Some people opt to use something like a Bed Buddy heat pack instead of a plug-in heated pet pad. I'm not sure why, because it generally seems like it is not worthwhile.
The Bed Buddy heat pack is warmed in the microwave for a few minutes, then used. Unfortunately, the various models hold their heat only for a short time. Some claim to keep the bed warm for an hour, but many report that it really only heats for about 20 minutes before needing to be re-heated.
I don't know about you, but I don't think my cat is going to like a bed that starts out super-warm and then loses its heat that fast. And I certainly am not going to keep running back and forth to the kitchen to re-heat a pad for my cat all day long.
Not only does it not retain its heat, but the power consumption is much higher. Running the microwave for two minutes uses more power than running an electric cat heating pad all day long.
The heated pet beds that are available today that you plug in take about the same amount of power to run as two christmas light bulbs.
With that in mind, I can see no reason to ever use a cat heating pad that needs to be re-heated, rather than one that simply plugs in and keeps kitty's bed warm all day and all night.
Christy - Information about a Cat Heating Pad from http://catheatingpad.net