Free lighting tool
Lighting controls savings calculator
This calculator estimates the annual kWh, cost and CO₂ a lighting installation would save from dimming, daylight harvesting and occupancy sensing, and the simple payback on the controls investment. It is for designers, M&E engineers, facilities managers and clients who need an indicative figure before commissioning a full LENI or Part L calculation.
By Nicholas RedicanLast reviewed
What is a lighting control savings calculator?
A lighting control savings calculator takes the installed lighting load, annual operating hours and electricity tariff, then applies percentage reductions for each control strategy to estimate how many kWh, pounds and kilograms of CO₂ the controls would save each year. Dividing the controls cost by the annual saving gives the simple payback period.
The tool models three strategies, each toggled independently. Dimming covers anything that runs luminaires below full output for most of the time: task tuning, constant-illuminance control or scheduled set-backs. Daylight harvesting dims luminaires near windows and rooflights while daylight is available. Occupancy sensing switches lighting off when a space is empty. The calculation runs on NR Lighting's servers; only the numbers you enter are sent, and they are not published.
How to use the lighting controls savings calculator
- 01Enter the installed lighting load in kW (circuit watts summed, or floor area × design W/m²), annual operating hours, tariff in £/kWh and the budget cost of the controls.
- 02Leave the grid carbon factor at 0.18 kg CO₂/kWh unless reporting for another year or region.
- 03Switch on the strategies you intend to install and enter their parameters: average dim level as a percentage of full output; daylight reduction and the share of hours with useful daylight; and the percentage of lit hours the space is empty.
- 04If strategies share the same luminaires, enter an interaction de-rate of 5–15% to allow for overlap.
- 05Select Calculate Savings for annual savings in £ and kWh, percentage reduction, a breakdown by strategy, payback, five-year net savings, CO₂ saved and the controlled annual energy and cost.
- 06Pro subscribers can add project details, see recommendations based on the result and export a PDF report.
The method and formula
Baseline kWh = installed load (kW) × annual hours. Dimming saving = baseline × (1 − dim level) × 0.85. Daylight saving = remaining load × daylight hours share × daylight reduction. Occupancy saving = remaining load × vacancy share × 0.90. Total saving = (sum of the three) × (1 − interaction de-rate).
The strategies are applied in sequence, not as independent percentages of the baseline: dimming acts on the full load, daylight harvesting on what remains, and occupancy sensing on what remains after that. The same kilowatt-hour cannot be saved twice, so a strategy later in the chain shows a smaller figure than it would alone.
Payback is the controls cost divided by the annual cost saving; five-year net savings are five times that saving minus the controls cost; CO₂ saved is total kWh saved × grid factor. The breakdown rows are shown before the de-rate, so with a de-rate applied they add up to more than the total.
| Factor | Value | Reason |
|---|---|---|
| LED dimming efficiency | 0.85 | Driver losses: dimmed LEDs save about 85% of the reduction in output |
| Occupancy sensor efficiency | 0.90 | Time delays mean sensors capture about 90% of the vacant time |
| Daylight harvesting | Reduction × share of hours, as entered | Applied to the load remaining after dimming |
| Interaction de-rate | 0% by default, up to 50% | Allows for overlap between strategies; 5–15% is typical |
| Grid carbon factor | 0.18 kg CO₂/kWh | DESNZ 2025 factor for UK electricity; editable |
Worked example
A 1,000 m² open-plan office lit at 8 W/m² has an installed load of 8 kW and runs 2,600 hours a year (ten hours a day, five days a week) at £0.28/kWh. DALI presence detectors, perimeter daylight sensors and constant-illuminance dimming are budgeted at £8,000. Inputs: 80% average dim level, 30% daylight reduction over 40% of hours, 25% vacancy, 10% interaction de-rate.
- Baseline: 8 × 2,600 = 20,800 kWh, costing £5,824 a year.
- Dimming: 20,800 × 0.20 × 0.85 = 3,536 kWh saved; 83% of the load remains.
- Daylight: 20,800 × 0.83 × 0.40 × 0.30 = 2,072 kWh saved; 73.0% of the load remains.
- Occupancy: 20,800 × 0.7304 × 0.25 × 0.90 = 3,418 kWh saved.
- Total: 9,026 kWh × (1 − 0.10) = 8,123 kWh, a 39.1% reduction worth £2,274.54 a year; controlled consumption 12,677 kWh (£3,549.46).
- Payback: £8,000 ÷ £2,274.54 = 3.5 years; five-year net saving £3,372.70; CO₂ saved 8,123 × 0.18 = 1,462 kg a year.
Limitations, and when to involve a lighting designer
The tool gives indicative design values, not compliance sign-off. Vacancy rate, useful daylight hours and average dim level are assumptions, and optimistic inputs produce optimistic paybacks: the defaults give a reduction close to 50%, at the top of what well-commissioned schemes achieve. Real installations are held back by sensor time delays, nuisance switching, occupants overriding scenes and controls that were never properly commissioned. The model also ignores control-system standby power, maintenance, tariff changes, the falling grid factor and any discount rate. For Part L, a LENI calculation to BS EN 15193-1 is the formal route.
I bring controls in early because the savings depend on zoning, sensor placement and commissioning rather than on the sensors themselves. On retail and hospitality projects, controls are also about the scene: a showroom or shop needs the right light on the product whenever the door is open, so daylight and presence strategies are written into a control narrative and witness-tested, not bolted on. If you need a LENI calculation for building control, a BREEAM submission or a control narrative and commissioning brief, get in touch.
Standards and guidance referenced
- Approved Document L, Volume 2 (2021): luminaire efficacy and lighting controls for buildings other than dwellings in England
- BS EN 15193-1: Lighting Energy Numeric Indicator (LENI), the formal method for occupancy and daylight dependency factors
- BS EN 12464-1:2021: maintained illuminance and uniformity that dimmed and daylight-linked schemes must still meet
- SLL Code for Lighting: design guidance, including lighting controls
- BS EN ISO 52120-1 (formerly BS EN 15232-1): impact of building automation and controls on energy performance
- DESNZ greenhouse gas conversion factors (2025): source of the default 0.18 kg CO₂/kWh grid factor
Frequently asked questions
How much energy do occupancy sensors save?
In this tool the occupancy saving is the vacancy share of lit hours × 0.90, applied to the load remaining after dimming and daylight harvesting. A meeting room empty for 60% of its lit hours saves about 54% of its remaining lighting energy; an office with 20% vacancy saves about 18%. Field results vary widely, so base the vacancy figure on how the space is really used.
How much does daylight dimming save?
The daylight saving is the reduction in artificial light while daylight is useful, multiplied by the share of operating hours it is useful, applied to the load remaining after dimming. A perimeter zone dimmed by 50% for 40% of the year saves 20% of its remaining energy; a deep-plan core with no useful daylight saves nothing. The daylight factor calculator shows how far into a room daylight reaches.
Are lighting controls required by Part L?
Approved Document L Volume 2 (2021), for buildings other than dwellings in England, sets minimum luminaire efficacy and expects new lighting to have appropriate controls, such as local switching, presence detection and daylight-linked control where the layout suits them; the alternative is to show compliance through a LENI calculation to BS EN 15193-1. Which controls a space needs depends on the building type and compliance route, so confirm with building control.
Can dimming, daylight and occupancy savings be added together without double counting?
Not by simple addition. This tool applies the strategies in sequence: dimming reduces the baseline, daylight harvesting acts on what remains, and occupancy sensing on what remains after that, so the same kWh cannot be saved twice. Because the three still overlap in practice, the interaction de-rate trims the total by a further 5–15% when strategies share the same luminaires.
What is a typical payback period for lighting controls?
Payback here is the controls cost divided by the annual cost saving, so it depends entirely on load, hours, tariff and quoted capex. Long operating hours, a high tariff and a space that is often empty give short paybacks; a small, well-daylit room with short hours may never repay a full DALI system. Beyond five years, consider phasing the work and use the lighting ROI calculator for a fuller appraisal.
Where does the CO₂ figure come from?
CO₂ saved is the total kWh saved multiplied by the grid carbon factor. The default 0.18 kg CO₂/kWh follows the DESNZ 2025 greenhouse gas conversion factor for UK grid electricity, which falls year on year as the renewables share grows; override it for another reporting year or a non-UK grid. Because the factor declines, today's saving will be worth fewer kilograms of CO₂ in future years.