LED Retrofit Savings Calculator
Work out what replacing existing light fittings saves, group by group. Enter how many fittings there are now and after, the input watts of each fitting including its ballast or driver, the hours they burn, your tariff, any change in maintenance cost and the installed cost.
What this tool estimates
Annual kWh and rupee savings from replacing light fittings, from each group's quantities, system watts and burning hours, net of any change in maintenance cost — and the simple payback when that net saving is positive.
What it cannot decide
- Whether the new fittings give the same maintained light level, uniformity and glare control — that needs a lighting calculation or lux readings.
- Savings from controls such as occupancy sensors or daylight dimming: hours stay the same before and after.
- The effect of driver power factor and harmonics on your panels, or any change in demand charges.
- How long the fittings will last or how they perform against their warranty.
Result
Check the light, not just the watts. The saving only holds if the new fittings give equivalent maintained illuminance at the working plane, with acceptable uniformity and glare. Verify with a lighting calculation or lux readings before relying on it.
| Group | Existing kW | New kW | Hours | Saved kWh |
|---|
| Scenario | Net ₹ / year | Payback |
|---|
Method version 6 October 2026. A screening estimate, not a design: see the limits below.
Check before you trust the saving
- Light level. Measure lux at working height before and after — BEE's lighting audit method does this with a lux meter alongside input-power readings — or run a lighting calculation for the new fittings.
- Like for like. Compare fittings on the light delivered to the task, uniformity, glare and colour rendering — not on watts or the lamp's label.
- Input watts. Take the whole fitting's input from a measurement or the datasheet, driver or ballast included.
- Hours. Use burning hours from the shift pattern or a logger; lights left on through breaks count.
- Drivers. Power factor, harmonic distortion and surge withstand affect your panels and the fittings' life — ask for the driver datasheet.
How this calculator works — and where it stops
What it calculates
Annual electricity and rupee savings from replacing existing light fittings — typically with LED — group by group, from the number of fittings, their input watts including ballast or driver, and their burning hours; net of any change in maintenance cost, with the simple payback when that net saving is positive. Negative savings are shown as they are.
Inputs
- For each group (an area or a fitting type): the number of existing fittings and the system watts of each; the number of replacement fittings and the system watts of each; the burning hours per year.
- System watts: the input of the whole fitting — lamp plus ballast, choke or driver — measured or from the datasheet, not the lamp's label.
- Electricity rate — ₹/kWh, or ₹/kVAh with the average billing power factor.
- Change in annual maintenance cost (new minus existing; negative if it falls) and the installed cost.
Method
- Existing kW = existing fittings × system watts ÷ 1,000; replacement kW = replacement fittings × system watts ÷ 1,000.
- kWh saved in each group = (existing kW − replacement kW) × burning hours; the total is the sum of the groups. A group where the replacement draws more gives a negative saving.
- Value = kWh saved × rate (on a kVAh tariff, kWh ÷ power factor first); net saving = value − change in maintenance cost.
- Simple payback = installed cost ÷ net saving, reported only when the net saving is positive.
- Sensitivity: 20% fewer burning hours, replacement fittings drawing 10% more than entered, and the rate 10% lower and higher.
Assumptions
- Burning hours are the same before and after, so the saving comes from watts alone. Controls are a separate measure.
- The replacement gives equivalent maintained illuminance where it matters — the tool does not check this.
- The rate is the marginal energy charge; demand charges do not change.
- The saving is the same every year: no change in hours, tariff or fitting performance is modelled.
Limitations
- No light-output, uniformity, glare or colour-rendering check. Verify maintained illuminance at the working plane with a lighting calculation or lux readings before relying on the saving.
- No standard saving percentage is assumed: every figure comes from your own fitting counts, wattages and hours.
- Driver power factor and harmonics, and the lower heat gain in air-conditioned spaces, are not modelled.
- Simple payback ignores the time value of money, financing and fitting life.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- One group: 100 existing fittings at 250 W each (system watts)
- 100 replacement fittings at 100 W each (system watts)
- 3,000 burning hours a year
- Electricity rate ₹8 per kWh
- Change in maintenance cost ₹0
- Installed cost ₹7,20,000
Working
- Existing load = 100 × 250 ÷ 1,000 = 25 kW; replacement load = 100 × 100 ÷ 1,000 = 10 kW.
- kWh saved = (25 − 10) × 3,000 = 45,000 kWh a year — 60% of these fittings' electricity.
- Value = 45,000 × ₹8 = ₹3,60,000 a year; with no maintenance change, the net saving is the same.
- Simple payback = ₹7,20,000 ÷ ₹3,60,000 = 2.00 years.
Result. 45,000 kWh and ₹3.60 lakh a year, paying back in 2 years before any maintenance change — provided the new fittings give equivalent maintained illuminance.
Sensitivity — what moves the answer
- Hours pass straight through: 20% fewer burning hours gives 36,000 kWh and a 2.50-year payback.
- If the replacement fittings draw 10% more than entered, the saving falls to 42,000 kWh and the payback stretches to 2.14 years.
- The rate moves the rupees, not the kWh: 10% lower gives a 2.22-year payback, 10% higher 1.82 years.
- A maintenance saving of ₹20,000 a year lifts the net saving to ₹3,80,000 and shortens the payback to 1.89 years.
How engineers use the result
To check a lighting-retrofit proposal's saving claim against the fitting counts, input watts and hours that actually apply, and to see how much of the payback rests on the hours. The light-level check comes next: the saving only counts if the new layout meets the task's illuminance.
When a professional design must replace it
Before ordering: the existing input watts and hours should be measured or taken from the fitting schedule and the shift pattern, the replacement checked by a lighting calculation — or a trial area with lux readings — for maintained illuminance, uniformity and glare, and the drivers' datasheets reviewed.
Sources
- BEE — Lighting System (Chapter 8 of the Bureau of Energy Efficiency's guide book on energy efficiency in electrical utilities) — §8.5 study method: inventory each location's lamp and ballast ratings, numbers and hours of use; measure lux at the working level and input power at the distribution boards; replace lamps with due consideration to the luminaire, colour rendering, lux level and expected life
- BEE — Energy Efficiency Solutions for Existing Buildings: Commercial, four climates (study by TERI for the Bureau of Energy Efficiency, first printed February 2025) — Table 1 lighting data for a retrofit: installed lamp types, the wattage of each, the total number of fixtures, usage hours per year and existing controls
Questions people ask before using it
What information do I need before using this LED retrofit calculator?
A fitting schedule for each area: how many fittings there are, what lamp and ballast or driver each has and its input watts (measured, or from the datasheet — not the lamp's label), the burning hours per year, the replacement fittings' quantities and input watts from the proposal, your energy rate per kWh or kVAh, the expected change in maintenance cost and the installed cost.
Which site conditions could change the LED retrofit savings?
The real burning hours (shifts, holidays, lights left on), supply voltage, which changes what magnetic ballasts draw, mounting height and layout if the fitting count or positions change, dust and heat that affect light output and driver life, and whether the space needs the same light level as today — an under-lit area may need more light, not less.
What should I send an engineer for a project-specific LED retrofit review?
The existing fitting schedule with photos of nameplates, the lighting layout drawings, any lux readings at the working plane, the operating schedule, details of the lighting distribution boards and their metering, recent electricity bills, and the replacement proposal with photometric data and driver datasheets.
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