LED Retrofit Savings and Payback Calculator
Energy, cost saving and payback from replacing old fittings with LED. With the formula, a worked example and typical values.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
This page calculates energy saved per year, cost saved per year and simple payback from number of fittings, existing fitting power, LED fitting power, operating hours per year and 2 more inputs. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Tariff and cost use any one currency. Include demand charges for a fuller picture.
Cross-check: 8 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Financial estimate from user-entered tariffs.
What this calculator checks
- The arithmetic of the simplified model for the values you entered.
What remains unverified
- Whether the simplifying assumptions listed on this page hold for your case.
- Any standard, manufacturer or site data. Treat the result as an estimate and confirm it by a further engineering check.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | Not applicable: Energy savings and simple payback are arithmetic estimates. Tariff, operating hours, lamp life and project costs are site-specific; no single standard governs the payback equation. |
| Standard review state | Not applicable |
| Scope-screening note | not recorded |
| Cross-check test cases | 8 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Number of fittings | 100 |
| Existing fitting power | 150 W |
| LED fitting power | 60 W |
| Operating hours per year | 4,000 |
| Energy tariff (per kWh) | 8 |
| Cost per new fitting | 2,500 |
- Saved = 100 × (150 − 60) W × 4,000 h = 36,000 kWh
- Cost saved = 36,000 × 8 = 288,000
- Payback = 250,000 / 288,000 = 0.87 years
Assumptions
- Fixed hours and tariff
- Maintenance and lamp-replacement savings not included (they shorten payback)
Tariff and cost use any one currency. Include demand charges for a fuller picture.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Number of fittings | (number) | 1 or more, up to 100,000 | |
| Existing fitting power | W | greater than 0 W | |
| LED fitting power | W | greater than 0 W | |
| Operating hours per year | (number) | 1 or more, up to 8,760 | |
| Energy tariff (per kWh) | (number) | greater than 0 | |
| Cost per new fitting | (number) | 0 or more |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Energy saved per year | kWh | Shown to 0 decimal places | |
| Cost saved per year | (number) | Shown to 0 decimal places | |
| Simple payback (years) | (number) | Shown to 2 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with existing fitting power
| Existing fitting power (W) | Energy saved per year (kWh) | Cost saved per year | Simple payback |
|---|---|---|---|
| 75 | 6,000 | 48,000 | 5.21 |
| 150 | 36,000 | 288,000 | 0.87 |
| 300 | 96,000 | 768,000 | 0.33 |
| 600 | 216,000 | 1,728,000 | 0.14 |
| 1,500 | 576,000 | 4,608,000 | 0.05 |
Other inputs are the same as in the worked example above.
Standards and further reading
- EN 12464-1 (lighting of indoor workplaces)
- CIE publications on photometry
Lamp and luminaire data come from the manufacturer; the illuminance targets that apply come from your project specification and local regulation. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter number of fittings, existing fitting power, lED fitting power, operating hours per year, energy tariff (per kWh) and cost per new fitting. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Fixed hours and tariff. Maintenance and lamp-replacement savings not included (they shorten payback).
Which calculators should I use next?
Common next steps are Appliance Running Cost (Energy and Cost) and Lighting Level (Lumen Method). The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 8 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Energy and Cost Formulas: kWh and Payback: Energy and cost formulas for kWh from power and hours, running cost, efficiency and payback of an upgrade
- Lighting Formulas: Lux, Lumens and Candela: Lighting formulas for lux, lumens and candela: inverse square law
- LED Formulas: Resistor Value and Power: LED formulas for the series resistor from supply voltage, forward voltage and current, resistor power, LEDs in series
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