Heat pump efficiency drops in cold weather primarily because air-source heat pumps (the most common type for homes) extract heat from outdoor air, and colder air contains less available heat energy.

This makes the system work harder thermodynamically, reducing the Coefficient of Performance (COP) — the ratio of useful heat output to electrical energy input.

The Core Physics

A heat pump works like a refrigerator in reverse:

1. Refrigerant cycle: The outdoor unit’s coils absorb heat from the air, causing the refrigerant to evaporate. The compressor then raises its temperature and pressure so the indoor unit can release that heat.
2. As outdoor temperature falls:
   • Less heat is available in the air → the refrigerant evaporates at a lower temperature and pressure.
   • The compressor must work much harder (higher compression ratio) to raise the refrigerant to a temperature high enough to heat the home (typically needing to reach 35–55°C / 95–130°F for radiators or air handlers).
   • This increases electrical consumption for the same heat output.
3. Defrost cycles: In cold, humid conditions, frost builds up on the outdoor coils. The system periodically reverses to defrost, using energy and temporarily stopping heat delivery.

Result:

COP falls as the temperature difference (lift) between the cold source (outdoor air) and the warm sink (indoor heating) grows.

Typical Performance Numbers

• Mild conditions (e.g., 7–10°C / 45–50°F): COP often 3–4+ (3–4+ units of heat per unit of electricity).
• Around freezing (0°C / 32°F): COP typically drops to 2–3.
• Colder (–10°C / 14°F or below): COP can fall toward 1.5–2.5 for standard units. Some cold-climate models maintain better performance.

At very low temperatures, many systems engage auxiliary electric resistance heat (strip heaters), which has a COP of ~1.0 — essentially the same as a basic electric heater and far less efficient than the heat pump mode. Performance curves show capacity (heating output) also declines with temperature, so the pump may not keep up with home heat loss in extreme cold without backup.

Factors That Influence the Drop

• Model type — Standard units drop off more sharply. Cold-climate heat pumps (with variable-speed compressors, enhanced refrigerants, or better heat exchangers) maintain higher COP and capacity down to –15°C to –25°C (5°F to –13°F) or lower.
• System sizing and design — Oversized or poorly matched systems, low indoor airflow, or high-temperature distribution (e.g., radiators vs. underfloor) worsen efficiency.
• Installation and maintenance — Dirty coils, poor insulation, or refrigerant issues amplify losses.
• Humidity and wind — Affect frost and heat transfer.

Real-World Implications

In regions with harsh winters, hybrid systems (heat pump + gas furnace) or dual-fuel setups switch to backup below a certain temperature (balance point). Modern cold-climate models and better home insulation help mitigate this.

Efficiency doesn’t collapse to zero — many units remain more efficient than resistive electric heating even at sub-zero temperatures — but the drop is significant enough that electricity demand rises sharply on the coldest days. This is a key reason for grid concerns with widespread adoption.

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Heat pump households face winter rationing threat

This refers to a recent UK news story (July 19, 2026) about proposed government measures for managing electricity demand from heat pumps during winter peaks.

What the Proposals Involve

Under the Smart Secure Electricity Systems scheme (now under consultation), electricity suppliers (e.g., Octopus Energy, British Gas) could remotely reduce power to heat pumps in participating households during periods of grid strain—typically cold winter snaps when heating demand spikes.

• This is described as “load control” to help balance the grid and reduce blackout risks.
• It would apply to homes that opt in voluntarily, in exchange for discounted tariffs.
• It also covers EV chargers and home batteries.
• Timeline: Suppliers could gain these capabilities by the end of 2027.

The Department for Energy Security and Net Zero (DESNZ) frames it as a practical tool for grid stability amid the push for electrification and renewables.

Context and Concerns

The UK has around 223,000–250,000 heat pump households, encouraged through subsidies as part of net zero goals (phasing out gas boilers). Heat pumps are efficient but draw significant electricity, especially in cold weather when efficiency can drop and resistive backup heaters may engage.

Critics call it “warmth rationing,” arguing it shows risks of rapid electrification without sufficient grid upgrades or reliable baseload power.

Concerns include:

• Reduced comfort or health risks for vulnerable people during cold snaps.
• Over-reliance on “smart” controls shifting burden to consumers.
• Broader fears of blackouts as more homes and vehicles electrify while intermittent renewables grow.

Supporters view demand-side flexibility (including smart controls) as essential for integrating more heat pumps and EVs without massive (and expensive) grid reinforcements. Similar load management exists for other appliances in various countries.

Broader Energy Picture

• Heat pumps can strain grids in extreme cold if widely adopted without efficiency improvements or backups.
• Studies (e.g., in Texas) suggest high-efficiency models can mitigate peaks better than low-efficiency ones.
• The UK is expanding renewables and interconnectors, but winter peaks remain a challenge with declining dispatchable generation.

This is a consultation proposal, not yet finalized policy—participation appears voluntary with incentives.

It highlights tensions in net zero transitions: balancing emissions goals, energy security, affordability, and reliability.

For the full article, check sources like The Telegraph.

Public reaction mixes skepticism about implementation with debates on energy policy.