– What Size Air Conditioner do I Need?

Are you considering buying a new air conditioner or upgrading an existing one for your home? While looking for residential air conditioners, you may have become familiar with the different types on offer, such as single split air conditioners, multi-split systems, ducted type and the various functions that they offer. You also might have encountered the term BTU but are not quite sure what it means or how to interpret it as part of your decision making. In this article we aim to explain BTUs and how to estimate what cooling capacity the correct sized air conditioner should have for your home.

Measuring an AC’s cooling capacity: BTU

When trying to understand an air conditioner’s power, it is common to find the acronym BTU. This stands for British Thermal Unit, an energy measurement unit; technically, 1 BTU is equivalent to the energy needed to heat one pound of water by 1ºF. In air conditioning, it is the unit used to indicate the quantity of energy used to remove heat from an area in an hour. To better understand BTUs, we can compare them to calories. Both are units that measure energy, and just like our bodies, air conditioners need the right amount energy to do daily tasks optimally. Too little and performance could be sluggish, too much and it could overload the system and space. As mentioned, air conditioners move energy from one place to another, and BTUs tell us the power of an AC. For example, a 12000 BTU unit can remove 12000 BTUs from a room in an hour. Simple.

So, the higher the BTU figure is, the more cooling capacity and more powerful an air conditioner is. This unit is also used for heat pumps, chillers and other heating and cooling appliances. One point to notice is that for air conditioners BTUs refers to the output, the cooling or heating your unit provides your home, as opposed to input capacity which measures how much energy is required to run an appliance, like a furnace, for example.

Let’s Calculate how many BTUs I need

At this point, you may be wondering what cooling capacity your air conditioner should have. The answer depends on the size of your home or room where the indoor unit of the air conditioner system will be installed. However, as an estimate based on size alone, ACs need 20 BTUs per one square foot and an average ceiling height (according to the recommendation of the US Department of Energy).

This is expressed with the following formula to calculate the required BTUs:

20 BTU x 1 sq/ft

That’s to say, if you want to cool a 600 sq/ft space with 8 ft ceiling, you will need a 12000 BTU air conditioner. This is an estimate for a room that is exposed to average climate conditions.

As another example, to cool a two-room apartment (living room and bedroom) of 800 sq/ft, you would need a 16000 BTU air conditioner. For a large family house of 12000 sq/ft, a 24000 BTU system will be needed. Calculating the capacity your home needs is easy once you understand the formula above.

Is a higher number of BTUs always better?

It may be logical to think that the higher the BTU capacity an air conditioner has, the better it is, no matter what our home size. This is a common misconception, just as a lower BTU than recommended won’t be enough to cool our home, a higher BTU also has its negative consequences. Just think of the calorie comparison we made above.

When an air conditioner’s BTU is too low: the air conditioner will run at its maximum power continuously and still will not reach the desired comfort level. Basically, there will be too much heat for the AC unit to handle. This will not only increase energy consumption, but also lower the air conditioner’s lifespan.

When an air conditioner’s BTU is too high: as it turns out, a higher BTU number is not always the best. When the air conditioner is too powerful, it will quickly cool the room off and turn off the cycle. Then it will cycle on again to keep the temperature. This continuous on and off cycling in such short periods of time makes the AC unit work harder than needed. It all goes without saying, a higher capacity unit will have a greater power consumption, wasting precious energy that could be spent elsewhere, and again, impacting its lifespan

How are BTUs related to energy efficiency?

When comparing two units with the same cooling capacity, one of the features that can influence our decision is their energy consumption rating.

There are different indicators for energy efficiency, some of the most common ones are EER (Energy Efficiency Ratio), SEER (Seasonal Energy Efficiency Ratio), COP (Coefficient of Performance) and color coding. An AC with higher energy efficiency rating consumes less energy than a counterpart with the same BTU and lower energy efficiency rating.

An air conditioner’s EER rating is the BTU capacity over its wattage. For example, if a 10,000 BTU AC unit consumes 1000 watts, its EER rating is 10 (10000 BTU/1000 watts). Appliances with a higher EER are usually more expensive, but they consume less. It is important to understand this and calculate how the upfront cost of the unit could translate into energy bill savings, and how long it would take for these savings to make up for the higher price point. Learn more about this in our article ‘Do energy-efficient appliances really save you money?’.

Other factors to take into consideration

As it is mentioned above, even though required BTU can be roughly estimated with a home’s surface in square feet and ceiling height values, there are other factors that need to be weighed in. These are some of them:

• Number and size of windows and sun exposure. Sunlight is usually considered a good thing, but windows can let a great deal of heat in through sunlight. If your home has many or very large windows, you will get greater sun exposure. This can be decisive in warmer climates, where your AC will need more power to cool your home off.

• The direction your home faces. Which way your home faces can affect the way it accumulates heat throughout the day. Generally, an East facing house gets the most sun exposure at the coolest time of the day, in the morning. Meanwhile, a West facing house gets sun exposure at the hottest time, from the afternoon onwards.

• The region’s climate. An AC won’t need to be as powerful in a cooler climate, compared to a warmer climate, where the AC will also need to be used more often for longer periods of time.

• Relative humidity. If you have experienced a hot humid summer, you know that higher humidity makes the air feel hotter and more uncomfortable. This is because hot air cannot dissipate in those conditions, it gets trapped by humidity. If you live in a high humidity region, look into AC units that feature the Dry Mode function, which helps to lower ambient humidity. 

• Insulation. A building’s insulation is critical for air conditioning and heating effectiveness. A properly insulated house will keep the cool air from escaping and keep the heat outdoors. The opposite happens with poor insulation, warm days can feel warmer and cold days, colder. As a first step, check your doors and windows for any gaps that could let the cool air out.

• House layout and distribution. As a factor that is not directly impacted by BTUs, but rather by the type of air conditioner, the house layout should be taken into consideration. It is very different to cool an open concept studio apartment than a flat with a separate bedroom. Usually, this establishes the type of AC recommended, as single split wall mounted air conditioners are ideal for single rooms, while multi-split units can be used in various rooms throughout the house, and both of these systems come in a range of different BTU cooling capacity.

Cooling power in different countries

BTU is one of the most widely used cooling power units, but, naturally, different countries use different units.

Along with BTU, one of the most common cooling power units you can encounter is tons or tonnage. As BTUs are a small unit that is measured in thousands, tons are used a larger unit for measuring an air conditioner’s cooling capacity. One ton of cooling is equivalent to around 12000 BTU. Knowing this, it becomes easy to convert BTU into tons, simply divide the BTU figure into 12000. So, when you’re looking at an air conditioner of 18000 BTU, it means it is a 1.5 ton AC.

In countries where the metric system predominates, such as in Europe and some Asian countries (France, Germany, China, Japan, etc.), the most common units for measuring cooling and heating power are watts and kilowatts, which are officially recognized in the International System of Units (SI). 1 watt is equivalent to 3.41 BTU; so, for example, a 12000 BTU unit would have 3,517 watts of cooling power. An estimate used by installers is that, for cooling, 100W are needed per square meter.

In Spanish-speaking countries, the term frigoría or frigorie is the most utilized. Despite its wide use in these countries, this unit is not recognized formally in the International System of Units. A frigorie is defined as the amount of energy needed to cool 1 gram of water by 1ºC. A frigorie is equivalent to 4 BTU, so a 12000 BTU air conditioning unit has 3000 frigorias.

Finding the ideal Hitachi AC for your home

We have seen that the perfect air conditioner cooling power for your home depends on various factors. Whatever the type you are looking for to suit your home, you can find the ideal solution in the range of Hitachi Residential Air Conditioners. From studio apartments to large family homes and beyond, there is a solution to match all your needs and that will keep your home comfortable.