For example, a 30,000 BTU, 2.5-ton air conditioner is designed to effectively cool a house between 1200 and 1500 square feet. A slightly larger 3,600, 3-ton unit is designed for homes between 1,500 and 1,800 square feet. The answer is a 5-ton air conditioner. Most homes of approximately 2500 to 3000 square feet require a 5-ton air conditioner. The two tables above verify that answer.
Next, if you need to calculate the power you need in a gas oven, simply multiply its efficiency by the input power indicated for the heat output in BTU. Do you see a pattern here? In the example above, if a furnace with 80% efficiency is used, the 2,000-square-foot house above would need a 100,000 BTU intake furnace that would produce the necessary heat output of 80,000 BTU. Does your house have two floors? If the answer to that question is yes, it will place a lower burden on the ground floor system, since the second floor acts as additional insulation. If your home isn't well insulated or if you have old-style windows and an obscenely high number of windows, you'll want to select a larger system that falls within the specified square footage range.
The less insulation there is and the more windows in the environment, the more likely you are to experience greater air and heat loss.