Cat v panel heater: Which is better?

How many cats do you need to heat an energy-efficient home?

It’s the question on everybody's lips. Well, maybe not everyone's lips, and it's possible this is a niche topic, but it is relevant.

A 3kg cat has a heat output of 14.8 watts, or 129.65 kilowatt-hours - the metric commonly used by power companies to show you how much energy you're using.

These numbers are important when it comes to the design and building of energy-efficient homes, as the heat output of random things like cats can lead to overheating.

For example, to be certified Passive House, a building must have an annual heating demand of less than 15kWh per square metre to maintain a comfortable temperature.

For a typical Kiwi house of 150sqm, the annual energy demand would be 2250kWh. The number of cats required, therefore, would be 17.35, but let's avoid chopping cats and round up to 18 whole cats.

This equates to 1 cat per 8.33sqm.

Easy as. Let’s bulk order a cat litter.

Cats cost about $250 per year to ‘operate’ which equates to a total running cost of $4,300 per year*. This is a bit more than $475 for gas or $675 for electric, although the installation cost is less for cats.

* The calculations have used an average heat load. In reality, the heat load will vary considerably throughout the year. You will need significantly more cats in colder weather and less during mild weather.

In hot weather, you can, of course, let the cats out, but this represents a significant waste of resources if you continue to feed them. And as cats cannot be used for cooling, you would also have to introduce an alternative cooling system such as lizards or other cold-blooded animals.

- James Powers is director of Oculus Architectural Engineering