Lighting Energy & Payback Calculator
Compare two luminaires, then estimate annual electricity use, costs and simple replacement payback.
Calculation inputs and results
Enable JavaScript to use the interactive calculator. You can also calculate directly: efficacy = lumens ÷ watts; annual kWh = watts × number of luminaires × hours per day × days per year ÷ 1,000; annual electricity cost = annual kWh × rate per kWh; simple payback = net project cost ÷ positive annual electricity savings.
Calculation method & comparison limits
Efficacy (lm/W) = complete-luminaire lumens ÷ total input watts. Use matching measurement conditions. CIE definition.
Percentage difference = (A − B) ÷ B × 100. Every percentage uses B as the baseline; a power reduction and an efficacy increase have different denominators.
Annual electricity use (kWh) = input watts × quantity × hours per day × days per year ÷ 1,000. This applies the DOE watt-hour conversion to the entered annual schedule. Values are rounded for display; calculations use the unrounded inputs.
Annual electricity cost = annual kWh × electricity rate per kWh. Annual electricity saving = B’s annual cost − A’s annual cost. A negative saving is shown as an extra annual cost.
Simple payback (years) = net project cost ÷ annual electricity saving, only when that saving is positive. Zero or negative savings give no electricity-based payback. This limited calculation excludes the broader cash flows and time value of money considered in a full retrofit financial analysis.
Lumens and efficacy do not establish task-plane illuminance, uniformity, glare or product suitability. Check photometry and the installation layout before treating luminaires as alternatives.