When growers consider to purchase growth lights for their own plants, they may want to know how much does it cost .For grow light beginner,may it is confused that how to calculate the cost of grow lights.

In this article,we will explore how to start it.The cost of grow light is depending on many factors,including the type of grow light,the planting area,the type of plant you will grow and the height of plant area etc.

We should first know some light terms such as:PAR,PPFD and DLI before calculating the energy consumption and cost of the grow light required by the greenhouse or plant factory.

**PAR**

Photosynthetically Active Radiation is also called PAR. only sunlight in the 400-700nm wavelength range is effective for plant photosynthesis.The wavelength of red light is between 630-700 nm, far-red light is 705-740 nm. The blue light is between 400-450 nm and the ultraviolet wavelength is between 315-400 nm.

**PPFD**

Photosynthetic photon flux density is also called PPFD,which is a unit for measuring the intensity of light. The unit is micromole/per square meter/second (µmol/m2/s).

**Daily light integral (DLI)**

DLI measures the total PAR received every day. The unit is mol/m-2/d-1 per square meter per day. The concept of DLI is similar to rainfall, which means rainfall in a certain area per unit time.

**The importance of DLI in greenhouse**

DLI is an important measurement index in greenhouse plant， because it will affect plant growth, development, yield and quality. Commercial growers usually monitor and record the DLI received by plants, and then decide how to use grow light.

This is especially important in the high latitudes of the United States, because from December to March, and the DLI of outdoor sunlight is between 5 and 30 mol/m-2/d-1. In the greenhouse, the value will be decreased by about 40% because of the shading of the greenhouse cover, structure and other factor.

If there are no effective data,it is recommend to install a intelligent monitor, which can help you monitor the status of plant light, temperature, moisture and fertility etc, after regularly recording the value, we can easily to calculate the DIP and other data,click here to know more __DLI ____requirements for various greenhouse crops__.

**How to calculate the cost of growth lights?**

There are many type of grow lights in the market,such as HPS light,metal halide light and LEDs light etc.So,There are too many factors affecting the size of grow light. Generally, we can follow the rule of thumb, or accurately calculate the two method,click here to know more what size of grow light do i need?

Fox example:

If we are planning to grow tomatoes in an area with 1000 square meter,and choose 1000W HPS lights as grow light.

Assuming the condition of light are below:

Model（HPS） | Power | PPFD | Price(USD) |

1000W | 1085W | 2100 µmol/s | 120 |

**The HPS light coverage area**

If we are planning to grow tomato in greenhouse,It is an ideal to reach PPFD = 200 µmol/m2/s. In the case of all light only from HPS light, we can calculate the coverage area by each light.(Cover Reflection rate 80%，space utilization factor 60%，Maintenance factor 75%,Distance loss 30% at height 1.2 meter)

2100 µmol/s*0.8*0.6*0.75*0.7/ 200 µmol/m2/s = 2.6 m2

**2.The number of HPS light**

If we have an area with 1000 m2 greenhouse or plant factory,

1000 m2 / 2.6 m2/piece ≈ 385（approximately equivalent to 385 piece）

Note:Of course,This is an idealized conditions to calculate it easily . (Regardless of the average intensity of the light, the height of grow light and loos rate)

**3.The Initial investment cost**

120USD/piece*385=46,200 USD

The cost of 1000W high pressure sodium light may be higher or lower when comes from different manufacturers, here is just an example.

**4.Energy consumption cost**

The price of electricity varies depending on whether you use commercial electricity or residential electricity in different area(Suppose 6.5 cents per kWh).

If a HPS light is used for 1500 hours a year, then our electricity bill:385 pcs x 1.055 kWh/hour x 1,500 hours/year x 0.08 $/kWh = $48,741 / year

**D****L****I calculation**

Under normal situation, you can record the instantaneous PPFD of each hour, then convert it into mol/m2/hour, and then sum up to get the DLI of one day’s sunlight.

For example:

Time | Umol/m2/s PPFD | Mol/m2/hour |

7:00AM | 25 | 0.1 |

8:00AM | 78 | 0.3 |

9:00AM | 153 | 0.6 |

10:00AM | 228 | 0.8 |

11:00AM | 443 | 1.6 |

12:00AM | 780 | 2.8 |

1:00PM | 823 | 3.0 |

2:00PM | 667 | 2.4 |

3:00PM | 454 | 1.6 |

4:00PM | 231 | 0.8 |

5:00PM | 128 | 1.5 |

6:00PM | 34 | 0.1 |

Total mol/m2/day | 14.6 |

Similarly, if a grow light with 250 µmol/m2/s PPFD,it can provide the same DLI as daylight for 16 hours a day under ideal conditions.

(250 µmol/m2/s x 1 hour x 3,600 seconds/hour) / 1,000,000 µmol/mol= 0.9 mol/m2/hour

Light up for 16 hours a day

0.9 mol/m2/hour x 16 hours/day= 14.4 mol/m2/day PAR

Through the above DIL monitoring calculations, we can calculate the actual DIL every day, and then control the light time at different growth stages of plants.

In summary, the above calculations are only under ideal conditions. We did not consider HPS light cover reflectivity, loss light intensity of distance between light and plants and environmental factors, etc.

In actual applications, we should measure the photon flux of the canopy of plants. If there are too many lights, the light between the light fixture and the light fixture will be superimposed on each other.So, it will be higher than the PPFD of a single light in the actual measurement.

**Grow Light ****Coverage Area**** Calculator**

**By Coverage Area**

The below are the approximate maximum coverage areas for the wattages of HPS bulbs:

150 watt: 2 x 2 feet (or 4 square feet)

250 watt: 2.5 x 2.5 feet (or 6.25 square feet)

400 watt: 3 x 3 feet (or 95 square feet)

600 watt: 4 x 4 feet (or 16 square feet)

1000 watt: 5 x 5 feet (or 25 square feet or 2.3 square meter)

From above,we should be able to calculate the number of grow light you need.

Assuming our growing area is still 1000 square meters, It is prefer to choose 600W or 1000W HPS light to get higher efficiency.

**The number of grow light by c****overage ****a****rea**

1000 m2 / 2.3 m2/piece ≈ 435（approximately equivalent to 435 piece）

**By Wattage**

The other way to calculate the number of HPS light is to use the wattage requirement. To grow a plant with a high light requirement, like tomato, you want 30 to 50 watts per square foot of canopy.

I would go for a middle of 40 watts per square foot(bout 0.09 square meter).Let’s look at the example from above again. The one with the 1000 square meter area.

**The number of grow light by wattage**

We need a total about 444,444 wattage{(1000/0.09)*40 wattage=1000}

So,That means we would need 445 piece of 1000 watt bulbs (444,444/1000≈445)

If we work backwards, the previous method would give us a coverage of 40 watts per square foot (8 bulbs x 1000 watts = 80,000 total watts / 200 sq ft area = 40 watts per square foot). That is still plenty of light, but we would get better results with more light.

Through the above three different algorithms, we have found that the number of plant lights range from 385 to 445. The theoretical calculation value is obviously too small. Using the coverage area and the wattage per square are not much different, so we can adjust the number of grow light according to the calculated reference value in actual application.

When the budget is limited, we choose at least 400 piece of light. If the budget is good, we choose 420 piece. If the budget is enough, we choose 450 piece. As for the actual number, we can measure the photon flux of canopy of plant.

**Affect the cost of grow light factor**

**The type of grow light**

One is determining the cost of grow light depends on what type of grow light. Whether the spectrum of grow light can be adjusted and additional functions. Sometimes requirements for the growth light are not only that it can supplement the light for the plants, but also it is waterproof, fireproof, and anti-leakage. These material be made of light will affect itself price.

**The ****size of ****grow light**

Growing lights come in different power. The high power unit price is high but the number of use will decrease, while the low power one is cheaper, but you need to install more.in other words,The higher the power, the larger the range. Taking it into consideration can effectively reduce the cost of installing the light. So,choose the proper the number of grow light according to the greenhouse area.

**T****he height of the plant**

In general, If the plant is relatively tall, it is necessary to place the growth lights at different heights, so that the plant can absorb the light energy more evenly. If the plant is low, consider using a single tube,sometimes two tubes. In other words, the higher the plant, the higher the cost of the plant growth light.

**Wrapping up**

In this article,we have introduced three method to calculate the cost of grow light and how to use DLI.It is better to choose the number of light from the closer to the minimum light requirement to high light requirements according to your actual requirement.