Choosing a Lamp for a Greenhouse
A greenhouse grower wants supplemental lighting for seedlings over a 14-hour photoperiod in winter. The bench is 4 m long and 1.2 m wide. Three options are available:
- Option A: 400 W high-pressure sodium (HPS), about 25% of input energy emitted as visible light, mostly orange-red.
- Option B: 320 W white LED array, about 40% of input energy emitted as visible light, broad spectrum including blue.
- Option C: 500 W incandescent, about 5% visible light, mostly red and infrared.
Electricity costs $0.14/kWh. The grower runs lights 14 h/day for 120 days. The seedlings are compact and need strong blue light to avoid stretching.
1. Calculate the electricity cost over 120 days for each option and compare them.
Read the worked answer
Energy per day (kW × hours):
- A: 0.400 kW × 14 h = 5.6 kWh/day
- B: 0.320 kW × 14 h = 4.48 kWh/day
- C: 0.500 kW × 14 h = 7.0 kWh/day
Energy over 120 days:
- A: 5.6 × 120 = 672 kWh
- B: 4.48 × 120 = 537.6 kWh
- C: 7.0 × 120 = 840 kWh
Cost at $0.14/kWh:
- A: 672 × 0.14 = $94.08
- B: 537.6 × 0.14 = $75.26
- C: 840 × 0.14 = $117.60
Option B is the cheapest to run, saving about $19 per season versus A and about $42 versus C. These figures cover electricity only; purchase price, lamp replacement, and cooling load from waste heat would change the total cost of ownership.
2. Recommend a lamp, going beyond cost to consider light quality and heat.
Read the worked answer
Recommend Option B (white LED).
- Light quality: the seedlings need blue light to stay compact. LEDs provide a broad spectrum including blue; HPS is dominated by orange-red and tends to promote stretching in blue-deficient conditions; incandescent is almost all red and infrared and would cause severe etiolation.
- Efficiency: at 40% visible-light conversion, LEDs deliver more usable photons per dollar of electricity than HPS (25%) or incandescent (5%). The incandescent wastes 95% of its input as heat.
- Heat load: incandescent and HPS dump large amounts of waste heat onto a 4.8 m² bench, which can scorch seedlings and raise cooling costs. LEDs run cooler at the canopy.
- Practical note: because the bench is small, the LED's lower wattage still provides adequate intensity if mounted at the recommended height; the grower should confirm with a light meter rather than assume.
Cost alone would also favour B, but the decisive factor is spectrum: only B supplies the blue light the compact seedlings require.