How to use it
- Enter the number of bulbs you plan to replace and the power of an old bulb and its LED replacement.
- Enter the hours per day they are on and your price per kWh.
- Enter the price and rated life of one old and one LED bulb. If you do not care about replacement costs, set both prices to the same value.
- Read the yearly savings, the payback in months and the net saving over ten years.
How it is calculated
- Hours per year = hours per day x 365.
- Energy saved (kWh) = bulbs x (old W - LED W) x hours per year / 1000.
- Electricity saved = energy saved x price per kWh.
- Bulb replacement cost per year = bulbs x hours per year x bulb price / rated life.
- Savings from fewer bulbs = old replacement cost - LED replacement cost.
- Payback (months) = bulbs x LED price / yearly savings x 12.
Worked example
Ten 60 W bulbs replaced with 9 W LEDs, 3 hours a day, electricity at 0.15 per kWh, LED 3.00 each, old bulbs 1.00 each with a 1,000 h life and a LED life of 15,000 h. Hours per year: 1,095. Energy saved: 10 x 51 x 1,095 / 1000 = 558.45 kWh, worth 83.77. Fewer bulbs to buy: 10.95 - 2.19 = 8.76. Savings per year 92.53. The LEDs cost 30.00 so they pay back in about 3.9 months, and over ten years you save 895.28 net.
Tips
- Start with the lamps that are on the longest, such as kitchen and hallway lights; they pay back first.
- Rated life is a manufacturer figure, so real bulbs may last more or less.
- Check dimmer compatibility before buying LEDs for dimmed fittings.