Greenhouse Solar Panel Calculator

How it works

How many solar panels do I need for a greenhouse?

It depends on what you run, for how long, and how much sun you get. A greenhouse that only runs a ventilation fan, a small circulation fan and a sensor in summer usually needs one 100 to 200 watt panel. Add grow lights, or ask it to work through December in the north, and you may need four panels or more. Electric heat is a different story, which we cover below.

The calculator works in three steps:

1. Daily energy (Wh) = Watts × Hours per day × How many

2. Panel watts = Daily energy ÷ (Peak sun hours × 0.75)

3. Battery (Ah) = Daily energy × Backup days ÷ Usable share ÷ Battery volts

Peak sun hours change a lot through the year. In the Southeast a south-facing panel gets about 5.8 hours in June but only about 3.5 in December, so a system that works all year needs far more panel than a summer-only one. The 0.75 covers losses in the wiring, controller and battery. The usable share protects your battery: lithium batteries can be used down to about 20%, lead-acid only to about 50%.

Approximate peak sun hours for a fixed panel facing south, tilted at about your latitude. Values for your exact location will differ.

Worked example: summer ventilation

A 40 W DC exhaust fan running 8 hours, a 15 W circulation fan running 12 hours and a 5 W sensor running all day use 320 + 180 + 120 = 620 Wh a day. In June in the Southeast (5.8 sun hours): 620 ÷ (5.8 × 0.75) ≈ 143 W of panels, so one 200 W panel. A day of backup from a 12 V lithium battery needs 620 ÷ 0.8 ÷ 12 ≈ 65 Ah, so a 100 Ah battery.

Heating

Can solar panels heat a greenhouse?

Not with an electric heater, for most people. A 1,500 W heater running 8 hours a night uses 12,000 Wh. In December in Georgia that takes about 4,600 W of panels (twenty-three 200 W panels) plus a battery bank of well over 1,000 Ah. That costs far more than the greenhouse.

Solar works best for the things that keep plants alive in the heat: fans, louvres, pumps and controls. For winter, keep the heat in with good glazing, bubble insulation and thermal mass, and use propane or grid power for the heater. Size it with our greenhouse heater calculator.

Get more from less

Ways to shrink your solar system

• Choose DC equipment12 V or 24 V fans and pumps run straight off the battery. An inverter for 120 V gear wastes 10 to 15% and adds cost.
• Let the thermostat decideA fan that only runs when it is hot uses a fraction of the energy of one that runs all day.
• Use non-electric vent openersWax-cylinder automatic openers open roof vents and louvres with heat alone, using no power at all.
• Aim panels southA panel facing south and tilted near your latitude gets the most sun. Keep it clear of tree shade, even for an hour.
• Pick lithium batteriesYou can use about 80% of a lithium battery, against 50% of lead-acid, so you need a smaller one and it lasts years longer.
• Add shade firstGood shade cloth lowers the heat your fans have to remove.