TotalPowerDemand vs TotalPower

[Anon123123]

Hello,

In my PM820 Niagra dashboard I see TotalPower and TotalPowerDemand. These values are similar but not the same. What is the difference between them? Is total power demand the kW and the total power kVA? (kVA^2=kW^2+kVAR^2). I also have Power (kW), Demand (kW), Power (kVAR), Power (kVA) and non-of these values are the same.

[Charles_Murison · Accepted solution]

Hello @Anon123123 ,

Total Power is the 1 second instant power value. Demand Power is a kind of average. Slightly more complex calculation but average will give the idea. If your load is stable the numbers will be close. If your load changes often, the values may be close or may be different depending when you read the Total power.

Regards,

Charles

[Anon123123]

Why do we need both values? Is it because German Electrical Utility companies or Network Operators calculate the Demand Charge based on an average value and not the instant 1sec Power value? Is Peak Demand in the table above the maximum instant 1 sec Power value or the maximum averaged "Demand" value? You can see below that the Demand and Power curves are similar but not the same: 

For demand management I would like to understand which curves data I need to use or if I need to use the  average of both values.

[Charles_Murison · Accepted solution]

Hello @Anon123123 ,
I am not sure it is the same for others but I cannot see your images. 
"Why do we need both values?" Each measurement has different importance. Total Power can show how much power your are currently using. Demand Power indicates how much power was used over the demand period. 

Lets say you have a big motor. When the motor is running the power is 25kW. However when the motor is first starting up the power draw is 75kW for 10 seconds. You have 15 minute demand windows. The motor is off and started 5 seconds before the next demand interval. The demand power for the end of the demand interval would be small because the motor has only just started however the instant power reading at the time would be high. Likewise, if the motor has been running for most of demand interval then turned off 5 seconds before the end of the demand interval. The total power would be low or 0 kW while the demand would be much closer to 25 kW.

Most utilities will charge based on the maximum demand and the total energy used. Note that different utilities have different rates so it is not a global statement. Some utilities will keep the peak demand for 12 months or other time periods, so a new max Peak in February would be charged in March April, May, ..., until February the next year.

Regards,

Charles

[Anon123123]

Yes, 

Thats what we have too. The utility company charges us the highest demand for 12month of the year. Even if the max is in December, we will retroactively pay as if we used that demand for the whole year. When you say maximum demand do you mean maximum power consumption averaged by 15min or the maximum highest instantons power? Basically, I'm trying to understand when I'm calculating savings from an energy conservation measure if I need to use the max value from the "Power" or max value from the "Demand" curve.

[Charles_Murison]

Hello @Anon123123 ,

I may suggest reaching out to your local community to confirm what the utility is using. I can say what is common however I do not have in depth knowledge of every possible utility pricing structure.  The most common power used is the demand (average) power. Assuming this is correct, be sure you use correct demand window. Some utilities use 5 minutes demand windows, others use 15 minutes.

Regards,

Charles