Morning Report: why are NZ homes so damn hot right now? Jason Quinn puts numbers on AC emissions

25 January 2024 by Jason Quinn

Morning Report rang me this week for comment about overheating homes, in the face of record temperature spikes. You can listen to the short segment here, or there’s a print version also.

For the physics geeks, here is how I worked out the numbers I provided about emissions. I used an average grid emissions factor of 0.12 kgCO2e per kWh of electricity. A typical hot water cylinder with local pipe install uses 80W every hour, on standby to provide hot water. That amounts to 84 kgCO2e/year.

A new heat pump with R32 accounts for 30 kgCO2e/year in emissions for the refrigerant leakage. I’ve used ASHRAE leakage rates for domestic heat pumps of 2% annually.

The average measured New Zealand home uses 6900 kWh of electricity annually. Roughly a third of that is heating, about 2300 kWh. If that is resistance electrical heating (100% efficient) that emits 280 kgCO2e/year. Switching to a heat pump with a COP of 3 (300% efficient) sees emissions drop to 93 kgCO2e/year.

Even if you assumed cooling used the same amount of energy as heating, less energy would be used by a heat pump to heat AND cool compared to heating with resistance electricity alone. But the worst performing house I’ve seen used less than a third of the energy for cooling than it did for heating.

Of course, I concur with the comments Nigel Isaac made about the need for better design. “We fail to recognise that limitless views come with limitless problems,” is a snappy line I’ll probably borrow.

Leave a Reply

Your email address will not be published. Required fields are marked *