard comparison assumes average grid carbon intensity. For 2025 GB average (~200 gCO2/kWh) and a heat pump with a COP of 3.5:
- Heat pump (average grid): 200 ÷ 3.5 = ~57 gCO2 per kWh of heat delivered
- Gas boiler (90% efficient): ~205 gCO2 per kWh of heat delivered (from direct combustion)
Heat pumps win on average. But the more interesting question is what happens when you time your heat pump use:
| Grid carbon intensity | Heat pump COP 3.5 | gCO2 per kWh of heat |
|---|---|---|
| 50 gCO2/kWh (sunny/windy) | 3.5 | 14 gCO2/kWh |
| 150 gCO2/kWh (typical midday) | 3.5 | 43 gCO2/kWh |
| 300 gCO2/kWh (winter evening peak) | 3.5 | 86 gCO2/kWh |
At a winter evening peak, your heat pump is producing heat with more carbon than a modern gas boiler. At a midday solar window, it's producing heat with one-seventh the carbon of gas.
Timing matters enormously.
How Heat Pumps Differ From Other Appliances
Heat pumps are not like dishwashers or washing machines. You can't simply delay them by four hours — your home needs to stay warm. The key insight is that buildings have thermal inertia:
- A well-insulated home can be heated to 21°C and then coast down to 18°C over 3–6 hours before anyone notices discomfort
- You can "pre-heat" the home during a low-carbon window and reduce or switch off the heat pump during the peak window
- Hot water tanks have even more thermal inertia — a well-insulated cylinder heated at noon will stay warm until evening
This thermal inertia is what makes heat pump scheduling possible and valuable.
The Setback Strategy: Pre-Heat and Coast
The most practical strategy for reducing heat pump carbon and cost is called pre-heat and setback:
- Identify the low-carbon window using the 48-hour forecast — typically 10am–2pm on sunny days, or overnight on windy nights
- Increase the target temperature by 1–2°C during the low-carbon window (e.g., heat to 21–22°C)
- Reduce or turn off during the high-carbon window (typically 5–8pm) — the home coasts from stored heat
- Resume normal operation after the evening peak
In a well-insulated modern home, this strategy can shift 30–50% of daily heat pump electricity consumption away from peak periods.
For hot water cylinders specifically:
- Set the cylinder to heat during the low-carbon window (10am–2pm or overnight)
- Disable or reduce the temperature during peak hours
- A 200-litre cylinder heated to 55°C stores roughly 7–8 kWh of useful heat energy — enough for 1–2 days of domestic hot water
Using the 48-Hour Forecast
The 48-hour carbon intensity forecast is the key tool for heat pump scheduling:
- Evening before: Check whether tomorrow morning has high or low carbon intensity. On a windy forecast, you can pre-heat early; on a still, overcast forecast, midday may be the lowest window
- Same day: Adjust setpoints based on live intensity — if carbon is running lower than forecast, keep the pump running; if higher, switch earlier to setback mode
- Seasonal awareness: In summer, morning windows are usually cleaner than winter; solar peaks are stronger; less heating is needed overall
Tariffs for Heat Pump Owners
Several electricity tariffs are specifically designed for heat pump households:
Octopus Cosy: A time-of-use tariff with very cheap "cosy hours" (4 periods per day, typically including 9am–11am, 2pm–4pm) designed to align with heat pump use. Rates during cosy periods can be 10–15p/kWh vs 25–30p/kWh at peak.
Economy 7 / Economy 10: Older overnight cheap-rate tariffs. Less optimal than Agile but simpler to manage — overnight charging of a buffer tank is a reasonable strategy.
Octopus Flux: Designed for solar + battery households, but also beneficial for heat pumps with solar, combining cheap import rates with export earnings.
For a heat pump running 3,000–5,000 kWh/year for heating, the difference between peak-rate and off-peak-rate charging can be £500–1,200/year.
Heat Pump Performance in Cold Weather
Heat pump COP falls as outdoor temperatures drop — this is a physical reality of the refrigeration cycle:
| Outdoor temperature | Typical COP (air source) |
|---|---|
| 15°C | 4.5–5.0 |
| 7°C | 3.5–4.0 |
| 0°C | 2.5–3.5 |
| -5°C | 2.0–2.5 |
Cold weather also tends to mean higher grid carbon intensity (high demand, low solar, often low wind during anticyclone cold snaps). This is the double-whammy: lowest COP coincides with highest grid carbon.
During severe cold spells, the carbon advantage of heat pumps over gas boilers temporarily narrows or reverses. This is an argument for:
- Good home insulation to reduce heat demand during cold periods
- A backup immersion heater only used sparingly during cold spells
- Accepting higher carbon during cold spells as unavoidable and focusing optimisation on milder weather when it's most impactful
Controls and Smart Scheduling
Modern heat pump controllers (Daikin Madoka, Mitsubishi MELCloud, Vaillant myVAILLANT, Nibe Uplink) all offer some degree of remote and scheduled control. Key features to use:
- Holiday mode / setback: Drop to a low temperature during away periods
- Time schedule: Programme daily heating profiles with setback during peak hours
- Weather compensation: Automatically adjusts flow temperature based on outdoor temperature — improves COP significantly over fixed flow temperatures
- Hot water scheduling: Set dedicated hot water periods aligned with low-carbon windows
Some systems can integrate with smart home platforms (Home Assistant, etc.) to automate scheduling based on live carbon intensity data from the Carbon Intensity API.
Summary
- Heat pumps are low-carbon on average, but high-carbon at peak winter evenings — timing matters
- Pre-heat during low-carbon windows (10am–2pm or overnight) and setback during 5–8pm peak
- Hot water cylinders offer the most thermal storage flexibility — time them to charge at midday or overnight
- Octopus Cosy tariff is purpose-designed for heat pump households
- Cold weather reduces both COP and grid carbon advantage — insulation and overnight scheduling help
- The 48-hour forecast shows the optimal heating windows for the next two days