Why Heat Pump Efficiency Matters: The Hidden Cost of Oversizing

When you invest in a heat pump, one of the key figures that determines how much you’ll spend on energy is the Coefficient of Performance (COP). This number represents how much heat energy your pump delivers compared to the electricity it consumes. Put simply, the higher the COP, the lower your running costs.

A well-sized and well-installed heat pump should consistently achieve a COP between 3.5 and 4, meaning for every unit of electricity, you’re getting 3.5 to 4 units of heat. That’s an efficiency of 350–400%. In fact, on some of our projects, we’ve seen systems performing at COP levels even higher than 4, delivering impressive energy savings.

Unfortunately, not all systems reach their potential. Some heat pumps run at a COP as low as 2 or 2.5 (200–250% efficiency), and the main culprits are oversizing and poor control setup.

Here’s why oversizing is such a problem: every time a heat pump starts up, it goes through a brief phase where it consumes electricity but doesn’t yet supply useful heat. During this period, its COP is actually negative. If the system is oversized, it will cycle on and off frequently—sometimes every 10–15 minutes. That means the pump is spending a lot of its time in this inefficient startup mode, dragging down overall performance.

The consequences don’t stop at higher bills. Frequent cycling puts extra strain on components, increasing wear and tear. Instead of lasting 15–20 years, an oversized heat pump might see its life expectancy cut dramatically—sometimes to as little as 5 years. That’s not just inefficient; it’s expensive and unsustainable.

So what’s the solution? Proper design and sizing. Matching the capacity of the heat pump to the actual heat demand of the building is critical. Controls must also be set up to allow the system to modulate smoothly, avoiding unnecessary stop-start cycles. When this is done correctly, heat pumps deliver the kind of efficiency that makes them one of the most powerful tools in cutting carbon emissions and reducing energy costs.

If you’re considering a heat pump—or already have one that seems to be underperforming—make sure sizing and controls are reviewed carefully. With the right setup, your heat pump should be a long-lasting, efficient investment in your comfort and sustainability.