Better designs that take into account environmental impact and technological advances. With higher SEER and SEER2 efficiency standards and modern, efficient compressors, the new air conditioning units are much more efficient than you're probably used to. However, BTUs refer to actual heat transfer and not to input energy, so the numbers remain constant. As others have said, if an older 18,000 BTU drive is right for your space, don't downsize it to a 12,000 BTU unit (you could try a 15,000 or 16,000 BTU drive, but you'll have to work hard) more).
Instead, a modern 18,000 BTU unit will provide the same amount of cooling as your old unit*, but will consume less electricity to do so. The current ones, in addition to the way in which functions and types of air conditioning can improve (or reduce) the actual efficiency of a given air conditioning unit. If you have a 25-year-old air conditioner in your Northridge, California home, its continued functionality is a testament to proper size and installation, diligent maintenance, and of timely air conditioner repairs.
