Here is what UL's web site
has to say about extension cords:
"Before deciding which extension cord to use, first carefully read the manufacturer's instructions for the power tools you will be using. These booklets contain important information about your tools and will provide instructions on their use. The booklets will also indicate whether the tools are suitable for use outdoors. Likewise, the first step in determining which extension cord you will need is to decide whether you will be using the appliance indoors or outdoors. Extension cords that can be used outdoors will be clearly marked "Suitable for Use with Outdoor Appliances." Never use an indoor extension cord outdoors; it could result in an electric shock or fire hazard.
To determine what size, or gauge, cord is needed, you will also have to determine the cord's length. A cord, based on its gauge, can power an appliance of a certain wattage only at specific distances. As the cord gets longer, the current carrying capacity of the cord gets lower. For example, a 16 gauge extension cord less than 50 feet in length can power a 1625 watt (W) appliance. A 16 gauge cord that is longer than 50 feet in length can only power an appliance up to 1250W."
They also offer this advice:
"Quick tip: if your appliance indicates that it uses 5 amps at 125 volts, then its wattage rating is 625W (5x125)."
Hope this helps.