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Solar Greenhouse Calculator

Enter your greenhouse loads and location — get solar panels, seasonal energy use, battery size for off-grid, and payback period.

Greenhouse setup
W
Seasonal usage & location
hrs/day
hrs/day
$/kWh
Solar sizing for your greenhouse
1 × 400W panel (sized for winter)
Peak load (all running)420 W
Winter daily usage5.04 kWh/day
Summer daily usage3.48 kWh/day
Annual grid usage1,554.9 kWh/yr
Annual electricity cost$202.14/yr
Est. system cost$2,120
Payback period10.5 yrs
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How to Use This Calculator

Configure your greenhouse loads

Start by selecting your greenhouse size — this helps you estimate grow light coverage (one 300W LED covers roughly one 4×4 ft area). Then toggle each electrical load: grow lights, ventilation fans, heating mats, and irrigation pump. Enter the total wattage for grow lights and heating mats based on what you actually have installed or plan to install.

Set seasonal operating hours

Greenhouses have dramatically different loads in winter vs summer. In winter, grow lights often run 14–18 hours per day to compensate for short days — this is usually the highest-consumption season. In summer, natural light reduces grow light runtime but ventilation fans run much longer (8–14 hours) to prevent overheating. The calculator sizes your solar system for whichever season draws more power.

Choose grid-tied or off-grid

Grid-tied systems offset your electricity bill with net metering — ideal if you have grid access. Off-grid systems use a battery bank sized for one day of autonomy, essential if your greenhouse is in a field or remote location. Off-grid greenhouses often use 48V systems for efficiency at the power levels involved.

The Formula

Winter Daily Wh = (Lights × Winter Hours) + (Fans × 2hrs) + (Mats × 16hrs) + (Pump × 0.5hrs) Summer Daily Wh = (Lights × Summer Hours) + (Fans × 14hrs) + (Pump × 1.5hrs) Worst-Season Daily Wh = max(Winter, Summer) System Watts = Worst-Season Daily Wh ÷ 1000 ÷ Peak Sun Hours ÷ 0.80 Panels = System Watts ÷ 400W (round up) Battery Ah (48V) = Worst-Season Daily Wh ÷ (48V × 0.80 DoD) Annual kWh ≈ (Winter×150 days + Summer×150 days + Shoulder×65 days) ÷ 1000

The system is sized for the worst-case season — typically winter for grow-light-heavy setups, or summer for fan-heavy setups. This ensures your panels generate enough energy year-round. Battery sizing assumes one day of autonomy — extend to 2 days for critical crops if you're in a cloudy climate.

Example

Maria — Year-round market garden, 12×20 ft, Atlanta

Maria runs a 240 sq ft greenhouse in Atlanta for year-round salad greens. She has 1,800W of LED grow lights (6 × 300W), ventilation fans, and a drip irrigation pump. She needs 16 hours of supplemental light in winter and only 4 hours in summer. She's grid-tied at $0.12/kWh.

Greenhouse12×20 ft (240 sq ft)
Grow lights1,800W LED, 16 hrs/winter
Ventilation120W fans
Irrigation pump150W, 1 hr/day
LocationAtlanta, GA (5.0 PSH)

Result

Winter daily usage~29.2 kWh/day
Summer daily usage~8.9 kWh/day
Panels needed8 × 400W panels
Annual grid cost~$760/yr
Est. system cost~$10,200
Payback~13 years

Eight panels handle Maria's winter-dominated load. The 5:1 ratio between winter and summer usage is common in grow-light-heavy greenhouses — solar is sized for winter, and in summer excess production offsets other home electricity. For Maria, the payback is longer than typical rooftop solar because greenhouse electricity use is seasonal and high in winter when Atlanta's solar production is lower.

FAQ

It depends almost entirely on your grow lights. A small 8×10 hobby greenhouse with a single 300W grow light running 16 hours in winter needs about 1–2 panels. A commercial 20×40 greenhouse with 6,000W of grow lights running 16 hours daily needs 15–20 panels. Ventilation fans, heating mats, and irrigation pumps add modest loads compared to grow lights. The key insight: grow lights are the dominant load in winter, and winter is your worst case for solar production.
Yes, but it requires careful sizing and a realistic battery bank. The challenge is winter: short days mean less solar production exactly when your grow lights run the longest. An off-grid greenhouse in a northern climate may need a large battery bank (10–20 kWh) to bridge cloudy winter days. Many off-grid greenhouse growers use a hybrid approach: solar + a small propane or diesel generator as backup for extended cloudy periods. For southern climates (Georgia, Florida, Texas), off-grid solar is much more viable year-round.
A 10×12 greenhouse (120 sq ft) with 2–3 LED grow lights (600–900W), ventilation fans, and heating mats typically needs 2–4 solar panels (400W each). Without grow lights (passive solar greenhouse relying on natural light), just fans and a pump need only 1 panel. With grow lights running 16 hrs/day in winter, 3–4 panels are needed. Location matters: in Denver (5.5 PSH) you'd need 3 panels; in Seattle (3.6 PSH) you'd need 4–5.
For a grid-connected greenhouse, batteries are optional. Net metering lets you export solar during midday peaks and draw grid power when your grow lights are running in early morning and evening. For an off-grid greenhouse, batteries are essential — plants need consistent lighting and temperature, you can't just turn off the lights when clouds roll in. Size your battery bank for at least one full day of autonomy, and consider 2 days for critical crops or unreliable weather regions.
Solar is most cost-effective for greenhouses that replace expensive grid extension (remote locations), or those with high year-round electricity use. For a grid-connected urban greenhouse, payback periods of 8–15 years are typical — reasonable given 25-year panel life. For a remote greenhouse where grid extension would cost $10,000–50,000 per mile, solar pays back in 2–5 years even before considering energy savings. The biggest lever: use efficient LED grow lights. Older HID/HPS lighting at double the wattage would double both your electricity cost and solar system size.

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