Calculators

Retrofit payback calculator

Simple payback for a lighting retrofit including the lines that are usually left out — access cost, avoided relamping, ballast losses and controls savings. Runs in your browser and shows every formula it applies.

Written by Long-Term New Media Limited · Last reviewed August 2026

Most retrofit business cases are built from two numbers: old watts, new watts. That comparison reliably produces a payback figure that is too optimistic on energy and too pessimistic on everything else, because it omits the access cost that dominates high-level installations and the avoided relamping that often matters more than the electricity.

The formulae

The four inputs that decide the answer

Circuit watts, not lamp watts

A twin 36 W fluorescent fitting is not a 72 W load. Magnetic ballasts add meaningful losses; electronic ballasts add less but not zero. Using lamp ratings understates the existing consumption and therefore understates the saving — usually by enough to matter. Where the number is worth getting right, clamp the circuit and measure a sample of fittings rather than adding up lamp labels.

Access is a one-off and it is often the largest single line

Reaching a 9 m high bay costs the same whether you are fitting a tube or a luminaire. On high-level and out-of-hours work, access can exceed the hardware cost outright, and once it is in the model the argument for doing the cheap version of the job largely collapses — you are spending the expensive part either way.

Avoided relamping is real money and gets forgotten

A site on a three-year group relamp cycle is paying for lamps, labour and access every three years. Removing that obligation is a genuine annual saving that belongs in the model. It also disappears from the model as soon as someone argues the LED product needs no maintenance — which is only true if nobody ever cleans it, and the maintenance factor discussion in maintained versus initial illuminance explains what that costs in light.

Controls savings belong to the replacement route only

A tube swap saves the wattage difference and nothing else. A new luminaire with presence detection and daylight linking saves the wattage difference plus a proportion of the running hours. In a space with real daylight and intermittent occupancy that proportion is substantial. If you are comparing routes, the controls percentage must be zero for the swap and non-zero for the replacement, otherwise you are not comparing the two things you are choosing between.

What simple payback deliberately ignores

This calculator gives simple payback, which is what most approval processes ask for and what most people mean. It is not a financial appraisal. It omits:

• The time value of money. No discounting; no NPV or IRR. For short paybacks the difference is small, for long ones it is not.
• Tariff structure. A single average price per kWh. Real tariffs have capacity charges, time-of-use bands and — for lighting that runs at peak — sometimes demand charges that change the answer materially.
• Cooling interaction. Lower lighting load means less heat rejected into the space, which reduces cooling energy in a conditioned building and slightly increases heating energy in a heated one. In a cooling-dominated climate this is a real additional saving that this model does not credit.
• Lifetime and replacement. A ten-year view that assumes nothing fails. Driver failures within that window are the usual reality; see driver matching for why driver life, not LED life, sets the interval.
• Incentives. Utility rebates, tax treatment and grant schemes change the capital line significantly where they exist.
• The reason to do it that is not money. Light quality, glare, uniformity and controllability. A retrofit that pays back in six years and fixes a space people complain about may be a better decision than one that pays back in three and does not.