You can get an estimate of energy use from the pump specifications. Normally pumps will specify the service factor amps for a given voltage (115 or 230). For example, my 1 HP Northstar pump has a 1.85 service factor which makes the pump braking HP 1.85 and is the actual HP of the pump. Labeled HP has little meaning so just ignore it. The amp rating is 7.8 amps at 230 volts which I verfied with a clamp on amp meter. So the wattage is volts times amps or 1.8 kilowatts. You will notice that this is very close to the braking horsepower. This can also be used as a good estimate.

A very good 2 speed pump is the Pentair Wisperflow but Hayward also makes a good 2 speed. In fact, you may be able to replace only the motor to get the 2 speed function of your existing pump. Just choose the same HP.

If you don't already have one, a 2 speed controller would be advisable. Most controllers have this funtion built in so if you already have one, it probably can support a 2 speed pump.