How Much Air Conditioning Do You Actually Need? The Ultimate Guide to Aircon BTUs and Heat Gain in Your Home

Your body is a furnace right now.

who’s a lil lightbulb

In this moment, 30 trillion of your cells are metabolising, transforming nutrients into energy. This happens even when you sleep, and keeps us alive.

As your cells metabolise, they release heat!

more activity + bigger body = more metabolism = more heat

Fun Biology Fact But a Bit Too Much Information

This heat is an inevitable byproduct of the chemical breakdown of ATP (Adenosine Triphosphate), a compound that is your body’s ‘currency of energy’. It’s needed to drive your muscle contractions, move your cells, circulate blood and even get your nerve impulses going!

In the world of aircon, one of the popular ways to measure this heat is with the BTU, The British Thermal Unit.

yum, energy

An indoor aircon unit with 18000 BTU capacity is able to remove 18000 BTU of heat (and humidity).

How much heat needs removing from your home?

Let’s start with the internal heat sources. These typically mean energy-consuming things, like appliances, lights and people.

1: Identify the heat sources inside your home

The first major heat source in your home is you!

That’s right – when you sleep, your body emanates about 18 satays of heat energy!

And as you read this, little oranges of heat are radiating from your body. (The healthy choice)

This heat leaves your body by convection and conduction from your skin to the air, which is only possible when the air is colder than your skin. This is usually the case!

Here’s a screenshot of my skin temperature ranging between 33.4 and 33.8°C , taken with this sensor I have!

My core temp at the time was 38, which was more than my battery level!

During a heat wave when the surrounding air is hotter than your skin, the body actually gains heat from conduction and convection – here it can only cool itself down by the evaporation of sweat. Because humidity limits this evaporation, heat loss becomes super difficult! This is why hot, humid climates (like Singapore) are considered much less comfortable than hot climates which are dry. (Like UAE, or certain parts of Australia!)

Other than humans, electrical and cooking equipment are a source of heat gain too. Studies from ASHRAE have been done to determine the recommended heat gain for a variety of these!

The list is long.

But it’s much simpler to assume that 100% of your appliances eventually convert electricity into heat gain.

Fun Facts about Entropy – Why All Your Electricity Becomes Heat

You might think that the light energy, the sound, and the processing power that appliances produce refutes this assumption. But light bounces around your walls until its radiation is absorbed by all surfaces in a room (some escapes out the window). Sound reverberates through the air and surfaces of your house, agitating the molecules which exert friction on each other as heat! In computers, the computation performed in CPUs are simply resistance to the circuit. They perform no energy storage and do no work – simply producing heat as electricity cascades through the billions of transistors to perform instructions.

For the laptop I’m typing this post on, that means 65W!

That means my laptop is emanating about 221.8 BTU/hour, or two plain pratas of heat during the day. Yum.

What about your entire home?

Looking at EMA’s stats on Average Monthly Household Electricity Consumption, the overall average Singapore home consumes about 470kWh of electricity a month!

That comes down to a heat generation about 470000 ÷ 30 ÷ 24 = 652.7 Watts.

Which is 2227 BTU/hour!

Plan ahead!

Because the average kWh consumption of 470kWh is quite high for smaller flats, I stratified the data like this!

Almost there!

Equally important are the sources of heat outside your walls.

2: Identify the heat sources outside your home

Even a closed, empty home heats up! Before cooling you and your family, heat enters from the hot, humid outdoor air and solar radiation outside your walls.

This happens even if you had no windows!

The amount of heat is proportional to your flat’s outer surface area, which will increase depending on your floor area:

This heat travels by conduction through your window glass and the solid concrete of your walls – when one side has a higher temperature, heat will inevitably transfer to the other side!

This heat (Q) is proportional to the wall material, the surface area of the walls (A) , their wall thickness (L) and the temperature difference (T1-T2) between the inside and outside!

A simple way to calculate the total heat gain through your walls and windows is to use the BCA’s Residence Envelope Transmittance Value (RETV).

(Sidenote: RETVs can be reduced with better insulation and materials! This can be a point of pride for residential buildings. To be eligible for a Green Mark Platinum, you need an RETV of 20 W/sqm or lower.)

Let’s take the maximum RETV – 25 W/sqm.

This leads us to the final table of internal and external loads!

I’m assuming that the floor area of a condo is about the same as a 4-roomer, gasp! As we all know, they’re going down in size.

The story doesnt end here!

The heat your house expends today may change over time, which would be wise to consider.

3. Changes over Time

Consider the changes to the number (and size) of people in your home over time.

If you want to minimize the number of times you upgrade/change air conditioning units, it’s a good idea to size for the future. Most air conditioning units last 15-20 years!

Within this time horizon, how will the loads in your house change? Should you size for the largest heat load in the next 20 years? Or the load your aircon serves 80-90% of the time? It might be tempting to just buy the biggest air conditioner you can, but over-sizing your unit can be tremendously wasteful depending on the part-load efficiency.

We’ll cover that… next semester!

I hope this post helped you understand what BTUs are and how the inside and outside of your house matters when you are looking for the right size of air conditioning!


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