...and this is part of the numerator of the LENI formula...
...but what do all those terms mean?
The simplest term is Pn, the total installed power for lighting, ignoring emergency lighting and charging of emergency lighting batteries. The n stands for "normal" I presume, as opposed to "emergency" or "charging".
Fc (more on this here) can reduce the weight of Pn because constant illuminance control (CIC) can reduce the power used to illuminate the room. This can happen because sunlight enters the room and electronic controllers react to it and reduce the power driving the luminaires.
Fo is the occupancy factor and takes into account that often lights are switched off when rooms are not occupied. For example a store cupboard may have 50W of lighting installed, but it is only ever switched on when someone goes into it to get a broom. Say 0.1 or 10% of the time. This would make the actual consumption be calculated at 5W (not 50W).
The terms in square brackets are a way of splitting the calculation into day-time and night-time periods.
td is day-time hours. tn is night-time hours.
td x Fd: is a term which can be thought of as: "how much lighting will I need during the day":
- Fd is 0.0 if there is always enough natural light during the daylight hours.
- Fd is 1.0 if artificial lighting is always required during daylight hours, for example in a room with no windows, no natural light.
- Fd is somewhere between 0 and 1 if, during the day, you sometimes need to switch a light on, but not always.
tn does not have a factor because you know you need lighting at night-time. And if nobody goes into the room at night-time is handled by Fo, the occupancy factor.
Fo[(td x Fd) + tn]
...handles the energy consumption change because occupancy of the room, requirement for lighting energy during the day and requirement for lighting energy during the night.