Calculating Pump Wattage for cost analysis

jgofnj

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Trying to do some cost analysis. I figured the wattage of my 1 HP single speed pump is 1725 watts (230 volts x 7.5 amps = 1725 watts). I assume this is the correct calculation. But then I saw on numerous sites that a 1 HP electric motor is only 746 watts. What am I missing? Which is right for my 1 HP single speed pump? And how can 1 HP always be 746 watts? Does it assume specific assumptions?
 

mas985

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Trying to do some cost analysis. I figured the wattage of my 1 HP single speed pump is 1725 watts (230 volts x 7.5 amps = 1725 watts). I assume this is the correct calculation. But then I saw on numerous sites that a 1 HP electric motor is only 746 watts. What am I missing? Which is right for my 1 HP single speed pump? And how can 1 HP always be 746 watts? Does it assume specific assumptions?
Here is a summary of motor terms form the Hydraulics 101 sticky in my signature:

Motor Nameplate Definitions (pre 7/19/2021)
  • Nameplate HP (NPHP) - This is the HP rating on the motor nameplate but is pretty much meaningless without the service factor.
  • Full Rated HP (FRHP) - Similar to nameplate HP and sometimes used when the pump is full rated.
  • Up Rated HP (URHP) - Similar to nameplate HP and sometimes used when the pump is max rated or up rated.
  • Nameplate KW = NPHP * 0.7457 - This is the KW rating and is similar to the nameplate HP and is generally used outside the US. Note that this is not the input power to the motor only the rating for the output power of the motor.
  • Service Factor - This is an overload rating for motors which states that the motor can be safely operated over the NPHP by the service factor for short periods of time. However, for pumps, this overload rating is typically used as the maximum load that a motor would need to deliver to the wet end. Because the load on a pump does not rapidly change over time, the service factor load is often used as the maximum design point for the pump.
  • Total HP (THP) or Service Factor HP (SFHP) = NPHP * Service Factor. This is the maximum load that can be safely driven by the motor and must always be greater than the maximum load from the impeller. A motor can be driven above the THP but will likely fail in a short period of time.
  • Electrical Horsepower (EHP) = Input Watts / 745.7 = Volts * Amps * Power Factor / 745.7 - Electrical power input delivered to the motor.
  • Brake Horsepower (BHP) = EHP * Motor Efficiency - Power delivered by the motor shaft to the impeller. This is not the same as THP or SFHP. BHP is a function of the load on the motor shaft and will change with Head, GPM and RPM.
  • Hydraulic HP (HHP) = BHP * Pumping Efficiency = Head (ft) * GPM / 3960 - Power delivered to the water. Sometimes called water HP (WHP) or pumping HP (PHP).
  • Motor Power Efficiency = BHP / EHP - I2R, magnetic and mechanical losses in the motor only.
  • Pumping Power Efficiency = HHP / BHP - Recirculation and internal friction losses in the wet end only.
  • Total Pump Power Efficiency = Motor Efficiency * Pumping Efficiency = HHP / EHP (note this is why total pump efficiency approaches 50%).
  • Energy Factor - Gallons/watt-hr = GPM * 60 / Watt-hr; A CEC definition used to quantify a pump's efficiency.
  • Service Factor Amps - The amp draw when the motor is loaded to the service factor. Also, multiplying the SF amps by voltage should also give a good estimate as to the upper limit for power draw. However, sometimes the motor is over dimensioned for the pump so it will not always be an accurate measure of input power. NEMA tolerance for this parameter is ±10%.
  • Full Load Amps - This can mean several things depending on the motor manufacture. It is either the amps at the NPHP or it can be the amps at the THP. I have seen it both ways so unfortunately, there is not a good standard for this one. NEMA tolerance for this parameter is ± 10%.
Here is a summary of the above:

Electrical Power >> Motor Output Power >> Hydraulic Power. >> Represents efficiency loss.

First, the 1 HP you mentioned is probably the label HP. Total HP is the actual motor rating and requires the service factor. THP = LHP * SF

Second, the conversion from HP to watts is 745.7 watts/HP. However, that conversion only works when you are talking about the same power measurement point (e.g. motor input or motor output power).

So for example, if your motor has a 1.65 service factor, then the THP rating is actually 1.65 HP, not 1 HP. So the output power of the motor would be 1230 watts at full load. Pump motors do not always run at full load because load depends on the operating point. However, if the motor was running at full load and the output power is 1230 watts and the input power is 1725 watts, then the motor efficiency is about 71% which is fairly typical for that size motor.

So yes, you can use 1725 watts as an upper limit in terms of energy use.
 
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