Solar panels for a greenhouse: User case from Maine

A greenhouse has to be built right so it doesn’t waste energy. Twin-wall polycarbonate can give you up to 50% energy savings compared to single-wall material. In addition, you can use bubble wrap sold at gardening centers to make your walls and roof even thicker. Use caulk, silicone or some other sealant to seal the gaps. Pay special attention to the holes between the foundation and frame.

Keeping your plants warm becomes essential during the winter. Heating requires a lot of energy and that’s where solar panels come in.

There are several ways you can add a PV array to your greenhouse. Here are some of the options:

• If your house already runs on solar and there is still space on the roof, you could simply add the needed capacity. Then you would hook up the hothouse and let net metering offset all the costs.
• You can install a ground-mounted solar system near the greenhouse. It’s an optimal solution if you don’t have a residential system or if the house is too far away. As an added bonus, you can employ a solar tracker and your panels will always follow the sun.
• You can use foldable solar panels for a small greenhouse. This would allow you to use them elsewhere when the heating is not needed. Take them on a camping trip, for example. All you’d need for that is a portable solar inverter.
• Finally, you could install solar panels right on top of your greenhouse but only if the structure can support it. Light and flexible thin-film panels are a better choice in this case. Keep in mind that your plants will be getting much less natural sunlight this way.

This is first-hand information, shared by my colleague Andrew. For him, a hardcore vegan living in a harsh climate, a hothouse was a dream. However, there was no way for him to hook up his greenhouse to the grid to power it. Instead of going with another noisy generator, he chose solar panels for greenhouse heating.

There are four key factors in figuring out solar capacity for a greenhouse:

1. the area of your greenhouse,
2. the wall material,
3. the average low temperature outside,
4. the temperature you want to maintain inside.

January is the coldest month in Bangor, Maine where Andrew lives. The average low during the month is 7ºF, while an optimal temperature for the plants starts around 60ºF. Andrew had to find a way to heat up his 50 ft² greenhouse in the winter. His father helped pick the systems for heating and heat distribution — the equipment would use about 2300 W. Together they calculated that a greenhouse made of 5/8" polycarbonate would consume around 100 kWh in January.

Solar panels get around 3.5 peak sun hours a day in Maine, and the days are mostly clear and sunny. Three 500 W solar panels would produce a little over 5 kWh in a day. This system would give Andrew over 150 kWh per month: the extras were just a safety margin. The battery would keep the heating system working at night.

Andrew decided that ground-mounted installation would fit his situation the most. The space was abundant, but the roof of the house didn’t have the proper inclination and angle. To provide solar power for greenhouse they opted for 440 W bifacial Aptos modules, a 3600 Wh KiloVault battery and a MidNite Classic charge controller. The whole lot cost them around $4,500. They mounted solar panels on a pole with the help of friends who had experience in installing solar.

Usually, a solar system requires an inverter to turn DC from panels to AC. Andrew didn’t need one because all he used his solar energy for was a DC heating system and lighting. That’s not the most common scenario, and If your equipment or appliances demand AC, add an inverter that matches your system type.

Since aesthetics rarely come into play when choosing solar panels for greenhouse, the most popular brands are those that offer the best price per watt. More powerful solar panels are also preferable. They are slightly more efficient and require less work and materials, since you get the same power but need to install fewer modules. However, saving shouldn’t come at the expense of quality. Below are some reliable and affordable brands that we can recommend:

• Aptos Solar makes great solar panels for greenhouse lights and heating. It is an American manufacturer with strong warranties, high quality standards and affordable pricing
• Axitec is another great brand. This German company sells solar panels with industry-leading performance warranties for less than $1/W
• Canadian Solar, another North American manufacturer, is known for adapting its modules to the harsh climates

Our engineer ranked the best panels for a greenhouse using his experience, feedback from customers and product specs.

A 440 W bifacial module can produce up to 30% more energy than standard 440 W solar panel, making the payback period shorter. The performance increase promised by bifacial panels stems from employing both sides of the module for generating electricity. Unlike monofacial solar panel systems, bifacials produce more energy when angled off the roof or ground. It is recommended to install the bifacial modules 1 m away from the ground or roof to maintain the energy yield of the module's rear side. This is easier to achieve in a ground-mounted system — and that’s exactly what Andrew did.

Aptos provides a 25-year product and a 30-year performance warranty, which says a lot about the expected lifespan of the panels. You’re unlikely to ever need this warranty though — the failure rate of Aptos solar panels is below 0.01%.

This monofacial panel will be a great choice for your solar greenhouse if you can’t create the optimal conditions for bifacial modules. Axitec provides a 15-year product and a 25-year performance warranty, which says a lot about the expected lifespan of the panels. Even 25 years later, your Axitec panels will conserve at least 85% of their original capacity.

A great greenhouse solar panel, this 370 W Suntech module has all it takes to become a reliable source of electricity for your plants. It is thinner and more compact than the two more powerful models above, but just as sturdy and efficient. Believe it or not, but it can handle a 5400 Pa snow load and winds up to 210 mph.