Running a greenhouse completely on solar power is an attractive option for many growers looking to reduce energy costs and environmental impact. However, determining the number of solar panels needed to fully power a greenhouse requires careful calculation based on several key factors. In this article, we will provide a step-by-step guide to calculating your solar panel needs for a greenhouse, including considerations for climate, greenhouse size, electrical load, and panel wattage and efficiency. With some basic information about your greenhouse, you can determine the ideal solar array size to make your operation energy independent.
Page Contents
How Much Electricity Does a Greenhouse Use?
The electricity usage of a greenhouse depends on several variables, including:
– Greenhouse size – Larger greenhouses require more lighting, ventilation, and controls. A small 100 sq ft hobby greenhouse may use 5-10 kWh per day, while a large 10,000 sq ft commercial operation can use 500-1000 kWh per day.
– Growing season – Electricity needs are higher in winter months for lighting and heating. Summer electrical loads are lower when relying more on passive ventilation and cooling.
– Equipment and appliances – Items like grow lights, exhaust fans, pumps, dehumidifiers, and heaters increase electricity consumption. More automated technology and controls also increase usage.
– Supplemental lighting – More light hours and use of high intensity grow lights dramatically increases electricity needs. Greenhouses focused on winter production will require much more lighting.
– Climate and location – Greenhouses in colder climates require more heating and lighting compared to warmer regions. Facilities in low-light areas also require supplemental lighting.
To determine your specific greenhouse electricity needs, track your power use over a full year and determine an average daily load measured in kilowatt-hours (kWh). Factoring in future equipment upgrades or expansions will also give you projected energy needs.
Sizing a Solar Array for a Greenhouse
Once you know your greenhouse’s average electricity requirements measured in kWh per day, you can calculate the solar array size needed to offset that energy use. The key factors are:
– Average daily electricity use (kWh per day)
– Hours of peak sun – The equivalent full sun hours per day in your location, accounting for seasonal variations
– Solar panel wattage – The power output rating of the panels you plan to install
– System losses – Loss of efficiency from dirt, heat, wiring, etc., usually estimated at 20%
You can then use this basic formula to calculate the needed solar array power rating in kilowatts:
Greenhouse Electricity Use (kWh per day) x 1.2 (system losses)
÷
Peak Sun Hours per Day
= kW of Solar Panels Needed
For example, if your greenhouse uses 300 kWh per day, and your location has an average of 5 peak sun hours per day, the math would be:
300 kWh x 1.2
÷
5 Sun Hours
= 72 kW Solar Array
This estimate tells you the total wattage of solar panels to meet 100% of your greenhouse’s electricity demands. If you install 300W panels, you would need approximately 240 panels rated at 300W each to achieve a 72kW system size.
Factoring in Battery Storage
Sizing your solar array only provides power when the sun is shining. To store excess power for nighttime and cloudy days, you will need to factor in a battery bank for your system.
Some key battery sizing considerations are:
– Days of autonomy – How many days the battery bank must supply power without sun. 3-5 days is recommended for a greenhouse.
– Battery efficiency – Lead acid batteries are ~80% efficient, lithium ion can be 90-95% efficient.
– Depth of discharge – Batteries should not be depleted more than 50-80% to prolong life.
– Greenhouse nighttime load – The kWh per night the battery must provide.
Using these variables, you can calculate the battery storage capacity needed using this formula:
Greenhouse Nightly Load (kWh)
x Days of Autonomy
÷ Battery Efficiency
÷ Depth of Discharge
= Battery Capacity Needed (kWh)
For example, if your nightly load is 100 kWh, you want 3 days of autonomy, using lithium ion batteries at 95% efficiency, not discharged below 50%:
100 kWh
x 3 Days
÷ 0.95
÷ 0.5
= 632 kWh Battery Capacity
Your solar array size must be large enough to meet daytime greenhouse loads while still charging the battery bank. A larger battery bank also allows the solar power system to better weather cloudy periods.
Key Considerations by Climate
Greenhouse solar requirements vary greatly depending on your location and climate:
Northern Climates
– Low peak sun hours – Need more panels for less daily solar gain
– Long winters – Need expanded lighting. Battery banks up to 10 days autonomy.
– Heating loads – Solar thermal panels may help reduce heating fuel needs.
– Snow – Must account for snow cover reducing solar gain through greenhouse glazing.
Southern Climates
– High peak sun hours – Solar arrays can be smaller for same energy output.
– Hot summers – Adequate roof and side venting is crucial to prevent overheating.
– Hurricane risks – Panels and supporting structure must withstand high winds.
– Higher ambient light – Supplemental lighting needs will be lower.
Coastal Climates
– Corrosion risks – Solar array structures and components must be corrosion resistant.
– Marine fog and clouds – Solar output can drop on overcast days. Larger battery bank recommended.
– Wind risks – Panels and support structures must withstand salt, moisture, and wind.
Arid Climates
– Excellent solar resource – High sunlight hours mean smaller solar arrays to meet needs.
– Extreme heat – Proper airflow and ventilation prevent greenhouse overheating.
– Dust – System maintenance is key. Dust buildup on panels will reduce output.
Example Solar Array Sizing for a 5,000 sq ft Greenhouse
Here is an example solar power calculation for a 5,000 sq ft greenhouse in the Northeast United States:
– Location: Concord, New Hampshire
– Greenhouse Size: 5,000 sq ft
– Average Yearly Electricity Use: 18,000 kWh
– Average Daily Use: 18,000 kWh / 365 Days = ~50 kWh Per Day
– Peak Sun Hours Per Day (Concord, NH): 3.5 Hours
– Nightly Load: 650W grow lights x 10 hrs = 6.5 kWh
– Lead Acid Batteries at 80% efficiency
– 3 Days Battery Autonomy
– 50% Maximum Depth of Discharge
Solar Array Size:
50 kWh Per Day x 1.2 Loss Factor
÷ 3.5 Peak Sun Hours
= 17 kW Solar Array
Battery Bank Size:
6.5 kWh Nightly Load
x 3 Days Autonomy
÷ 0.8 Battery Efficiency
÷ 0.5 Depth of Discharge
= 65 kWh Battery Capacity
For this 5,000 sq ft greenhouse in Concord, NH, a 17 kW solar array with 65 kWh of battery storage would meet the electricity demands.
This is a simplified estimate focusing just on energy needs. When installing a full solar system, you need to factor in panel tilts, orientation, wiring, voltage, controllers, inverters, chargers, and other design considerations beyond just sizing the solar array capacity. Consulting with an experienced solar installer is highly recommended.
Solar Greenhouse Costs
Installing a solar power system for your greenhouse represents a major investment that can pay off substantially in the long run through lower energy bills and grid independence. Here are some rough costs estimates:
– Solar Panels – $0.75-$3.50 per watt depending on type, brand, and panel wattage. Economy of scale applies to larger systems.
– Racking – $0.70-$1.50 per watt for mounting hardware, rails, purlins etc.
– Inverters – $0.20-$0.60 per watt, more expensive for off-grid tied systems. Automatic transfer switches add cost for grid-tied systems.
– Batteries – Lead acid around $150-$300 per kWh stored. Lithium ion $400-$1000 per kWh. Upfront cost is higher but lithium batteries have longer lifetime and superior performance.
– Charge controllers and wiring – $1000-$4000 depending on system complexity and size
– Labor, permits, drawings – At least 10-25% of hardware costs.
A very rough initial estimate is $3-$5 per watt for a complete off-grid solar array with battery storage. On the low end a 10 kW system would be around $30,000. On the high end, a 50 kW system could be $250,000 or more.
Federal and state incentives like the ITC (Investment Tax Credit) can offset 26% of the system cost. Some states also provide additional solar incentives. Grants or financing may also be available depending on your location and specific project details.
Over the long term, a solar greenhouse can provide excellent return on investment by stabilizing and reducing your energy costs. Carefully size your system to match your specific needs and maximize savings over time.
Solar Greenhouse Design Best Practices
Here are some key solar power system design considerations for greenhouses:
– Face solar panels south for maximum exposure in northern hemisphere. Angle panels for optimal seasonal performance.
– Maximize roof and side wall area for solar panels while accounting for summer shading needs.
– Use high transmission glazing materials like tempered glass to allow sunlight to penetrate into the growing space.
– Allow for open roof ventilation to prevent overheating under the panels. Panels shed heat and can cause temperature spikes.
– Use high efficiency mono- or polycrystalline panels rated for long life in harsh outdoor environments.
– Keep panels clear of snow, leaves, dust or any obstruction that would reduce output.
– Use tilting racking systems to angle panels for seasonal sun angles and allow gravity shedding of snow and debris.
– Locate batteries in a temperature controlled enclosure for optimal performance and safety.
– Include grounded surge suppressors to protect electronics from lightning strikes.
– Follow all local codes and obtain permits. Engage with utilities for interconnection requirements if tying into grid as backup.
Conclusion
Installing a properly sized solar array with battery storage provides an ideal way to offset electricity usage in the greenhouse. With some basic information about your facility’s location, size, and electrical loads, you can calculate the total solar panel and battery capacity needed to operate self-sufficiently. While requiring a sizable upfront investment, solar power systems provide excellent long-term return through energy savings. When designed and installed correctly, a solar PV system can meet all the electrical needs of your greenhouse sustainably and economically.