Unless you are doing far more extracting than most of us, the electrical cost of running an extractor is almost negligible.  Other electrical devices like water heaters, heaters, boilers, lights etc. are likely to use more power.  A large plama TV uses more juice than most extractors.

Figure out the total running time of your extractor per year and multiply by your total cost per KWH.  Include the energy, the delivery and the tax in that price.  Chances are it is less than $0.20 per KWH, and perhaps as low as 1/3 that.

At a steady eight hours per day with a motor drawing one KW (That would be more than one HP, but is a convenient number), you are looking at $1.60 max per day.  

Typical duty cycles run the extractor much less than eight hours in an eight hour day and most non-production line extractors draw much less than 1 KW, so that $1.60 estimate is quite high -- especially considering power costs as little as $0.065 in some areas, not $0.20.  1HP is actually 746 watts, but there is some loss, so this is, once more, a generous estimate.

Let's say you extract 5 days a week for a month each year.  5 x 4.2 x 1.60= $33.60 per year.

Remember that this is a very high estimate.  To the extent that your power costs less, you run fewer hours or your extractor motor is under 1 HP, your current costs are probably well under $20 per year and maybe just $10 a year -- or less.

The main considerations in choosing an extractor motor are

1.) Will it start without blowing the breaker or dimming the lights?  
2.) Will it run the extractor without overheating? 

Ideally , a clamp-on ammeter should be used to assure that the current draw at maximum load is under the rated current on the plate attached to the motor, and that the starting current is not excessive for the circuit.  but that said, I have run motors well above rating for extended periods with no apparent harm except possible inefficiency.

For hard-starting equipment, capacitor start motors are generally recommended.  If starting a heavy load dims the lights or blows a breaker, many of us just slacken the belt to allow some slip.  This is not kosher, but it works, and if not done to excess, most belts last a long, long time.  I always keep spares nearby if I do that and never seem to need them.

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