ELECTRICAL DEMAND

THE HIDDEN FACTOR

Stephen F. Kelly P.E.

Most of us are aware that there is an energy consumption charge based on how much electrical energy we use. The more energy we use, the more we pay. For instance, the longer we keep on a light, the more energy the light uses to stay lit. In addition to the energy Usage charge, there is also a Demand Charge on the building you work in. The Demand component may come as a surprise to most of us – and can be very confusing to understand. Please review the definitions and example below regarding electrical billing criteria.

EXAMPLE – As you can see in the below, you can go 500 miles (or use 500 kwh) in any number of ways. Whether you can go at the rate of 1000 miles/hr (1000 kwd) depends on the equipment you have. You are billed by the power company on how fast you want to get there as well as how much energy it takes to go the distance. In this example the distance is 500 whatevers and the time it take to get there is shown anywhere from 30 minutes to 10 hours. If you buy a car that is capable of doing 1000 miles/hour, drive it for ½ hour to go 500 miles, and then let it sit for the next 9 ½ hours, what have you achieved? You’ve paid for a car that didn’t need to go 1000 m/hr, only went 500 miles, then let it sit unused. If we only need a car to go 50 m/hr, why buy one that goes faster than we need. The utility company has to build their power plants based on how “fast” we “drive” – all of us together at one time. When we “drive” our buildings faster than we need to we end up paying for it through our power bills. The key is to make the most wise use of what we need – We need electricity just like we need transportation – but we must be careful not to exceed the economic speed limit! If we do, one thing is for sure; we will pay for it.

EXAMPLE CHART

Electricity		Automobile
500 kwh kwh Meter	----------Distance----------	500 Miles Odometer
Demand Meter Reads (Rate of Speed)	Number of 30 minute intervals	Speedometer Reads (Rate of Speed)
kwd = 1000 x 30/30 = 1000 kwd	1	MPH = 1000 miles/hour
kwd = 500 x 30/30 = 500 kwd	2	MPH = 500 miles/hour
kwd = 250 x 30/30 = 250 kwd	4	MPH = 250 miles/hour
kwd = 100 x 30/30 = 100 kwd	10	MPH = 100 miles/hour
kwd = 50 x 30/30 = 50 kwd	20	MPH = 50 miles/hour

BUILDING ELECTRICAL DATA / DEFINITIONS

kwh = Kilowatt-Hour – The total amount of electrical energy used during a time period. (usually one month) Consider this to be like MILES on a trip.

Kwd = Kilowatt Demand – The average maximum rate of electricity required during a specific interval. This interval for all City of Charlotte buildings is a 30 minute demand interval period.

HrsUseDmd – Hours Use Of Demand – This is calculated by dividing Kilowatt-Hours used by the Kilowatt Demand. This number represents the hours used at the full demand. There are 720 hours in a typical 30 day month. The calculated hours use for a month can be used to compare against this 720 available hours. The higher the hours use, the more effective the use of demand. This is actually a comparison of “Load Factor.”

AKWD – Actual kwd – The actual highest demand recording for a building interval during a monthly billing cycle.

BKWD – Billing kwd – The billing demand can vary from the actual demand if the actual demand is lower than the minimum demand required in the contracted rate schedule. If the actual demand is greater than the contract minimums, it is the billing demand.

In many instances energy usage conservation can conflict with and be more than offset by increasing the demand component. A perfect example is the “let’s leave the air conditioner off until the last minute” case. If this is done, and then the full air conditioner load is on with a full lighting load, the two demands will compound on top of each other. The better (and cheaper) solution is to bring on the air conditioning system early enough so that it will barely have the building under control and have begun to cycle down just before the building lights are turned on adding its load to the space.

Usually when we purchase things, we know in advance what we are getting for our hard-earned money. Electricity and other types of metered energy only show up as a bill after it’s too late! We’ve already used it – we didn’t see ourselves using it – and on the day we received our utility bill we sometimes fail to remember how miserable the weather may have been, the extended operating hours, or possibly even the malfunction that could have occurred in some piece of equipment such as a time clock on lights or an air conditioner. If we sit back and think for a minute, none of us intentionally wastes energy, and if we knew it was going down the “wires” we would try to plug the “hole” immediately. You certainly let us know when the roof leaks and you would let us know if you were “leaking” electricity. On the positive side, this is one of the primary reasons that together we now have a tool for more closely monitoring and evaluating our energy usage. In conclusion, energy management is a combination of many factors: information analysis; technological improvements and operating improvements just to name a few. We look forward to working with you in the future in this area.

MAJOR KEYS TO DEMAND AND/OR ENERGY REDUCTION

Keys	Demand	Energy
Non-Simultaneous Operation of Loads	Yes	No
Load Reductions During the Building Peak Times	Yes	Yes
Excessive Outdoor Air Quantities During Peak Times	Yes	Yes
Common-Sense Lighting Reductions	Yes	Yes
Properly Applied and Operated Building Automation Systems Should Be Used Whenever Possible to Automatically Control Building Functions for the Purpose of Demand Reduction as well as Energy Conservation	Yes	Yes

The concept of the demand component coupled with the key ideas shown above should get you thinking more about energy conservation. This is only the tip of the iceberg!