Two major decisions should guide your purchase of a room air conditioner. Buy a correctly sized unit. Buy an energy-efficient unit. If the room is very sunny, increase capacity by 10%. If the unit is for a kitchen, increase the capacity by 4,000 Btu per hour.
Decorate with pale colors on walls, ceilings and floors. Soft tones reflect more light, so you can use lower wattage bulbs and delay turning on lights until later in the day. Using high-gloss paint can help as well.