Free lighting tool
LENI calculator
This free LENI calculator works out the Lighting Energy Numeric Indicator of a lighting installation in kWh/m² per year from the floor area, installed lighting power, annual operating hours and the control strategy, adds the parasitic energy for controls and emergency lighting, and compares the result with the BS EN 15193-1 benchmark for the building type. It is aimed at lighting designers, M&E engineers and energy assessors preparing Part L evidence.
By Nicholas RedicanLast reviewed
What is LENI?
LENI, the Lighting Energy Numeric Indicator, is the annual energy used by a building’s lighting divided by its floor area, expressed in kWh/m² per year. It is defined in BS EN 15193-1 and is the figure used to compare lighting installations of different sizes on the same basis, and to show that a lighting design meets an energy target.
Unlike a simple watts-per-square-metre figure, LENI includes how long the lighting runs and how much the controls reduce that, plus the small amount of standby energy that control systems and emergency lighting draw around the clock. Two schemes with the same installed load can have very different LENI values if one is switched by presence and daylight and the other is left on manually.
How to use the LENI calculator
- 01Enter the floor area of the space or zone in square metres and the total installed lighting power in watts (luminaire circuit watts, including drivers).
- 02Enter the annual operating hours. The default of 2,500 hours suits a typical office; retail and hospitality run longer.
- 03Choose the control strategy: manual switching (factor 1.0), absence detection (0.9), daylight dimming (0.85), combined absence and daylight (0.75) or full DALI scene control with scheduling (0.65).
- 04Keep or adjust the parasitic energy for controls (default 5 kWh/m² per year) and emergency lighting charging (default 1 kWh/m² per year).
- 05Select the building type. The calculator shows the BS EN 15193-1 benchmark for that type and reports the LENI, the total annual energy in kWh and the margin above or below the benchmark.
The LENI formula
LENI = (installed power × operating hours × control factor ÷ 1,000) ÷ floor area + parasitic energy for controls + parasitic energy for emergency lighting, all in kWh/m² per year.
BS EN 15193-1 splits the annual lighting energy into the energy used to provide light (installed power, adjusted by occupancy, daylight and constant-illuminance factors, multiplied by the operating hours) and the parasitic energy drawn by control systems in standby and by emergency lighting batteries on charge. The calculator combines the occupancy and daylight effects into a single control factor for the strategy you choose, which is the quick method most designers use at concept stage.
The control factors are typical values for each strategy. In a detailed assessment the occupancy dependency factor, the daylight dependency factor and the constant-illuminance factor are calculated zone by zone from the building’s geometry, glazing and sensor coverage, which is the work a lighting designer does for the final Part L submission.
Worked example
A 500 m² open-plan office has 6,000 W of installed lighting, which is 12 W/m². It runs 2,500 hours a year with combined absence detection and daylight dimming, control factor 0.75. Lighting energy is 6,000 × 2,500 × 0.75 ÷ 1,000 = 11,250 kWh a year, or 22.5 kWh/m². Adding 5 kWh/m² for controls and 1 kWh/m² for emergency lighting gives a LENI of 28.5 kWh/m² per year.
The same office with manual switching (factor 1.0) would use 15,000 kWh, a LENI of 36 kWh/m² per year. With full DALI control and scheduling (0.65) it drops to 25.5 kWh/m² per year. That spread is why controls, not just efficient luminaires, decide whether a scheme meets its target.
LENI and Building Regulations Part L
For buildings other than dwellings in England, Approved Document L sets a minimum luminaire efficacy for general lighting and requires effective automatic controls. Where a design needs to demonstrate its energy performance more precisely, or where the project’s energy strategy sets a lighting energy target, LENI calculated to BS EN 15193-1 is the metric used. BREEAM and many client briefs also ask for a LENI figure.
The benchmarks in the calculator come from BS EN 15193-1 Annex F for each building type. Project-specific limits from an approved energy strategy or a client’s sustainability brief may be lower, so confirm the target that applies before relying on the comparison.
Limitations, and when to involve a lighting designer
The calculator treats the space as one zone with one control strategy and uses typical control factors. It does not calculate daylight dependency from the glazing and orientation, it does not model constant-illuminance control, and it does not check the luminaire efficacy route. It is an early-stage design check, not a compliance certificate.
For a Part L submission, a BREEAM assessment or a planning energy statement, I calculate LENI zone by zone in the Dialux Evo model with the real luminaire loads, sensor coverage and daylight availability, and provide the report with the calculation. Pro users can export a PDF of the quick calculation for their own records.
Standards and guidance referenced
- BS EN 15193-1: energy performance of buildings, energy requirements for lighting (LENI method and Annex F benchmarks)
- Approved Document L, Volume 2 (England): luminaire efficacy and lighting controls for buildings other than dwellings
- BS EN 12464-1:2021: the illuminance targets the installed load has to deliver
- BREEAM UK New Construction: Ene 01 and lighting energy reporting
Frequently asked questions
What is LENI?
LENI is the Lighting Energy Numeric Indicator: the annual energy used by a building’s lighting divided by its floor area, in kWh/m² per year. It is defined in BS EN 15193-1 and includes both the energy used to provide light and the parasitic energy of controls and emergency lighting.
How is LENI calculated?
Installed lighting power multiplied by the operating hours and by the control factor gives the lighting energy; divide by the floor area, then add the parasitic energy for controls and emergency lighting. The full BS EN 15193-1 method derives the control factor from occupancy, daylight and constant-illuminance factors for each zone.
What LENI value passes Part L?
There is no single number. Approved Document L relies mainly on minimum luminaire efficacy and automatic controls; LENI is used where a more detailed energy demonstration is needed or where the project sets a lighting energy target. The calculator compares your result with the BS EN 15193-1 benchmark for the building type; project-specific limits may be lower.
LENI or the luminaire efficacy route: which should I use?
Use the efficacy route when the luminaires comfortably exceed the minimum lumens per circuit watt and the controls meet the Approved Document requirements. Use LENI when the scheme relies on controls to justify higher installed loads, when the client or BREEAM asks for an energy figure, or when the efficacy route is marginal.
Do daylight dimming and occupancy sensing reduce LENI?
Yes, directly. In the calculator, absence detection reduces the lighting energy by about 10 per cent, daylight dimming by about 15 per cent, the two combined by about 25 per cent and full DALI scene control with scheduling by about 35 per cent, using the typical control factors from the standard.
What are the parasitic energy values?
They are the standby energy of the lighting control system and the charging energy of emergency lighting, both expressed in kWh/m² per year. The calculator uses 5 kWh/m² for controls and 1 kWh/m² for emergency lighting, the default values in BS EN 15193-1, and both can be changed.