Size an LED grow light by canopy area and see what running it 12–18 hours a day actually costs — the two questions every indoor grower asks.
Sizing light for plants
Indoor plants live on light the way outdoor plants live on weather — and the working rule for modern quality LEDs is 25–40 watts per square foot of canopy: the low end for herbs and leafy greens, the high end for fruiting and flowering plants. Enter the canopy footprint, a watts-per-square-foot target, the daily photoperiod and your electricity rate, and this calculator returns the fixture size and the honest running cost.
How to use it
- Enter the canopy area the light must cover — the plants’ footprint, not the room’s.
- Pick a watts/sq ft target for what you’re growing.
- Set the daily hours (leafy ≈16, fruiting stages often 12) and your $/kWh.
The formulas
A worked example
A 2 × 4 ft canopy at 30 W/sq ft wants a 240-watt LED. Sixteen hours a day is 3.84 kWh — about $20 a month at $0.17/kWh, and roughly $235 a year: for most hobby growers the electricity quietly costs more than the light did. The wattage rule is a proxy for the real currency — photon flux (PPFD) — so between two fixtures of equal watts, the one publishing a proper PPFD map is the one built by people who measure.