Two numbers matter. Watts describe how much power the lights draw at a given moment. Kilowatt-hours (kWh) describe how much energy they use over time, and kWh is what appears on a power bill.
The conversion is:
kWh = watts x hours on / 1000
So 100 watts running for 10 hours uses 1,000 watt-hours, which is 1 kWh.
Addressable LED systems list power in a few ways: watts per pixel, watts per foot, or watts per section at full brightness. Use the number from your system's spec sheet, and note what setting it describes. Full-brightness white, with every LED on, is usually the highest draw. Single colors and dimmed settings draw less.
To get total watts:
Total watts = watts per pixel x number of pixels
Or, if the sheet lists watts per foot:
Total watts = watts per foot x feet of roofline
The numbers below are illustrative, chosen to make the math easy. They are not measurements of any product. Plug in your own spec sheet values.
Suppose a 150-foot roofline uses 6-inch spacing. That is 2 pixels per foot, or 300 pixels. If the spec sheet says 0.3 watts per pixel at full white:
300 x 0.3 = 90 watts at full white
Run that for 6 hours a night:
90 x 6 / 1000 = 0.54 kWh per night
Over a 30-day month:
0.54 x 30 = 16.2 kWh per month
If you dim to half brightness most nights, draw falls roughly with it, so the same month might be closer to 8 kWh. LED drivers are not perfectly linear, so treat that as an estimate.
To compare, find the wattage of a familiar appliance and run the same formula. A single 60-watt incandescent bulb left on for 6 hours uses 0.36 kWh. That makes it easy to see how a whole roofline at a low everyday setting compares to a few old porch bulbs.
Find your rate per kWh on your electric bill, then multiply it by the monthly kWh from the math above. Your utility's rate, plus any fixed charges, is the only thing that turns kWh into a bill line, so do that step with your own bill in hand.
Also check how much the controller and power supplies draw when the lights are off. Standby draw is small, but it runs all day, all year. Your spec sheet or installer can tell you the figure.
If you want help reading a spec sheet for a Nashville-area home, Permanent Lighting Solutions is where to reach us.