Start with the lights you actually use
Read the electrical input watts for one complete string or device, or use an appropriate measured average. Count identical strings together. LED is a product description, not a wattage: two strings with different lengths or ratings need their own inputs.
Gather five numbers before opening the calculator: watts per string, identical string count, actual on-hours per night, operating days in the modeled month and your electricity price per kWh. Replace the illustrative starting values in the tool.
A worked December example
Imagine six identical 12-watt strings lit for eight hours on each of 31 December nights. These are fictional inputs, not a claim about a particular product or an average household. The combined input is 72 W, or 0.072 kW. Energy is 0.072 × 8 × 31 = 17.856 kWh.
At an illustrative $0.20 per kWh, the energy charge rounds to $3.57 for that month. In the Equipment Electricity Cost tool, enter 12 W, six devices, 100% average fraction, eight hours, 31 days, 0.20 per kWh and zero additional monthly charge for this incremental-use example.
Compare a shorter schedule on the same basis
With the same strings, rate and 31 days, five hours per night uses 11.16 kWh and costs $2.23. The difference between the displayed monthly charges is $1.34. That is a scenario comparison, not a forecast of your bill.
For mixed lights, calculate each wattage-and-schedule group separately and combine compatible monthly energy amounts. A display running across November and December needs separate month scenarios; one field accepts at most 31 operating days.
Read the monthly output for a seasonal display
Use the monthly energy and monthly cost outputs. The annual output repeats the entered month twelve times; it is not the cost of a Christmas season. Do not present twelve identical Decembers as a household annual forecast.
Use 100% when the watts already represent average draw or the entered hours are actual on-time. Reducing both the hours and load fraction for the same timer schedule counts the reduction twice. Add a monthly charge only if it is attributable to this comparison, rather than allocating the whole utility standing charge to the lights.
Know what this calculation leaves out
The flat-rate model does not reproduce time-of-use billing, tiers, demand charges, taxes or other unentered charges. If your rate varies by time, use the Time-of-Use tool with compatible total energy and your actual tariff assumptions.
This guide estimates energy cost. It does not determine electrical loading, outdoor suitability or safe installation. Save or print your chosen inputs with the result so you can distinguish the example from your actual display.
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