kilowatt hours

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Ambient44

Member
Location
Oak Lawn, IL
Can anyone please explain how one might go about calculating the amount of wattage used by a 20hp motor at 480V, the amps are listed at 25amps at 460V. We are looking to charge the customer for using the pump per hour.
Thank you,
Tom
 

broadgage

Senior Member
Location
London, England
No exact answer can be given without knowing if the motor is fully loaded or not, and without knowing the motor efficiency.

As an approximation though, one HP=746 watts.
Therefore a 25 HP motor will have an output of 18.65 KW
The input will be more than this because the motor is not 100% efficient, we dont know what the losses are, but 10% might be a fair estimate.
That would give an input of about 20KW.
Multiply the prevailing cost per KWH in cents by 20, and that is the APPROXIMATE running cost in cents/hour, presuming that the motor is fully loaded.

It would be more accurate to fit a KWH meter and bill for the actual energy used.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Can anyone please explain how one might go about calculating the amount of wattage used by a 20hp motor at 480V, the amps are listed at 25amps at 460V. We are looking to charge the customer for using the pump per hour.
Thank you,
Tom

Not taking into account in rush current or duty cycle, name tag apparent power would be 11.5 kVA.

That will have to be adjusted for power factor (for example a pf of .7 would mean that only about 8 kW is being used), duty cycle (what percent of the day will be pump actually be running) and peak power during start up that may be in excess of the name plate rating.

At 20 cents per kW, the pump costs approx. 40 cents of electricity to run per hour at a 100 percent duty cycle.

40 cents per hour may not seem like much, but it works out to 288 dollars per month.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
No exact answer can be given without knowing if the motor is fully loaded or not, and without knowing the motor efficiency.

As an approximation though, one HP=746 watts.
Therefore a 25 HP motor will have an output of 18.65 KW
The input will be more than this because the motor is not 100% efficient, we dont know what the losses are, but 10% might be a fair estimate.
That would give an input of about 20KW.
Multiply the prevailing cost per KWH in cents by 20, and that is the APPROXIMATE running cost in cents/hour, presuming that the motor is fully loaded.

It would be more accurate to fit a KWH meter and bill for the actual energy used.

That is not legal in most states as the sale of electricity must be overseen by the state's Public Service Commission. That is why landlords can't charge their tenants per kwh as well.

I doubt anyone would balk at paying a buck an hour and that would certainly be enough to compensate for the amount of additional electricity being used.
 

gar

Senior Member
K8MHZ:

Your calculation of 11.5 KVA is not correct.

Convert the 460 to the equivalent line to neutral which is 266.
Multiply 25 * 266 = 6.65 KVA.
Then by 3 because there are three phases which produces 3 * 6.65 = 19.95 KVA.

I agree with your comment on selling electricity. Instead sell the services of the pump and embed the electric cost in the services. And a dollar per hour seems like a good add on.

.
.
 

Besoeker

Senior Member
Location
UK
Can anyone please explain how one might go about calculating the amount of wattage used by a 20hp motor at 480V, the amps are listed at 25amps at 460V. We are looking to charge the customer for using the pump per hour.
Thank you,
Tom

Wattage?
Assume you mean power.
Are you supplying the power? If not, what would your basis for charging for that?
Anyway, 20hp is about 15kW. Losses might be around 10% so power supplied if the motor is running at rated load would be about 16.5kW.

By the hour that would be 16.5kWh.
Obviously.
 

Ambient44

Member
Location
Oak Lawn, IL
Thanks

Thanks

Thank you for the help in this matter, I assumed there was a power factor involved I was asking how to translate it into hours of use. I believe I got the answer I was seeking. For calculating three phase kilowatts what is the watts X 1.732 divided by 1000 then?
 

gar

Senior Member
110706-1900 EDT

Ambient44:

Following is an intuitive way to think thru some 3 phase problems.

If you have a Y source with a Y resistive load, then it is obvious that the current in any phase is in phase with the voltage from line to neutral. Why? because the line to neutral voltage is the voltage across the resistor and the current thru a resistor is in phase with the voltage across the resistor.

So now if you know the line to neutral voltage and the current in the line, then you can calculated the VA to the load in that one phase. For a resistive load the VA and watts will be identical.

Assuming everything is balanced and you know the power in one phase load, then 3 times this is the total power in the 3 phase circuit.

You also know that line to neutral voltage is less than line to line. Thus, the way to use 1.732 (George Washington's birth year) is to divide the line to line voltage by 1.732 to get the line to neutral voltage. 1.732 is derived from drawing a triangle whose base is 1/2 of the line to line voltage, the hypotenuse is the line to neutral voltage, and cos 30 deg is the relationship between base and hypotenuse. cos 30 is (1/2)* sq-root of 3. Sq-root of 3 is 1.732 .

.
 

readydave8

re member
Location
Clarkesville, Georgia
Occupation
electrician
That is not legal in most states as the sale of electricity must be overseen by the state's Public Service Commission. That is why landlords can't charge their tenants per kwh as well.

I may be wrong but I believe that in Georgia it is perfectly legal for a landlord to charge tenants per kwh, as long as landlord doesn't mark up price and sells at or below cost?
 

Electric-Light

Senior Member
sub-metering is legal in many places, but just as scales used at farmers' market must be legal for trade and built to resist calibration tampering, sub-metering instruments are subject to restrictions by governing bodies.

So, you can't buy some kWh meter off the internet and use that for the purpose of billing.
 

kwired

Electron manager
Location
NE Nebraska
Wattage?
Assume you mean power.
Are you supplying the power? If not, what would your basis for charging for that?
Anyway, 20hp is about 15kW. Losses might be around 10% so power supplied if the motor is running at rated load would be about 16.5kW.

By the hour that would be 16.5kWh.
Obviously.

If energy cost is 10 cents per kWh one hour costs 1.65. Those that suggested a buck an hour are losing money.
 

kwired

Electron manager
Location
NE Nebraska
Thank you for the help in this matter, I assumed there was a power factor involved I was asking how to translate it into hours of use. I believe I got the answer I was seeking. For calculating three phase kilowatts what is the watts X 1.732 divided by 1000 then?

Watts is energy, and is not dependent on voltage, current or number of phases. Think of it as how much work is being done not how many amps are being drawn. Kilowatts is just 1000 watts.

Change voltage or current or number of phases and you will have different levels of the others if the same wattage is being consumed.

Two identical prime movers one supplying single phase generator the other supplying three phase generator - each supplying loads that provide the same amount of work should use the same amount of fuel to get the work done assuming all efficiencies are also equal.
 

Besoeker

Senior Member
Location
UK
Watts is energy, and is not dependent on voltage, current or number of phases. Think of it as how much work is being done not how many amps are being drawn. Kilowatts is just 1000 watts.

Watts are a measure of power, not energy. Energy is billed in kWh, not W or kW.
 

kwired

Electron manager
Location
NE Nebraska
Watts are a measure of power, not energy. Energy is billed in kWh, not W or kW.

I knew that, I was focusing more on the fact that he was trying to multiply watts by 1.73 to convert to three phase values.

A better statement is that power and/or energy is same for a given amount of work (assuming same efficiencies of equipment) no matter what voltage or number of phases is or even if AC vs DC.
 

gar

Senior Member
110707-1048 EDT

I made a mistake in supporting the $1 per hour charge. In my mind I may have been thinking of $1 per KWH. In any event the real cost of electricity for the area or conditions (gasoline powered generator) needs to be used to arrive at a price.


kwired:

i somewhat support your loose use of power relative to what you were trying to explain. I think in loose conversation it is easier to say power than energy. We usually speak of power companies, but the use of energy companies is becoming more common.

.
 

Besoeker

Senior Member
Location
UK
I knew that, I was focusing more on the fact that he was trying to multiply watts by 1.73 to convert to three phase values.

A better statement is that power and/or energy is same for a given amount of work (assuming same efficiencies of equipment)
Energy and work are equivalent quantities.

Power is not the same as work.
Power is the rate of doing work.


In SI (metric) units, the unit of work (and energy) is the Joule.
One Joule is a force of one Newton moved through a distance of one metre.
Do that in a time of one second and that gives you the rate of doing work which, in this case in one Watt. Which also happens to be one Amp times one Volt. Which also happens to be one of the reasons I like using SI.
:D

For Imperial, if you move a force of one lb(f) through a distance of 550 feet or 550lbf through a distance of one foot, you will you will have done 550 ft-lbf of work. Do either in a second and your rate of work will be one horsepower. Or about 746 Watts.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I think that before getting too far off on tangents, just stick to the facts you have and don't add too many things into the mix. Those are that
1) Motor electrical power consumption = 746W per HP, plus losses,
2) The only information you have about the motor is the full load HP, you don't know what the actual load is on the motor.

But it's a relatively safe bet it's probably around 80-90% loaded, that's a good design practice. So since you don't know how loaded the motor is, but you also don't know the efficiency (losses), you can pretty much trade the unknown loading for the unknown losses and call it a wash.

So for NON-REGULATED energy charges, I would just go with the 15kW and if you want to charge for 1 hour of work, that's 15kWh. Keep it simple.

If you want to be gnat's ass accurate, buy a kWh meter.
 

iwire

Moderator
Staff member
Location
Massachusetts
That is not legal in most states as the sale of electricity must be overseen by the state's Public Service Commission.

You can still install a meter to monitor kwh and in most areas you can charge what you paid for the electricity.

In other words the illegal part would be reselling the power for profit.
 
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