Tools
Energy cost and payback calculator
What the lighting costs to run now, what it would cost after, and how long the difference takes to pay for the change.
The arithmetic is straightforward: fixtures multiplied by watts, multiplied by hours per day, multiplied by 365, divided by 1,000 gives kilowatt hours a year. Multiply by your rate per kilowatt hour and you have the annual cost. Your actual rate is on your utility bill and is usually a little higher than the headline number once delivery charges are included.
What the calculator leaves out generally works in your favour. It ignores labour, which matters on a large retrofit. It ignores the reduction in air-conditioning load that comes from fixtures running cooler, which is not trivial in South Florida. It ignores lamp replacement cost and the labour of getting to a fixture on a high ceiling. And it ignores utility rebates, which for commercial projects can cover a meaningful share of the changeover.
For commercial work the honest answer is that a payback under two years is usually easy to justify, and most lighting retrofits in older buildings land well inside that once controls are included.
Before you rely on it
This is arithmetic, not a photometric calculation. It will get you to a sensible starting point and tell you when something is obviously wrong, but a real layout gets run in AGi32 against the actual IES files. Send me the plan and I will run it properly.