kWh to kW

Status
Not open for further replies.

Besoeker

Senior Member
Location
UK
How can I convert the above?

Thanks
You can't.
kWh is energy. Power times time.
kW is power.

If you know the duration over which the kWh was measured (in hours) you could divide by thos hours to get average kW. But note that it is average. If there are periods of lower power use, then there will be periods of higher than average power use.
 

mgookin

Senior Member
Location
Fort Myers, FL
For the OP:

1Wh = one watt for one hour
1kWh = one thousand watts for one hour

A 400 watt load for 3 hours = 400W x 3h = 1200Wh = 1.2kWh
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You can't.
kWh is energy. Power times time.
kW is power.

If you know the duration over which the kWh was measured (in hours) you could divide by thos hours to get average kW. But note that it is average. If there are periods of lower power use, then there will be periods of higher than average power use.
Absolutely. That calculator is somewhat pointless, I'm not sure what an"'average" kW value is good for.

10kW for 2 hours steady is 20kWH, which looks like an average of 10kW according to that calculator

2MW for 36 seconds followed by zero power for 3564 seconds is also going to show as a value of 20kWH on a meter.

What did having an average of 10kW in the first example tell you then, as it relates to the possibility of 2MW potential peak use?
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
Trying to get something through plans review using 220.87. Told customer to get me max demand for the last year and they sent my shop a years worth of POCO bills. Called building dept this morning saying all I had was the average but it was low enough that they will except it. That was a verbal, we'll see what happens when I actually submit this.

Just a note, the Building dept is very familiar with me and the structure in question.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Trying to get something through plans review using 220.87. Told customer to get me max demand for the last year and they sent my shop a years worth of POCO bills. Called building dept this morning saying all I had was the average but it was low enough that they will except it. That was a verbal, we'll see what happens when I actually submit this.

Just a note, the Building dept is very familiar with me and the structure in question.

As others have said, you can't compare KWH to KW. When using 220.87 it is looking for KW or KVA demand as that is what will more directly affect the line current imposed on the service. For example, the line current for a load that consumes 1000 KWH over a 30 day period will be vastly different than a load that uses the same 1000 KWH in a 1 day period.
In residential POCO billing, the demand values are not typically available so the bill is worthless for applying 220.87. Most light commercial and above POCO bills typically have demand values that can be used for this.
As a side note, is the AHJ Miami-Dade Building and Zoning? If so, I hope you have a good working relationship with them. I am well versed in dealing with them. I've been known to have done battle there, and it can be challenging.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
131203-2129 EST

Average power is a very useful measurement.

For example: If I measure the running power used by my furnace blower motor, which is about 400 W, then I can estimate my energy consumption for some period of time when the home and outside conditions are about the same. I also need to know the duty cycle. For 23 April 2011 my motor power was 400 W based on a measurement using a TED system. The motor on time was 11.82 minutes, and off time was 5.15 minutes. The duty cycle was 69.7%, and thus average power was 279 W. If these conditions are constant thru the day, then the energy used per day by the motor is about 6.7 kWh.

Lets work in reverse. Assume I have no power measuring equipment except my smart meter. I measure the time for 3 dot sequence steps at 27 seconds while the motor is running. Each step is worth 1 Wh of energy. Thus, one dot step is 9 seconds. There are 3600/9 times 1 Wh units of of energy that would be measured in one hour, 400 Wh, or 0.4 kWh. This can be produced by an average consumption of 400 W or 0.4 kW when the motor is runfor one continuous hour.

Now measure the duty cycle, the run time divided by the total cycle time. Multiply the running average power by the duty cycle and the result is an average running power level assuming the motor ran continuously, and used the same amount of energy.

A very useful tool.

.
 

Sahib

Senior Member
Location
India
The relationship between average power and maximum demand is given by

Load factor=average power/maximum demand.........(1)

The load factor for single family residence is in the range 10-15%.
By calculation of average power from POCO's energy bills and use of (1) above, the maximum demand may be estimated which may serve your purpose.
 

rayp1

Member
Location
Phoenix, Az, USA
kW from kWH

kW from kWH

Many utilities just measure power consumption in kWH without any kW component or the utility does not have smart meters yet.

All you can do is following: High month XX,XXX kWH used in the month divided by 720 hours in the month or the actual days converted into hours for the meter read period. This is the utility conversion practice typical of southwest USA utilities.
 

Besoeker

Senior Member
Location
UK
Many utilities just measure power consumption in kWH without any kW component or the utility does not have smart meters yet..
kWh is energy consumption, not power.
It's an important distinction particularly in the context of this thread.
 

kwired

Electron manager
Location
NE Nebraska
Many utilities just measure power consumption in kWH without any kW component or the utility does not have smart meters yet.

All you can do is following: High month XX,XXX kWH used in the month divided by 720 hours in the month or the actual days converted into hours for the meter read period. This is the utility conversion practice typical of southwest USA utilities.

That still only assumes the load was spread out over the time period. What if you had a month's worth of kWH but the bulk of the consumption was during only a few days? If you tried to size a supply system based on the average, it could be well under capacity of what is needed during those few days of heavy loading.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
That still only assumes the load was spread out over the time period. What if you had a month's worth of kWH but the bulk of the consumption was during only a few days? If you tried to size a supply system based on the average, it could be well under capacity of what is needed during those few days of heavy loading.
There's the rub. You have to size a system based on the highest instantaneous power it will require, and merely the kWh in a month or year is not enough data - or should I say, it's not the right kind of data - on which to base a system design.
 

kwired

Electron manager
Location
NE Nebraska
There's the rub. You have to size a system based on the highest instantaneous power it will require, and merely the kWh in a month or year is not enough data - or should I say, it's not the right kind of data - on which to base a system design.

Churches are a good example. Most of the time there is little to no load. One day a week for anywhere from 1 to maybe 6 hours the load increases dramatically, maybe even a second day or two the load increases but only for a few hours each time. Then comes the occasional big wedding, funeral, fundraising event, or similar where peak demand is maybe reached - but again for only a few hours. That is the time you don't want the service to fail because it couldn't handle the peak demand, though it was well oversized if you only considered average demand.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Churches are a good example. Most of the time there is little to no load. One day a week for anywhere from 1 to maybe 6 hours the load increases dramatically, maybe even a second day or two the load increases but only for a few hours each time. Then comes the occasional big wedding, funeral, fundraising event, or similar where peak demand is maybe reached - but again for only a few hours. That is the time you don't want the service to fail because it couldn't handle the peak demand, though it was well oversized if you only considered average demand.

Another is a transmission line from a wind farm.
 
Status
Not open for further replies.
Top