How quickly does the swap pay for itself?

Divide the cost of the bulbs by the yearly saving. Ten LEDs at 4 each is 40, and against 186.15 a year that is paid back in under three months; against the one-hour-a-day case it takes just over a year. Both are good returns by any ordinary standard, which is why the useful question is not whether to switch but in what order — the fittings that are on longest repay first, and doing those before the rarely-used ones brings the whole saving forward by months.

Does the heat from old bulbs count as heating?

Partly, and only in winter. An incandescent bulb turns most of its electricity into heat, so in a heated room during the cold months that heat does displace some of what the heating system would have supplied — the saving from switching is genuinely smaller then. The offset disappears in summer, when the same heat has to be tolerated or removed, and it is worth little where the heating is cheaper per unit of heat than electricity, which is the usual case for gas or a heat pump. Over a full year the saving stands; it is simply not evenly spread across it.

The Bulb Worth Changing Is the One That Is On

Published 9/29/2026 · 3 min read · Everyday calculators

Marco Bianchi — Home, DIY & motoring writer at OneKitly

Renovation · Materials

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In short

The calculation is watts × hours × 365 × bulbs ÷ 1000 × the price of a kilowatt-hour. Ten 60 W bulbs running five hours a day burn 1,095 kWh a year, which at 0.20 per kWh is 219; the same ten in 9 W LED burn 164 kWh and cost 32.85, so the saving is 186.15 a year. The number that actually decides this is the hours, not the wattage difference: run the same bulbs one hour a day and the saving collapses to 37.23, because the wattage is only ever multiplied by the time it is switched on. That is the whole method for deciding which bulbs to replace first — walk the house and rank the fittings by how long they are lit, not by how bright or how old they look. The hall light nobody turns off is worth several bedroom lamps.

Ten 60 W bulbs on five hours a day cost 219 a year; the same ten in 9 W LED cost 32.85, a saving of 186.15. Cut the hours to one and the saving falls to 37.23 — the hours matter more than the wattage.

Match the lumens, not the watts

The 60 W and the 9 W in this calculation are only comparable because they produce roughly the same light. Watts measure the electricity going in, lumens measure the light coming out, and the equivalence that used to be automatic — everyone knew what a 60 W bulb looked like — stopped being reliable the moment efficiency started varying between products. Buy on the lumen figure printed on the box, keep the wattage for the running-cost arithmetic, and check the colour temperature separately: a replacement with the right lumens and the wrong colour will be judged a failure by everyone in the room regardless of what it saves.

The price per kilowatt-hour is the other lever

This example uses 0.20 per kilowatt-hour because a number is needed, but yours is on the bill and it varies by more than a factor of two between places and tariffs. At 0.35 the same swap saves 325.76 a year rather than 186.15 — the same bulbs, the same hours, a completely different case for acting today. Look up the figure before deciding, and take it from the total bill divided by the kilowatt-hours rather than from the headline unit rate, since standing charges and tiered blocks mean the two are rarely the same number.

Ten bulbs, 60 W against 9 W, at 0.20 per kWh
Hours a day	Old bulbs	Yearly saving
5	219.00	186.15
1	43.80	37.23

Worked with our own calculator

LED savings calculator

Given

Old bulb (W): 120
LED bulb (W): 18
Hours per day: 10
Number of bulbs: 20
Price per kWh: $0.40

Result

Old bulbs cost/year: $3,504.00
LED cost/year: $525.60
Yearly saving: $2,978.40

These figures are produced by the calculator below, not typed in by hand — they are recomputed whenever the tool changes.

Run it on your own figures →

Frequently asked questions

How quickly does the swap pay for itself?: Divide the cost of the bulbs by the yearly saving. Ten LEDs at 4 each is 40, and against 186.15 a year that is paid back in under three months; against the one-hour-a-day case it takes just over a year. Both are good returns by any ordinary standard, which is why the useful question is not whether to switch but in what order — the fittings that are on longest repay first, and doing those before the rarely-used ones brings the whole saving forward by months.
Does the heat from old bulbs count as heating?: Partly, and only in winter. An incandescent bulb turns most of its electricity into heat, so in a heated room during the cold months that heat does displace some of what the heating system would have supplied — the saving from switching is genuinely smaller then. The offset disappears in summer, when the same heat has to be tolerated or removed, and it is worth little where the heating is cheaper per unit of heat than electricity, which is the usual case for gas or a heat pump. Over a full year the saving stands; it is simply not evenly spread across it.

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Sources

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