How to calculate Lighting Power Density

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Energy efficiency has always been an integral part of lighting design, but it has rapidly become one of the top concerns that lighting designers and electrical engineers deal as lighting loads accounts about 40% of the total load in any commercial building. The load used by the lighting system can be analysed in terms of LPD.

Light Power Density -The lighting power density or LPD is the lighting load in watts/sq ft (or watts/square meter). Lighting Power Density technically represents the load of any lighting equipment in any defined area.

 To certify any building under LEED certification the standards of ASHRAE must be followed.

ASHRAE 90.1 offers two approaches to calculating LPD – the Building Area Method and the Space-by-Space Method.

Building Area Method

The Building Area Method lists about 30 different building types (Office, School, Hospital, etc.) and provides an LPD for the entire square footage of the building.

In this method the energy consumption of all lighting fixtures in the building are added up and then divided by the overall area of the building.

The LPD for each building type is roughly equivalent to the LPD that would result if each space of that building type was calculated individually using the Space-by-space method and added together.

Eg. If a building has total Gross Floor Area 53000 sq.ft , provided with light fixtures shown in table below

Type of Fixtures Wattage Quantity Energy Consumed (W/hr)
LED 6 256 1536
LED 10 152 1520
LED 15 150 2250
LED 20 70 1400
Total 51 628 32028

Thus LPD will be 32028/53000= 0.6 w/sq.ft

Space By Space Method

The Space-by-Space Method generally takes a similar tactic but considers each room/space of the building individually.

The Space-by-Space method lists approximately 100 space types (Open office, Enclosed office, Classroom, Restroom, Corridor, etc.) and provides an LPD for each space of that type in a building.

This makes it more complex calculation and requires a large number of very individual decisions to be made.

Most of the designers opt for the space type with the higher LPD. This permits them to make fewer difficult decisions, but this comes at the expense of higher energy consumption.

Following are the steps to calculate LPD by space by space method:

• Input manufacturers lighting fixture data [coefficient of utilization (CU) for three room cavity ratios (RCRs)].

 • Calculate CU for the fixture type which is calculated by an average of the CU’s of each individual fixture that exists within the type.

 • Input characteristic data including lamp lumen depreciation, luminaire dirt depreciation, room surface depreciation and lumen efficacies in the fixture type data database.

 • Calculate Total Efficiency Factor (TEF) values from CU, light loss factors (LLF), and luminous efficacies (LE) for each RCR condition. TEF = CU*LLF*LE

• Create models with user inputs for each space type including space and building type descriptions, general and task light levels [footcandles (fc)], percentage of space for general and task lighting, fixture type inputs (up to 3) with descriptions and average CU data, percentage of lighting provided by each system, the expected RCR for that space, and model number.

 • Calculate LPD for the three RCR values for each model described above but display only the one associated with the expected RCR. The program makes use of up to three lighting fixture types for each space selected from a library of lighting fixtures. It then calculates the LPD for the space using the equation below. LPD = (Average Weighted Illuminance * % of light from fixture type 1/TEF for fixture type 1)+ (Average Weighted Illuminance * % of light from fixture type 2/TEF for fixture type 2)+ (Average Weighted Illuminance * % of light from fixture type 3/TEF for fixture type 3)

The LPD for different space types prescribed by ASHRAE are shown in table below

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