SEER is the acronym for “Seasonal Energy Efficiency Ratio”. It is a maximum rating and not a constant value which can make it hard to really understand in every situation. Higher numbers represent higher efficiency. Currently, you can find AC units with SEER ratings as high as 25.
Basically, SEER is the average expected energy use of your home’s AC unit if you keep a constant inside temperature while dealing with the outside fluctuations in temperature during the season (so 15 – 35 degrees C in Alberta).
You can think of the SEER value the same way as the milage (L/100km) your vehicle gets. Under optimal conditions, without driving aggressively (cooling too much) you should average out to the manufacturers rating.