With a simple amp meter, you are neglecting the power factor. This is caused by amps getting out of phase with the voltage. Typically, power factors for motors are about 90% so you can subtract about 10% off of that bill.

The most accurate way to measure your power is to use the electric meter on the side of your house since that is what the power company uses. Each meter is a bit different but a call to your electric service or web site will show you how to convert to kwh.