Lighting power density:
watts of lighting per square metre of floor

What the ratio captures

Lighting power density, often shortened to LPD, is the installed lighting load spread over the floor area it serves. It tells you how electrically hungry a lighting scheme is per square metre, independent of the room's size, so a design for a small studio and one for a large open-plan floor can be put side by side.

Energy codes and green-building schemes often set limits in W/m² for different space types, and designers use LPD during design to show compliance. The limits, and the way the area and the watts are counted, differ between regions and standards, so use the one that applies to your project.

It is a measure of connected power, not of actual energy use or of how bright the room is. Two rooms with the same LPD can use very different energy if one is lit for 12 hours and the other for 2.

The calculation step by step

1. 1List every luminaire in the space and its input power from the datasheet.
2. 2Sum the watts, including control gear.
3. 3Measure the floor area of the same space, excluding areas served by different circuits.
4. 4Divide watts by area and compare with the target.

What goes into the watts and the area

The two inputs look simple, but the way they are counted changes the result. For the power, use the input power of the whole luminaire, including the driver or ballast, because a lamp marked 18 W may draw 21 W at the wall. For the area, use the floor area of the lit space, measured within the walls, and not the gross area including thick partitions or neighbouring rooms.

• Emergency and exit lighting are often counted separately; check the rule for your scheme.
• Decorative and display lighting may have an allowance of their own under some codes.
• A space that is partly open, such as a mezzanine, needs a clear boundary for the area.
• Where one circuit serves several rooms, split its load by the number of fittings in each.

Writing down how the watts and the area were measured makes a later check or a retrofit comparison much easier, and avoids arguments about whether two figures are like for like.

Comparing different spaces

The ratio becomes meaningful when you compare several rooms. The table below shows how a few typical spaces might look. These are worked illustrations of the arithmetic, not limits.

A corridor may show a high LPD simply because it is narrow and long, with few square metres per fixture. Codes recognise this by setting different limits for different space types, so a fair check compares each space with its own category rather than with the building average.

How LPD relates to light level

Power is not light. The light delivered to the work plane, measured in lux, depends on the lumens produced per watt of the source, the fraction that actually reaches the plane, and how the light has been maintained. A more efficient source needs fewer watts for the same lux.

This shows why LPD limits fall as lamp efficacy improves. Replacing 50 lm/W fittings with 100 lm/W units halves the watts needed for the same lux, and it is the reason retrofits can meet both the light level and the energy limit at once.

A retrofit comparison

LPD is a convenient way to show the effect of a retrofit. Imagine the same 80 m² room, first with 40 W fluorescent fittings and then with LED panels of 24 W giving similar output.

The first change cuts connected power by 40% and the second reduces the average load by a further 30%, though the dimmed figure is an average in use, not connected load, so it will not count for code compliance the way the connected value does.

Bringing a high figure down

If a space exceeds its target, the options are to reduce the watts while keeping the light, or to reduce the light where it is not needed. The first is usually the better path.

• Switch to higher-efficacy sources and efficient drivers.
• Use task lighting at desks and lower ambient levels in the rest of the room.
• Choose luminaires with good optical control, so more of the output reaches the work plane.
• Use controls such as daylight dimming and occupancy sensing; they cut energy use even where they do not change connected load.
• Light-coloured finishes bounce more light and may allow fewer fittings.

A lighting power calculation that totals the fixture wattage of a room gives the numerator of this ratio, so it is a handy companion step, with the area as the other half. Remember to count the whole control-gear load when comparing with a code.