Saving Money on Greenhouse Lighting

Paul Morin offers a new plan for lighting in the greenhouse.

Greenhouse owners are always on the lookout for energy-saving ideas, and here's one that might benefit you. Normally every grower who uses auxiliary lights in his or her greenhouse will be using a time clock. I have one operating from 6 am to 8 pm, to which I added a photocell fixed on a pole outside the greenhouse. This causes the lighting system to turn off automatically when the sun reaches it, no matter if the time clock is still running. If the sky gets cloudy, the lights will turn on instantly if the timer is on. The sensitivity of the photocell - which can be rotated and adjusted to different angles - is a key element in the system. In winter, when the angle of the sun is low, I adjust the inclination of the photocell to the same angle. It is not necessary to move the unit very much for it to be effective. Orient the photocell to the southeast, so when the sun is high enough (about 8 or 9 am) the photocell will operate automatically to turn off the lights if it is sunny enough. Conversely, it will turn on the lights if the sky gets cloudy. Before beginning this project, consult with your local building official to find out if a permit is necessary or if there are special requirements. All wiring must follow the National Electrical Code (in the United States) or the Canadian Electrical Code (in Canada) and meet local electrical codes. Consult with an electrician to clarify any questions.

Equipment Required

The sales representative at your local hardware store can work with you to choose the best equipment. I completed my setup for $100 (1995). The following elements were involved: [1] Photocell (2,000-watt) with sliding device. This instrument has a 1/4-inch hole in the center and a sliding device to reduce the size of the hole that controls how much light reaches the cell. I use a 2,000-watt photocell to accommodate the total light wattage in my greenhouse (three 400-watt sodium lights). Photocells with higher and lower wattage are available; choose one suitable for the lighting system in your greenhouse.

  1. A rectangular waterproof electric box suitable for use outside. This will be positioned beneath the photocell and wired to it.
  2. Electrical-rated conduit, 1/2-inch diameter (looks like a metal pole). This extends from the exterior electric box to inside the greenhouse.
  3. UV-resistant electric wire (gauge 12 with three conductors with ground). This wire is run from the photocell through the conduit to inside the greenhouse where it is connected to the electric box and timer.
  4. Electric box. Inside the greenhouse, connect the electric wire to an electric box large enough to accommodate the wires. Prepare the box to receive the wires from the photocell and those from your greenhouse lights. Use the same gauge wire to link the electrical box to the time clock. Then plug the timer into a regular grounded house outlet.

Reprinted from Orchids July 1996.