Shorten a porch light’s overnight schedule
A household uses a 20 W porch light for 12 hours each night. They compare a timer that leaves it on for six hours during periods when people are most likely to arrive.
Open CalculatorInputs and calculation
| Tool input | Value |
|---|---|
| Currency | USD |
| Power unit | W |
| Power | 20 |
| Hours per day | 12 |
| Days or equivalent sessions | 30 |
| Duty cycle (%) | 100 |
| Energy price (USD per input kWh) | 0.3 |
| Alternative power (W) | 20 |
| Alternative hours per day | 6 |
| Alternative duty cycle (%) | 100 |
Baseline: 20 ÷ 1,000 × 12 × 30 × 100% = 7.2 kWh. Alternative: 20 ÷ 1,000 × 6 × 30 × 100% = 3.6 kWh. Multiply both by 0.3; the baseline minus the alternative is USD 1.08.
Modeled difference · USD 1.08
What the comparison shows
The light uses half the modeled energy after the timer change. The cost difference is small because the original draw was low.
Assumptions and limits
The example assumes a fixed timer and the same lamp. Confirm that the shorter schedule meets the household’s lighting needs.