Always use an air conditioner that’s correctly sized for your home. An oversize unit may cycle on and off too quickly, making it ineffective at removing humidity and maintaining uniform temperatures. On the other hand, a too-small unit will need to run constantly, which will drastically shorten its lifespan. Determine your needed cooling capacity by doing a load calculation, taking into account each room’s square footage, layout, insulation, function, and typical occupancy. To learn more, download this worksheet from the Energy Star website, or consult an air-conditioning contractor.