The light math
Plant light is counted in photons, not lumens. PPFD is how many micromoles of photosynthetic photons land on a square meter each second. Multiply by the seconds of light in a day and convert to moles, and you get the daily light integral (DLI): DLI = PPFD x hours x 3600 / 1,000,000.
Leafy greens such as lettuce are commonly grown at a DLI in the low to high teens. The range shown here is a general guideline for teaching, not a recipe for a particular cultivar.
Try it: hold the DLI near 14 two different ways, bright for fewer hours or dimmer for more hours. Longer, dimmer days often use the LEDs more efficiently because leaves saturate at high intensity.
Energy is the real budget
On a spacecraft every watt counts. The energy readout assumes LEDs that turn electricity into plant light at about 2.5 micromoles per joule over a 0.17 square meter growing area. Both numbers are simplified assumptions for this model.