From most accurate to least accurate there are several methods:

1) Use a watt meter to measure the power. Not too many people have access to this.

2) Measure the current and voltage on line, multiply them together and take 90% of that number for a power factor. Again, requires an AC Volt/Amp meter.

3) Use the maximum amp rating on the pump and multiply it by the voltage rating and again multiply by 90%. This gives you watts but tends to over estimate.

4) Use braking HP (HP * service factor) for kwh. Could have up to 25% error.

Multiply watts by the run time in hours over the month, divide by 1000 and you get kwh for the month. This is what the power company uses for billing. Your rate should be on your last bill.