The Air Con Efficiency Rating You Should Really Care About

Pop quiz!  What’s more important when it comes to buying an air conditioner?

• (a) Cooling efficiency
• (b) Heating efficiency
• (c) Purple monkey dishwasher

Is Air Con Heating Or Cooling Efficiency More Important?

If you replied “(a) Cooling efficiency” and live in the hotter northern parts of Australia, that’s correct!  But if you, like most Australians, live south of Byron Bay or west of Brisbane, then “(b) Heating efficiency” is the right answer.  And if you chose “(c) Purple monkey dishwasher” then you need to take me more seriously and respect my authority.

In this blog post, I’ll explain:

1. Why heating efficiency is more important than cooling efficiency for most Australians when buying an air conditioner.
2. How to read air conditioner energy efficiency labels.
3. Why air conditioner labels always say cooling efficiency is higher than heating efficiency.
4. Why this doesn’t matter.
5. Why having rooftop solar increases the importance of good heating efficiency.
6. Why having a battery doesn’t change the importance of heating efficiency.

That’s six points to cover.  I promise I won’t add any more.  This is partly because it would make the post too long, but mainly because I’d have to watch some Sesame Street first to learn more letters.

Air Conditioners: The Cheapest Way To Heat Your Home

If you want to heat your home, an air conditioner is the most cost-effective method.  This is because it’s a type of heat pump that moves thermal energy around, which takes less energy than directly creating it.  In summer, it can take heat from air inside your home and move it outside, while in winter, it can move heat from outside air into your home.  While outdoor winter air may feel frosty, air at 0ºC still has 93% as much thermal energy as air at 20ºC, and a heat pump can move a portion of that inside, despite the outside temperature being lower.

But you will need to use a reverse cycle air conditioner, which means it can work backwards.  When you switch one of these from cooling your home to heating it, the refrigerant fluid inside reverses direction, so it does the opposite of cooling.  There are still air conditioners available for sale that don’t have a reverse cycle, but I don’t recommend them.  You might save a few bucks, but it’s only going to be a few.  Very little extra is required to make one reverse cycle.  Even if you live in Townsville and only heat your home a few days a year, the minor extra expense can easily pay for itself over the air-con’s lifetime.  (One day, 75 years ago, it got down to 1°C in Townsville.  If it had gotten any lower, locals may have discovered it’s possible for ice to exist outside of freezer compartments.)

The efficiency of reverse cycle heating depends on the air conditioner and the location, but even easily affordable air conditioners can be three or more times as energy efficient as an electric resistance heater.

A reverse cycle air conditioner that is three times as energy efficient as electric resistance heating will also be around three and a half times as energy efficient as gas heating.  You can save plenty of money by using your air con for heat instead of gas, but how much will depend on what you pay for gas and how much your solar system powers your air conditioner.

Air Con Labels

It’s easy to compare air conditioners because they now come with groovy Energy Rating stickers that give heating and cooling efficiency based on climate location.  Here’s an example, freshly stolen from a government website:

Unfortunately, the government clearly did not spend the $300 it takes to hire a skilled Slovakian graphic designer.  The number of efficiency stars is not obvious.  If you look at the star ratings for the hot climate zone, it looks like there are 3 for cooling, but it actually has 4.5.  It also looks like it has 2 stars for heating, but it really has 3.5.  While saving money is generally a good thing, sometimes it’s worth letting a Slovakian feed his family and/or his Lady Gaga obsession.

I first wrote about these new labels five years ago, and now we’re in the distant future year of 2026, every new air conditioner should finally come with one — but don’t blame me if they don’t.

Air Conditioner Zones

These Zoned Energy Rating Labels, or ZERL labels, show heating and cooling efficiency in three climate zones:

These zones are different from the zones used for solar and for hot water heat pumps.  They also have names instead of numbers:

• Hot Zone: This covers the northern third of Australia, including the coastal strip of South East QLD and a little bit of Northern NSW.  It includes Brisbane — the capital of Queensland, Darwin — the capital of the NT, and the City of Townsville — the capital of sounding stupid when literally translated into another language, because it becomes Big Town Town Small Town.
• Average Zone:  This zone can get hot in summer but winters are cool.  Capitals include Sydney, Perth, and Adelaide.
• Cold Zone:  These are the parts of Australia that get really cold in winter and the whole of Lewd Zealand.  Included capitals are Melbourne, Canberra, Hobart, and Wellington.

Efficiency Stars

The more stars a label has for a climate zone, the more efficient it is at heating or cooling.  They also give estimates of how many kWh the air con will typically consume in each climate zone over a year:

As the example label above shows, in every zone except the hot one, homes use a lot more electricity for heating than cooling.  The actual ratio will depend on the household, but for the above example:

• Hot Zone:  10 times more energy is spent cooling than heating.
• Average Zone:  Twice as much energy is spent heating than cooling.
• Cold Zone:  8 times more energy is spent heating than cooling.

The zones aren’t magical.  Just because you’re in the Average Zone doesn’t mean you will definitely use more energy for heating than cooling.  If you rarely or never heat your home, either because you have effective passive heating or because you’re like Elsa from Frozen and the cold never bothered you anyway, then the figure won’t match your consumption.  It also may be off if you live at the edge of a zone.  But, generally, they’re a decent guide for those whose heating and cooling habits aren’t too far off normal.

More Stars For Cooling Than Heating

Because most Australians live in climate zones where air conditioners use more electricity for heating than cooling, it means heating efficiency is usually more important than cooling efficiency.  But if you look at air conditioner labels, or go to the government energy rating page and click on air conditioners, you’ll see that — except for the worst ones available — they always have more stars for cooling than for heating.

Below, I’ve put the best and the worst energy star ratings available in the three climate zones.  While one of the worst is equally bad at both heating and cooling, the best all have 2.5 to 3.5 more stars for cooling than heating:

If you avoid the least efficient air conditioners  — which is something I so recommend doing — you’ll always end up with more stars for cooling than heating.  But this doesn’t alter the fact that, outside of the Hot Zone, an extra star for heating efficiency will save more money than an extra star for cooling efficiency for typical households.

It may seem strange that air conditioners have more stars for cooling than heating, since they’re basically doing the same thing, they’re just doing it backwards for heating.  But there are two good reasons why:

1. The tests aren’t equal, so it’s harder to get stars for heating than for cooling.
2. When the outside temperature is below 5.5ºC, ice can form on air conditioner radiator piping and reduce efficiency.

On a hot 34ºC day, you might set your air conditioner to cool your home, or part of it, down to 24ºC.  That’s a 10ºC difference.  But if it’s 10ºC outside and you warm your home up to 24ºC, that’s a 14ºC difference.  With a larger temperature difference, air conditioners have to work harder, and that reduces their efficiency.  Because the efficiency tests assume a larger average temperature difference for heating, air conditioners end up with fewer heating efficiency stars than cooling stars.

Ice forming on air conditioner heat exchange piping also lowers heating efficiency.  This happens because, when heating in reserve cycle mode, heat is moved inside and cold is moved outside, so the outside radiator pipes can be below freezing and ice up when the external air temperature is below 5.5ºC.  This can require an energy consuming deicing cycle.  Air conditioners can vary in how much ice builds up and how efficiently they get rid of it.  While this isn’t an important consideration in the tropics, in the Cold Zone it can significantly affect the number of heating energy stars.

It’s In The Stars

By poking around the government energy rating page, I figured out how much electricity air conditioners with different energy star ratings will consume.  Here’s the difference half a star makes for heating and cooling under the following conditions:

• An air conditioner large enough for a typical living room.
• In the Average Climate Zone (Adelaide, Perth, Sydney).
• With what the testers consider normal use.

The above graph clearly shows…

• With expected use, homes in the Average Climate Zone use far more electricity for heating than cooling.
• An extra half a star doesn’t save a huge amount of electricity – but still helps.

If we do the same thing for the Cold Climate Zone, we get…

This shows that in places like Melbourne, they go easy on cooling and use less than half as much as in the Average Climate Zone.  But they go hard on heating, and use around 75% more.

Savings

For the Average Climate Zone example above, the amount of electricity saved over a year by having an extra half an efficiency star is:

• Average Zone cooling: 17 kWh
• Average Zone heating: 25kWh
• Average Zone total: 42 kWh

How much money this will save you isn’t easy to work out — unless you have a flat tariff without solar.  In that case, it’s really easy.  Even I can do it.  A Sydney home paying a flat tariff of 36c per kWh, without solar, will see the following savings:

• Average Zone cooling costs: $6.12
• Average Zone heating costs: $9
• Total annual savings: $15.12

A saving of around $15 a year doesn’t seem like much — because it isn’t.  But if your air conditioner lasts 15 years, it will come to around $225 and can make paying extra for a slightly higher efficiency rating worthwhile.

For the Cold Climate Zone example, the total number of kWh saved by having an extra half an efficiency star for cooling and heating is:

• Cold Zone cooling: 10 kWh
• Cold Zone heating: 42kWh
• Cold Zone total: 52 kWh

If this home was paying the same 36c per kWh as the Average Climate Zone home, then the annual savings would come to $18.72 a year.  But in Melbourne, where the largest number of Cold Zone Australians live, on a flat rate, electricity is around 23c per kWh.  This makes the savings:

• Cold Zone cooling costs:  $2.30
• Cold Zone heating costs:  $9.66
• Cold Zone total annual savings:  $11.96

While total savings are less in Melbourne, savings from higher heating efficiency are greater.  So higher heating efficiency is still useful, even in the state with the lowest electricity prices.

Savings With Solar

If you have solar and/or a time-of-use tariff, savings become more difficult to determine, but the importance of heating efficiency increases.  The greatest cooling demand occurs on sunny summer days of high solar output.   While there’s variation between households, it’s easy for solar to cut cooling costs by half or more.  But for heating, it’s the other way around, as the greatest heating demand is on cold, cloudy winter days when solar output is low.  This means that, without an intentional effort to shift heating to the middle of the day in a well-insulated home, the reduction in air conditioner heating costs from solar is usually modest.  So if you have solar, it makes even more sense to prioritise heating efficiency over cooling, outside of the Hot Climate Zone.

If you have a battery, you can store solar energy and use it for heating when solar output is minimal.  But unless your solar system is massive, you still run the risk of running low during cold, cloudy periods.  So even with a battery, heating efficiency remains important if you don’t live in the stinking hot zone.

That’s It!

I’ve covered all six points I said I would, and they basically boil down to:

If your location is cooler than Brisbane, air conditioner heating efficiency is normally a more important consideration than cooling efficiency.

Of course, if you don’t use air conditioning to warm your home, even though it’s the lowest cost method — especially if you have a battery — then I can’t help you.

Be sure to catch my next article, which will be on the benefits of powering your dishwasher with solar electricity, and I’ll complete it as soon as I manage to catch that purple monkey.

In the meantime, read our detailed guide to air conditioning for more tips, and our reviews page to browse different air conditioner brands.