Compare light output before comparing watts
Watts measure electrical power, not brightness. A lower-wattage lamp is only a useful substitute if its light output, distribution, color characteristics, dimmer compatibility, and fixture suitability meet the need.
ENERGY STAR explains the efficiency advantages of LED lighting and the importance of choosing an appropriate product. Actual performance depends on the lamp and how it is used; an energy-cost estimate does not certify a particular product. [1]
Use the rated input wattage of each option. A “60-watt equivalent” label describes an approximate brightness comparison, not a 60-watt electrical load.
Calculate annual energy use
For identical lamps:
Annual kWh = number of lamps × watts ÷ 1,000 × hours per day × days per year
Annual energy cost = annual kWh × electricity price per kWh
Use the marginal electricity rate relevant to the additional consumption, where known. A bill's average cost per kWh may include fixed charges that do not decrease when a lamp uses less energy.
For time-of-use pricing, calculate usage in each rate period separately if the difference matters. A single blended rate is an approximation and should be labeled as such.
Worked example: replacing ten lamps
Assume ten 60-watt lamps are replaced by ten 9-watt LEDs that meet the lighting requirements. They operate three hours per day for 365 days, and the assumed electricity price is $0.18 per kWh.
| Measure | Existing lamps | LED lamps |
|---|---|---|
| Total power | 600 W | 90 W |
| Annual electricity | 657.00 kWh | 98.55 kWh |
| Annual energy cost | $118.26 | $17.74 |
| Annual energy savings | — | 558.45 kWh |
| Annual dollar savings | — | $100.52 |
These are hypothetical operating assumptions, not a claim about every home or current local electricity prices.
Calculate simple payback with the right cost
If the ten LEDs cost $50 in total and no other immediate cost is incurred, simple energy-only payback is $50 ÷ $100.521, approximately 0.50 years or six months.
The appropriate cost depends on the decision. Replacing still-working lamps uses the full new purchase cost unless there is a relevant offset. Choosing between two new alternatives may use the incremental purchase cost between them.
Add installation, adapters, controls, or fixture replacement if needed. Subtract a rebate only after verifying the offer and eligibility. Do not assume that an advertised incentive applies to the selected product or purchaser.
Lifetime savings need a replacement model
A basic annual-energy calculation does not include failures, replacement labor, changing electricity prices, financing, or the time value of money. A more complete life-cycle comparison places each expected cost and saving in the year it occurs.
Rated life is not a promise that every lamp will last exactly that long in every fixture. Heat, controls, usage, and product quality can affect actual performance. Avoid multiplying one optimistic lifespan by annual savings and labeling the result guaranteed profit.
In a commercial space, maintenance access and labor may matter as much as lamp price. In a home with very low-use fixtures, the energy-only payback may be slower even when the wattage reduction is substantial.
Check the edge cases
At zero operating hours or a zero electricity rate, the model produces no energy-cost savings and therefore no finite energy-only payback for a positive purchase cost. If the replacement uses more watts, the energy difference is a cost increase rather than a saving.
Controls can also change hours. Compare the complete before-and-after setup rather than attributing every change to the lamp itself.
Use the result as a transparent estimate: these lamps, these hours, this rate, and this purchase cost. That statement is more actionable than a broad percentage claim without a usage basis.
Frequently asked questions
Should an energy-cost comparison use equivalent wattage or actual wattage?
Use actual electrical input wattage. A “60-watt equivalent” LED may draw far less than 60 watts.
Does the payback include replacement labor and rebates?
Only if you include them in the net incremental cost input. The energy calculation itself does not verify rebates or model future maintenance.
When is there no finite energy-cost payback?
A positive cost cannot be recovered through energy savings when modeled annual dollar savings are zero or negative.
Sources & calculation notes
Primary references are linked below. Dates, limits, and product terms can change; confirm the applicable details before acting.
Use this guide thoughtfully. Quantity and budgeting guidance only. These calculations do not approve a structural design, determine code compliance, or establish a safe work method. Confirm plans, product instructions, and applicable requirements with qualified professionals.