If you plan to do only low fire (Cone 1 and below), this usually isn't an issue.  But if you plan to fire at Cone 6, we recommend buying a Cone 8 or 10 kiln.  When offered, heavy duty elements and thermocouples are also highly recommended for Cone 6 and above.  

If you really want to fire Cone 10 in an electric kiln, we recommend a TRUE Cone 10 kiln.  Examples are the Paragon Viking and Dragon Kilns, and the L&L DaVinci kilns.  One good way to compare different kilns is to compare their watts per cubic feet.  If two kilns have the same interior space, the one with higher watts (voltage times current) will get hotter.  3" brick (rather than 2.5") also helps the kiln reach hotter temperatures, and available on most kilns.  

Traditionally the problem has been that a kiln is spec'd at best case.  If the voltage on your line is very good (normal to high) and the elements are brand new, you can reach that temperature.  

These are all reasons why your kiln might not be able to reach the temperature you thought it would.

Not all manufacturers and retailers will tell you this.  But we want you to be happy with your kiln.  When I bought my kiln I was told it would fire to Cone 10 repeatedly with no problems, and I since learned that wasn't true.  I think when you purchase a kiln, you should do so knowing the truth.   

So this is why we HIGHLY recommend that you always buy a kiln capable of firing higher than you intend to fire.

Excellent Cone 10 Kilns:

Paragon Dragon, Viking, and Iguana kilns
Skutt PK Kilns
L&L DaVinci Kilns
Olympic Front Load Production Kilns
Most ConeArt Kilns
Many Olympic H Model Kilns
Many smaller and medium sized kilns with 3" brick
This is not an exhaustive list, but a good place to start

Cone Temperature Chart

Kilns are not fired just to a temperature.  They are fired to a "cone" level, which accounts for time as well as temperature.  Think of it as heat absorption rather than just temperature.

Cone's come in different numbers, each of which corresponds to a heating rate / temperature combination which will make that cone deform.  At the beginning of the firing the cone is standing at an 8 degree angle.  A perfectly fire cone will be best to a 90 degree angle.  If the cone is bent less, the kiln was under fired.  If the cone is bent more, the kiln was over fired.

Notice the way the cones are numbered.  The hottest is 10, going downward to 1.  Then as it continues to getting cooler it continues at 01, 02, etc.  So there is a BIG difference between cone 5 and cone 05!

Automatic electric kilns such as the Skutt Kilnmaster series have a thermocouple which continuously measures the temperature, records it over time, and shuts off when the appropriate heat absorption has been met.  So if the ramping temperature is fast, the kiln will go to a higher temperature before it turns off than if the ramping temperature is slow (thus allowing the clay to absorb more heat along the way.)  The final temperature is most affected by the rate of temperature increase over the last 300 to 400 degrees of firing.

Think of the 0 in a cone number as meaning "minus".  So 06 is much cooler than 6 because it is like a "minus 6".