Input watts per fixture
Input W = lamp W x (1 + loss fraction)
Driver or ballast losses are added to the lamp or LED load to reach the watts at the panel.
Guide
Lighting Running Costs: kWh, Rates, Emissions, and What Controls Really Save
How to get from connected watts and hours to an annual bill you can defend, and where controls change the number most.
Get the connected load from the panel, not the box
Connected watts are the input watts on the fixture schedule, including the driver or ballast loss, and they should reconcile with the panel schedule. Nameplate lamp watts understate the load by five to fifteen percent depending on the technology, and that difference flows straight into the kWh figure.
Hours are the number worth arguing about
Operating hours come from the schedule, the occupancy pattern, or a log, and they vary more than any other input. A retail floor at twelve hours a day and three hundred and sixty days is four thousand three hundred hours; a warehouse single shift is nearer two thousand five hundred. Using the wrong profile changes the answer by tens of percent, so write down the basis and keep it with the analysis.
Use the all-in price from your own bill
National average prices are a starting point, but commercial accounts add demand charges, fixed fees, and sometimes time-of-use periods. Lighting is a daytime load in most buildings, which is exactly when many commercial tariffs are most expensive, and demand charges add to the first-year saving whenever connected kilowatts come down.
Model controls as a reduction in hours or output
Occupancy sensing removes hours, daylight dimming reduces output when there is daylight, and scheduling removes whole periods. Apply them as multipliers on the load or hours, then sanity check the total: a saving above about forty percent in a typical office usually means two of the assumptions are counting the same effect.
Emissions follow the grid, not the fixture
Kilograms of CO2 per kWh come from the generation mix serving the building, so the same saving is worth different carbon in different regions. Report the emission factor and its source with the result, and avoid claiming a carbon number that is really a national average applied to a local project.
Formulas used
Annual energy
kWh = fixtures x Input W x hours / 1,000
Connected kilowatts times annual operating hours, with any controls reduction applied to the hours or the output before multiplying.
Annual cost
Cost = kWh x price / 100
Price in cents per kWh from the actual tariff, not from a national average, once the analysis is being used for a decision.
Worked examples
Two hundred office fixtures
Inputs: 200 fixtures at 40 W, 10 percent driver loss, office profile of 2,500 hours, 13 cents per kWh.
Input watts are 44 per fixture, so the connected load is 8.8 kW. At 2,500 hours that is 22,000 kWh and, at 13 cents, $2,860 a year, or $14.30 per fixture.
The same fixtures with occupancy and daylight control
Inputs: The same 22,000 kWh baseline with a 25 percent combined controls saving.
Energy falls to 16,500 kWh and the bill to $2,145, a saving of $715 a year. Applied to the same fixture population it is 7.8 cents per fixture per day, which is the figure that makes a controls quote easy or hard to justify.
Questions readers ask
Should I use the retail or the commercial price for an office building?
Neither national average belongs in a final decision. Take the blended rate from a year of bills for that account, include demand charges where they apply, and record the source. Average prices are only useful for a first screen.
How much can lighting controls really save?
Occupancy sensing in intermittently used rooms, daylight dimming at the perimeter, and scheduling outside opening hours each contribute in the range of ten to thirty percent on their own, and rarely add up beyond about forty percent on a single load. Stacking savings on the same hours is the commonest way to overstate a controls case.
Does a lower connected load reduce the demand charge?
Only if the lighting is running during the demand window. In offices, lighting coincides with the peak, so a kilowatt removed from the lighting load usually removes a kilowatt from the billing demand, which is why demand charges belong in the business case.
Put an annual cost on your lighting load
Sources
- Electric Power Monthly U.S. Energy Information Administration · 2026-10-15
- Lighting and energy saving resources U.S. Department of Energy, Energy Saver · 2026-10-15
- Electricity Data Browser U.S. Energy Information Administration · 2026-10-15