you need "watts in" to calculate the cost of operating the circulating pump. The hp value on the pump or motor is the mechanical output power and multiplying that by 746 just gives watts out; watts in will always be higher because of internal pump/motor losses. AC Watts in = volts x amps x power factor; power factor does not apply to purely resistive loads, which a motor is not, so there is a power factor, but this is usually only identified on larger three phase motors, not small single phase motors like here. In the electric utility world, there exists Apparent power (volts x amps) , Real power (watts), and Reactive power (look up power triangle for those more interested). Reason for mentioning this is that the utility company charges you for Real power which is "Watts in" to your house through your meter. So without knowing the power factor, can we still calculate the cost to operate your pump? Of course. It won't be dead on, but will get you in the ballpark. Example: I have a B&G circulating pump for my house run of my owb, and the nameplate says 2.1 amps, so 120 volts x 2.1 = 252 VA; I have no idea what the power factor is on my pump so we'll just continue on to arrive at the estimated cost, so 252 watts (er VA) x $.12061/kWhour (my rate here in WI) x 24 hours/day x 30 days/month x 1kW/1000 watts = $21.88 per month. Power factor for motors is always less than 1, so the actual cost will be somewhat less, for example, if my pump has a pf of .78, then my actual cost for watts consumed would be $21.88 x .78, or $17.07.