How much light do I need in the greenhouse?
Nothing will take the place of sunlight but sometimes, due to the shorter days of winter or greenhouse placement you need supplemental lighting. Greenhouses should receive at least 6 hours of direct sunlight a day. If you get less than that you should add lighting.
Symptoms of low light are slow growth, elongated or spindly plants, and plants bending towards the light source.
Plants use mostly the red and blue spectrums of visible light. Incandescent bulbs don't efficiently supply the needed light for plants. A good way to get full spectrum light is to use one 'cool white' and one 'daylight or warm white' fluorescent bulb or special grow light (full spectrum) bulbs. These bulbs can be used in an inexpensive 'shop light' or you can purchase specially made grow light fixtures. Remember that fluorescent bulbs must be within a 3-4 inches of the plant to be an effective source of light. They work best for seedlings or small plants for this reason.
There are special 'High-Intensity Discharge Lamps' for the serious (and rich) greenhouse grower. These are expensive to buy and more expensive to use. They are beyond the scope of most hobbyists.
Light is measured in foot-candles or lumens depending on how you're measuring. If you are looking at the light source itself it's measured in lumens, if you're looking at the measurement at the plant itself it's measure in foot-candles.
Here are some average light requirements:
Low light houseplants 100-200 foot-candles,
Most seedlings and flowering bulbs 750 foot-candles,
Orchids, roses, succulents 1,500 foot-candles,
Leafy vegetables 2,800 foot-candles,
Fruits and vegetables 5,000 foot candles,
You can purchase light meters in greenhouse supply stores to help you determine if you are getting enough light for your environment.
Remember, nothing beats sunlight so plan your greenhouse to get as much natural light as you can.