Calculating Costs$$$$$

All-
i am trying to calculate the costs to run 1 pc of equipment per day. the equip spec is 480v 3phase 100A and my rate is $.071 per KWh here in PA.

the formula i have is:

1.73 x volts x amps x Power factor / 1000 = kw

kw x 24 = kwh/day

kwh x $rate = costs per day

SO....

1.73 x 480 x 100 x .8 / 1000 = 66.432 kw

66.432 x 24 = 1594.36 KWh

1594.36 x $.071 = $113.20/day to operate

does this look right??

thanks for your time

You need to divide by power factor, not multiply. Lower power factor makes the cost go up, not down. And technically, you would do the same with motor efficiency.

rate * hours * (1.73 * V * A)/(1000 * pf * eff)

Thanks and more

i recieved this from my air conditioning vendor, may be of use to some.

1. Supply & Exhaust/Return Fan Energy:
Cost Hour Fan Energy = {Supply Fan BHP (18.2) + Exhaust Fan BHP (8.8)} x 0.746 kW/HP x $/kWH (?)
$1.21=(27 x 0.746 x $0.06)

2. Summer Cooling Energy (Mechanical) Based On Average 40% Loading:
Cost Hour Mechanical Cooling Average = Nominal Equipment Tons (46) x Load factor (40%) x kW/Ton (1.2) x $/kWH (?)
$1.32=(46 x 40% x 1.2 x $0.06)

You have it right in your first post.

Ozee, your equation doesn't make sense to me. I think you mixed the motor power equation with the Power equation.

Total Power = KW + KVARs = V x A x 1.73
Total Power x Pf = KW

(Mech. Motor HP x 746)/(Pf x Eff) = Elec. Input Total Power

I agree the your first post is correct
See power triangle below
http://en.wikipedia.org/wiki/Electric_power

The question I have is the unit price of 0.071 $ / Kw appears low even taking into account differences in our countries exchange rates, have you added all costs in,
Eg. Unit costs , KVA / KW , Peak load , Line charges , Taxes ,
You know how this list grows, more branches than a tree.

Since you are in USA and with 100 HP motor I assume youre utility bill is a total of
KW
Demand charge
Power factor charge
Service fee
Taxes.

I would get a copy of a utility bill and divide the total cost by Kw. Now you know your overall cost and averaged cost per Kw. Not 100% accurate but hopefully just a little high for any application. This will work for motors resistive load etc etc.

Cost of running a load is V x A x 1.73 (if 3 ph) x hours x rate
I do not think you need pf because the utility is billing for it anyway and it is in your calculated rate - maybe a rough estimate but should come close.

The other costs should maybe also be added in - compressed air, cooling water etc etc. you could also add the rejected heat to the cost of air conditioning if an air conditioned space.

Dan Bentler

I didn't see that you said it was a 100HP motor, Just 100 amps. Now to be more exact, is your machine running at 100 amps of load, or is the main breaker a 100 amp breaker. If the main breaker is 100 amps, then you will pull less than 100 amps during running - sometimes a lot less. The best thing is to get an average load over a typical 24 hour period. This will give you a better calculation of what it costs to run that piece of equipment. Factor in downtime for PMs and plant shutdowns and you can get a better cost to run that piece of machinery for a year.