Power Requirements for Greenhouse Controllers

A careful analysis of power requirements can help optimize the power installation for a greenhouse operation. The goal should be to size the power feeds properly to avoid overspending on unnecessary capacity on one hand and inadequate power availability on the other. The following information is to serve as a general guide that will have different applications depending on local conditions, electrical codes, etc.

The two primary considerations for powering greenhouse controllers are total power consumption and capacity. Total power consumption is important when considering the greenhouse’s economics, and capacity is important in determining the size of the power feeds.

Controllers

Controller power consumption and demand are straightforward; the controller runs constantly and consumes a predictable amount of power. The two power-consuming elements of an Advancing Alternatives controller are the display and control circuitry and the relays that control outputs. The relay current varies with the number of active outputs and is usually greater than the display current. The approximate power consumption of Advancing Alternatives controllers (DISREGARDING motor current) with all outputs active is:

• VCU2-24 – 8W
• AegisTec – 14W
• AegisTec Plus – 20W

Vent Motors

Vent motor current is very intermittent and difficult to characterize; each installation has its own characteristics. By “vent motors”, we are discussing motors that are used to roll up side curtains, or to raise ridge vents, etc. It is important to understand that the power consumption of the motors is directly related to the load placed on the motor; a shorter curtain, for example, will consume less power to roll than a longer one. For a rough estimate, or a starting point for calculating motor power consumption, the typical consumption for various Advancing Alternatives low voltage DC motors under a moderate to full load is:

• LVM-60 – 95 W
• LVM-100 – 125 W
• LVM-180 – 200 W
• LVM250 – 300 W

When calculating power requirements for the motors, the fly in the ointment is duty cycle. How often and for what duration do the motors run? Every site and every greenhouse is different. The simplest example is one greenhouse with two side vents that run simultaneously. In this case, the necessary capacity is the total of the consumption of the controller plus the two motors. The total consumption over a period of time depends on the duty cycle of the vent motors. An interesting side note here is that in a typical one-greenhouse installation, the consumption over time is greater for the controller than for the motors since the motors run a small fraction of the time.

For larger installations with multiple greenhouses, it may not be economical or necessary to install power feeds that provide the capacity to run all motors simultaneously; this is where local conditions come into play. Fast temperature changes will cause more motors to run simultaneously, however with the individual greenhouses it remains unlikely that all motors will run at once.

There is no universal standard for calculating feed capacity for multiple loads, however one common method of estimating is:

Total Power Requirement = (Load 1 Power x Load 1 Duty Cycle) + (Load 2 Power x Load 2 Duty Cycle) + … + (Load X Power x Load X Duty Cycle)

…for the total of all the motors. For greenhouse use, the duty cycle must be calculated based on the expected activity during a period of changing temperature, when determining feeder size. It is of no help that the motors may sit idle for hours or even days, the important thing is what is needed during periods of activity.

For a simple calculation, assume that duty cycle will be 15%, and motor consumption is 125W, with 100 motors. 15% X 125 = 18.75W. 18.75W X 100 motors is 1875W. The system should be oversized based on the level of confidence in the assumptions. A 2:1 oversize (1875W X 2 = 3750W) might be a good measure, and much more economical than simply multiplying motor power (125W X 100 = 12,500W) and installing more capacity than needed.

This is a matter of estimation and prediction, therefore it is important to carefully consider all factors, and to have some level of contingency corresponding to the accuracy of the assumptions.

Heaters, Fans, etc.

The same duty cycle principles and calculations apply to other greenhouse assets.

Inflation fans have a low power demand but significant consumption since they may run at 100% duty cycle.

Electric heaters are the other extreme, having both a high demand and high total consumption over time. They will likely be the most significant consideration for sizing the system. With propane or natural gas heaters it must be remembered that even though the heat source is not electric, there will still be significant power consumption to run the heater’s fans.

Fans and Louvers can be considered in the same way as vent motors, taking into account local conditions that affect the duty cycle and overall operating time.

Local electrical issues are best referred to qualified local electrical contractors with experience in industrial electrical installations. Advancing Alternatives has a wealth of knowledge based on years of experience with greenhouses and can offer advice to local contractors who are ultimately responsible for the adequacy and integrity of the electrical service.