Demand Determination

Whenever demand charges or demand block energy charges are used in a rate structure, the peak demand must be determined for each billing period. For electric rates the integrated power use over a 15, 30 or 60 minute period is typically used. For fuel rates, the peak hourly fuel consumption, or peak daily fuel consumption is used. In the simplest cases the measured peak demand is used directly to compute the demand charge. In other cases, however, the measured demand is adjusted by one or more clauses to determine a "billing" demand used to calculate the charge. For example, some rate structures impose a minimum demand clause. The billing demand is either the measured demand or the minimum demand, whichever is larger.

Clauses used to derive "billing" demand from the measured demand are referred to as "Demand Determination" clauses. Usually the utility rate sheet will include a "Demand Determination" section that spells out these clauses. In other cases the clauses are provided as fine print below the demand charge statement

Each utility company defines clauses in different ways, but most fall into one of the following five categories:

 Minimum demand clauses  Rachet clauses

 Trailing window clauses  Demand multiplier clauses

 Power factor multiplier clauses (electric rates only).

Utilities will never refer to the clauses by these names. Instead, these are simple descriptive names that make explaining the clauses easier. To determine which kind of demand clauses are used in your rate structure, match the clause defined on your utility rate sheet with the following descriptions.

Minimum Demand Clause. Utilities often specify that billing demand may not be less than a certain

demand level.

Sample Demand Clause:

The billing demand shall be the larger of:

a. The maximum 30-minute integrated demand measured, or b. 50 kW.

Example: In a particular month the measured demand is 35 kW. Using the sample clause above, billing demand would be determined as:

Measured Demand Minimum Demand Billing Demand

35 kW 50 kW 50 kW

Program Input: To model this clause in HAP, select the Minimum Demand Clause option and specify the minimum demand value.

Rachet Clause. A rachet clause introduces a penalty for large swings between monthly demands. The

key to recognizing the rachet clause is that it compares measured demands with a percentage of the highest demand found during a fixed set of months.

Sample Demand Clause:

The billing demand shall be the larger of:

a. The maximum 30-minute integrated demand measured, or

b. 75% of the highest demand determined during the billing months of June through August

Example: The measured demand for November is 100 kW. The highest measured demand during the months of July through August was 200 kW. Using the rachet clause above billing demand is determined as follows:

Measured Demand Rachet Demand Billing Demand

100 kW 0.75 x 200 = 150 kW 150 kW

Program Input: To model this clause in HAP select the Rachet Clause option and then specify the months during which the rachet peak is determined ("peaking months"), the months in which the rachet applies ("Applies In") and the multiplier factor. For the sample rachet clause the following inputs would be used:

 Peaking months = June to August

 Applies in months = January to December  Multiplier = 75%.

Trailing Window Clause. A "trailing window" clause also introduces a penalty for large swings

between monthly demands. The key to recognizing the trailing window clause is that it compares the measured demand in the current month with a percentage of the highest demand found within a series of preceding months. This series of months is referred to as the "trailing window".

Sample Demand Clause:

The billing demand shall be the larger of:

a. The maximum 30-minute integrated demand measured, or

b. 50% of the highest demand measured during the preceding 6 months.

Example: The measured demand for November is 100 kW. The highest measured demand during the previous 6 months was 250 kW in July. Using the trailing window clause above billing demand is determined as follows:

Measured Demand Rachet Demand Billing Demand

100 kW 0.50 x 250 = 125 kW 125 kW

Program Input: To model this clause in HAP select the Trailing Window Clause option and then specify the size of the trailing window and the multiplier factor. For the sample rachet clause the following inputs would be used:

Chapter 6

Design Applications

Example: The measured peak demand for one of the winter billing months is 140 kW. Using the demand multiplier clause shown above, the billing demand would be determined as follows:

Measured Demand Demand Multiplier Adjustment

Billing Demand

140 kW 140 kW x 0.60 = 84 kW 84 kW

Program Input: To model this clause in HAP, select the Demand Multiplier option and specify the multiplier factor and the season and period in which the multiplier applies. For the example clause above the inputs would be:

 Multiplier = 60%  Season = Winter  Period = All

Power Factor Multiplier Clause. This clause introduces an indirect charge for excessive reactive power use. It is only used in electric rate structures. Power used in alternating current circuits is classified as "working" and "reactive". "Apparent" power is the vector sum of working and reactive power. Working power can be measured by a wattmeter. Reactive power is used to generate the magnetic flux in inductive machinery such as electric motors. It must be measured with separate metering equipment. Rather than measure it directly, utilities sometimes spot check buildings and impose a penalty if reactive power use is excessive. The reference value for the penalty is the "power factor" which is the ratio of working power to apparent power and therefore indirectly indicates the magnitude of the reactive power component. The lower the power factor, the larger the reactive power use.

Sample Demand Clause:

Customers shall maintain a lagging power factor of 90% or higher. For each 1% by which the average power factor lags below 90%, the demand charge shall be increased by 1%.

Example: For a certain month the measured peak demand is 200 kW. A spot check indicates the building power factor is 80% lagging. Using the demand clause above, the building would be penalized by increasing the demand charge by 1% for each 1% the power factor is below 90%, or a total of 10%.

Measured Demand Power Factor Multiplier Adjustment

Billing Demand

200 kW 200 kW x 1.10 = 220 kW 220 kW

Program Input: To model this clause in HAP, select the Power Factor Multiplier option and then specify the multiplier factor. For the example clause above the multiplier would be 110%.

This appendix describes procedures used to perform common tasks in HAP such as entering or editing data and generating reports. This information may be useful for new users learning the program and for occasional users who need a refresher on operating procedures.

While designing and analyzing HVAC systems with HAP, a common set of procedures is used to enter data, modify data and generate reports. Using common procedures to operate the program makes the program easier to learn and simpler to use. Whether you are working with walls, spaces or systems, for example, the same basic procedures are used.