Ideally, countertop microwaves should have 1000 or more watts of power for the most efficient cooking. As of 2015, most microwaves cook with between 600 and 1200 watts of power.
Microwaves at any wattage cook food; however, food cooks more quickly at higher wattages. Microwaves with under 700 watts of power may cook food too slowly and unevenly. Recipes that use microwaves for cooking generally assume that a microwave has at least 800 watts of power. While a typical family-size microwave had 700 watts of power in 1993, many family-size microwaves in 2015 have over 1000 watts of power.
As of February 2015, the Kenmore 73092, with 900 watts of power, is considered the "Best Bang for your Buck" by Best Reviews. The site also considers the Panasonic NN-H765BF Genius, with 1250 watts, to be the "Best of the Best."