Lighting Systems in Summer: How to Reduce Consumption
in Your Offices and Retail Spaces
For energy managers, facility managers, and property directors: reduce your electricity bill by acting on lighting and its impact on air conditioning. Simple and compliant with regulatory requirements.
🔎 The problem: in summer, lighting systems generate additional heat load in offices. Air conditioning systems must compensate for this heat, leading to a simultaneous increase in both lighting and cooling consumption.
Good news: simple and quick optimization solutions can be implemented without major renovations.
- Why lighting costs more in summer
- The domino effect on air conditioning
- Photometric quantities (lux, lumen, efficacy)
- Estimation methods & sector shares
- The LENI indicator (EN 15193)
- Maximum savings with daylight regulation
- 3 simple solutions for your offices
- Regulatory framework & obligations
- Wattnow: monitoring & analysis
- Frequently asked questions
Why lighting weighs on your summer bill
Contrary to popular belief, lighting consumption doesn't automatically decrease with sunny days. In offices, bank branches, and retail spaces, several factors explain this:
- Areas with insufficient natural light: deep spaces (building cores, corridors, meeting rooms) require permanent artificial lighting.
- Conflict with solar protections: blinds and shutters closed to limit heat... but which also block natural light.
- Extended operating hours: occupancy periods (often 7am-7pm) far exceed peak useful daylight hours.
Lighting heats your spaces: impact on air conditioning
A luminaire converts part of its energy into heat. In summer, this heat adds to solar gains and forces air conditioning to work harder. This is called the domino effect.
Old technologies (fluorescent, halogen)
Up to 90% of electricity consumed is converted to heat. For 100W consumed, 90W heat the room. Air conditioning must evacuate this additional heat.
High-efficiency LEDs
Only 20 to 30% residual heat. The rest is converted to light. Thermal impact is divided by 3 to 4 compared to old technologies.
💡 Summer reflex: reducing unnecessary lighting also reduces the load on air conditioning. Each kWh saved on lighting reduces your AC bill by approximately 0.5 kWh additionally.
Understanding photometric quantities: lumen, lux, luminous efficacy
To optimize a lighting system, it's essential to master the measurement units that allow objective comparison of light sources:
- Luminous flux (lumen - lm): total amount of light emitted by a source. A candle emits ~12 lm, a 35W fluorescent tube ~3650 lm.
- Illuminance (lux - lx): luminous flux received per square meter. An office requires 500 lux (EN 12464-1 standard).
- Luminous efficacy (lm/W): ratio between emitted flux and power consumed. A modern LED reaches 150-180 lm/W, compared to 12 lm/W for incandescent.
- Color temperature (Kelvin - K): 3000K (warm / reception areas), 4000K (neutral / offices), 6000K (cool / workshops).
- CRI (Color Rendering Index): >80 for commercial buildings, >90 for laboratories or quality control.
📐 Key takeaway: a lamp with high luminous efficacy (lm/W) consumes less electricity to produce the same illuminance level. Moving from 70 lm/W (old fluorescent) to 150 lm/W (LED) allows you to halve the installed power for equal light output.
Estimation method #1: sector benchmarking
Before even taking precise measurements, you can estimate the share of lighting in your electricity bill based on benchmarks specific to each sector.
| Building category | Lighting share of electricity consumption |
|---|---|
| Individual housing | 15 to 20% |
| Collective housing (common areas) | 50 to 60% |
| School building with gym | 50 to 65% |
| Administrative building / Offices | ~50% |
| Hotel-restaurant | 30 to 40% |
| Retail store / Commerce | 40 to 65% |
| Hospital | ~40% |
| Industry | Highly variable (20 to 70%) |
📊 Interpretation for commercial buildings: in office or administrative buildings, lighting represents about half of electricity consumption (excluding heating/cooling). This is the first area to act upon.
Method #2: rough calculation
A simple approach is to count luminaires, read their power rating, and multiply by estimated operating hours.
Example: a 200 m² open space equipped with 40 fluorescent tubes of 36W each (with ballast, actual consumption ~48W). Operation 2000 h/year (8h/day, 250 days).
Annual consumption = 40 × 48W × 2000h / 1000 = 3840 kWh
Switching to 18W LED (actual consumption 18W) reduces consumption to 40 × 18W × 2000h / 1000 = 1440 kWh, a -62% reduction.
LENI: lighting energy performance indicator (EN 15193)
The Lighting Energy Numeric Indicator (LENI) expresses the actual energy consumption of a lighting installation in kWh per square meter per year. It is the reference metric for evaluating and comparing lighting system performance in commercial buildings.
Simplified LENI formula:
- Pn : installed power (W)
- tD : daytime operating hours (h/year)
- tN : nighttime operating hours (h/year)
- FD : daylight control factor (≤1)
- FO : occupancy control factor (≤1)
- FC : constant light control factor (≤1)
- A : evaluated area (m²)
Maximum savings with daylight-dependent regulation
Optimal use of natural light is one of the most effective levers for reducing artificial lighting consumption. Depending on the level of automation and regulation, achievable savings vary considerably. Here are typical observed gains:
- Manual control (baseline)0% savings
- Manual control + automatic switch-off during lunch break25% savings
- Constant illuminance regulation, without standby40% savings
- Automatic ON/OFF switching40% savings
- Constant illuminance regulation with standby50% savings
- Automatic switch-off based on daylight + manual activation50% savings
💡 Key lesson: moving from purely manual control to automatic regulation with occupancy detection and daylight harvesting allows you to halve lighting consumption (50% savings). In areas near windows, light sensors adjust LED power, guaranteeing constant illuminance (e.g., 500 lux) without waste.
Concrete application example
A 300 m² open space with a glass facade. Without regulation: annual consumption = 12,000 kWh. With constant illuminance regulation and standby (50% savings): consumption = 6,000 kWh.
Annual savings: 6,000 kWh → approximately €900 to €1,200 depending on electricity rates, not including savings on air conditioning (reduced heat load). Typical ROI: 2 to 3 years.
3 levers to reduce consumption without major renovations
1. Switch to high-performance LEDs
Modern LEDs consume 4 to 5 times less than fluorescent tubes and produce very little heat. Return on investment is typically 2 to 3 years based on lighting alone, faster when including air conditioning savings.
2. Occupancy sensors
In corridors, restrooms, meeting rooms, and passage areas, sensors automatically turn off lights after 10-15 minutes of absence. Typical savings: 20 to 40% in these areas.
3. Daylight dimming near windows
Light sensors adjust LED power near windows based on sunlight. Simple, comfortable, and effective. Waste reduction without effort (up to 50% savings).
Obligations for commercial buildings
Progressive reduction targets (French Tertiary Decree)
- -40% by 2030 compared to baseline year
- -50% by 2040
- -60% by 2050
Annual declaration is mandatory on the OPERAT platform. Lighting is a priority action lever.
Energy management policies (Tunisia)
- Mandatory energy audits for large consumers
- Financial incentives for energy efficiency investments (ANME funds)
- National carbon intensity reduction targets
With rising energy costs (STEG), lighting becomes an immediate and easy-to-activate savings category.
Wattnow: monitor, analyze, alert – without remote lighting control
What Wattnow provides concretely
Track lighting consumption per zone (floor, wing, branch) from a simple and intuitive dashboard.
Get notified if lighting remains abnormally on at night or on weekends.
Wattnow calculates your LENI indicator from actual consumption data.
Exports adapted to declaration platforms (OPERAT, ANME reports).
Compare consumption before/after installing detectors or dimming.
30 to 50% reduction in lighting consumption within the first year.
What commercial building managers ask us
Lumen (lm) measures the total amount of light emitted by a source. Lux (lx) measures the amount of light received on a surface (1 lux = 1 lm/m²). For the same luminous flux, the larger the surface, the lower the illuminance in lux.
Depending on the automation level, savings range from 25% (lunch break shut-off) to 50% (constant illuminance regulation with standby). In perimeter areas near windows, the savings potential is highest.
Simplified method: LENI = (Total installed power in kW × annual operating hours) / area in m². For refinement, apply coefficients: ×0.8 for occupancy sensors, ×0.7 for daylight dimming. Wattnow automates this calculation.
No. Wattnow is a measurement, analysis, and alert solution. It flags anomalies but does not control luminaires. Action decisions remain yours.
On average 15 to 30% of total electricity consumption (excluding HVAC). In retail stores, this share can reach 65% (display and shelf lighting).
Official resources & references
Further reading
Ready to master your lighting consumption?
Wattnow helps you monitor and analyze lighting in your offices and branches. Automatic LENI calculation, drift alerts, simplified regulatory reporting.
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