One, two or 3 days over - it doesn't matter once you go past your due date, you can't pay you Electric Bill thru the same channels. Normally you can pay your 7-Eleven or Tesco Lotus, but they'll just give you blank look and a "mai Dai" if you bring it in late. Your only option? Trek to the nearest Electricity Authority office and stand in line. This could take you a good hour or more of waiting, if they are open and if you get there in time.
The next option is your motorcycle Taxi stand. They are used to running around doing odd jobs, but if your bill is a few thousand you might not feel so comfortable handing over the money. Best if you have an existing relationship with them.
The Best Way to Pay
Finally the secret option that so far I've not see any other people post. It involves online transfer with a Thai bank. But not any bank. You must use SCB online banking. They have a form that looks like this:
The key here is they manually accept your Customer Number and Bill Reference number. Manually enter that data as well as a name and amount. SCB is not using the same connection as Kbank where they look up the value you owe, present you with it and pay it.
After you're done, the money will leave you account an you'll get a confirmation. You'll want to make a screen capture or print of this page incase and kind of dispute would need to take place.
Let me know in the comments if you find this trick useful!