More Power! How much air-conditioning is enough?

I took a risk buying Fujitsu’s least powerful ducted air-conditioner. One installer flat-out refused to install it, saying it wasn’t powerful enough for our house.

Luckily it’s paid off: We’ve stayed cool during the days since it was installed, all of which have exceeded 40°C.

If you want to buy an AC, and would rather not gamble, read on. Here’s what I’ve learnt about selecting an air conditioner powerful enough to suit my needs and my home:

First: calculate your house’s “heat load”. List the rooms in your house. Include any space you want air conditioned (I included my hallway but not my bathroom, toilet, or laundry). Go to the FairAir.com.au size calculator and use it to work out how many kw (kilowatts) of cooling power each room needs, one room at a time. It will ask you about almost everything that effects the temperature of the room, including room size, window size and orientation, insulation, adjoining rooms, number of people in the room, etc. The results for my 3 bedroom house (100m²) were:

But you’ll need to use the calculator yourself, as these will vary a lot based on what insulation you have, which way your windows face and their size, how much external shade (like trees or nearby houses) you have, etc.

Second: Decide what rooms should be cooled and how often. You may not need to cool the whole house at once. What rooms do you actually use when you’re at home? You’ll probably want the AC most on the hottest days, but how many of these days will there be in a year? For example, can your family just spend most of their time in just the living room on 40°C days? Unless your funds are unlimited, decide what you really need, versus what you want.

So far we’ve used our AC mostly in the bedrooms. As you can see above, fairair.com.au recommends 4.2kw for our bedrooms, so our 8.8kw has been adequate even in extreme 40+°C heat of the last few days. We had some guests over on one of those days, in the heat of the late afternoon, and it did take a long while (45 minutes or so) to cool the living room, playroom and kitchen areas (which need 8.7kw according to Fairair) but cool down it eventually did.

Third: Decide how much you can afford to spend. A larger unit will cost you more to purchase (and possibly in electrical costs, although apparently it can be cheaper to run a suitable unit at a moderate level than to strain an underpowered unit all day long). The next cheapest Fujitsu we looked at had 12.5kw cooling power for only $1000 more, which is a lot of extra cooling bang for your buck. It seemed like a sensible choice when already spending $5800 for the 8.8kw unit. But $5800 was already $1800 more than we’d planned to spend, so we decided to save the money, and are glad we did.

Some final thoughts: the numbers given to us by FairAir ended up being a pretty decent indication of how much power we’d need. If we ever want to cool the whole house down on a very hot day (like the stinkers we’ve had so far), our 8.8kw unit will definitely struggle to cool our 12.4kw house. But even in extreme heat, it cooled an 8.7kw area in less than an hour, and a 4.2kw area in about 15 minutes.