When the star Sirius appears in the night sky it signals the start of the "dog days of summer," 40 of the hottest, stickiest days of the year. July 3rd to August 15th is a period also unofficially dubbed "air-conditioner appreciation days" -- when Americans decide "maybe it's time to buy one." Which brings up the question: how powerful an air conditioner does one need to cool off during the dog days? Try this formula. Cooling is measured in BTUs (British thermal units). To determine how many you'll need, multiply room length by width and height to determine the total cubic feet that need to be cooled. If facing north, multiply by 16 to compensate for the heat of the sun, 17 if facing east, 18, if south and 20, if west. Then divide by 4, 5 or 6 depending on how good you think your insulation is -- 4 being poor and 6 being good. The final number will tell you how many BTUs to buy. And that's the On The House tip for today.