The extension cord is very important when buying an electric lawn mower with a cord. Your quality of cut is directly affected depending on the length and size of the cord. This page is probably the most important one you will run into while looking for a corded electric lawn mower.
This pdf will help with choosing the right cord. On many pages at Amazon I find people buying 16 gauge cords that are 100 ft long. I could not find a single electric lawn mower where this would be adequate. Do your mower and your pocketbook a favor and buy the right cord. You prematurely wear out your brushes and risk burning out the motor when using the wrong size cord. Feel free to download this PDF.
If you would like to see how to use the PDF I walk through a couple examples on my post about buying the best extension cord for an electric lawn mower. This works for all power equipment, not just electric lawn mowers.