Choosing the right HVAC unit for your home is all about striking the perfect balance between what you need and what a system can do. If you buy one that's too small, it'll have to work too hard to cool your home - thus costing you money you don't need to spend in excess utility bills. Choose one that's too big and you're also spending more money than you really should be. Luckily, the process of sizing the right unit to meet the requirements of your home is a lot more straightforward than you might expect.
Note that as a general rule of thumb, an HVAC unit will require about 20 BTUs for every square foot of space in your home you want to heat and cool. Go through your house with a tape measure and a calculator and find out the length and the width of each room and multiply those numbers together to find the square footage. In this case, a 10 foot by 10 foot room would translate to 100 square feet.
Next, add up the square footages of all rooms together and multiply that number by 20. This will give you the approximate size of the system you need to buy to meet your needs all year round. If your home gets a lot of shade during the summer, you'll likely want to reduce the overall capacity by about 10% or so. Likewise, if your home gets a lot of direct sunlight, you would want to increase by about 10% just to be safe.
Note that if you really want to be 100% sure you're getting the right sized system for your home, nothing will match having a contractor come out to perform a precise load calculation. In addition to total square footage, this will take into consideration things like building construction, the orientation of the sun and more - all things that you won't be able to truly nail down yourself.
If you are considering getting a new HVAC unit, it is recommended that you hire a professional. Coleman-Mitchell Heating has been serving Pennsylvania since 1969. That is experience that you can count on!