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Solar Grow Light Calculator

Enter your grow light setup, photoperiod, and ventilation — get monthly electricity cost, solar panels needed to offset, and payback period.

Grow lights
fixtures
hrs/day
Ventilation & climate
W
W
Location & system
$/kWh
Solar sizing for your grow operation
8 × 400W panels to offset 12.6 kWh/day
Total light watts600 W
Light daily kWh10.8 kWh/day
Ventilation daily kWh1.8 kWh/day
Total daily kWh12.6 kWh/day
Monthly electricity cost (grid)$49.80/mo
Cost per 60-day grow cycle$98.28/cycle
Solar system size2.9 kW (8 panels)
Est. system cost$10,160
Annual savings$598/yr
Payback period17.0 yrs
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How to Use This Calculator

Enter your grow light setup

Select the number of fixtures and fixture type. The wattage listed is the actual draw — not "equivalent" watts. A 300W LED draws 300W from the wall. Select your grow cycle: vegetative plants need 18 hours of light per day; flowering plants need 12 hours. These settings are pre-filled but can be manually overridden with the daily hours input.

Add ventilation and cooling loads

Inline fans and carbon filter fans are always on during the photoperiod. Enter the combined wattage. If you're running HPS lights, you almost certainly need AC cooling — HPS converts ~35% of electricity to light and 65% to heat. Modern LEDs run much cooler, but large operations still need supplemental cooling. Enter 0 if relying only on passive air exchange.

Choose grid-tied vs off-grid

For indoor grows connected to the utility grid, grid-tied solar is almost always the right choice. Off-grid solar for an 18-hour photoperiod requires a massive battery to power lights through the night — often 15-50+ kWh, which is expensive and typically impractical. Grid-tied solar offsets your electricity bill without the battery complexity. The calculator flags when an off-grid setup would be impractical (battery >30 kWh).

The Formula

Total Light Watts = Number of Fixtures × Fixture Watts Light Daily kWh = Total Light Watts × Daily Hours ÷ 1000 Vent Daily kWh = (Fan Watts + AC Watts) × Daily Hours ÷ 1000 Total Daily kWh = Light Daily kWh + Vent Daily kWh Monthly kWh = Total Daily kWh × 30.4 Monthly Cost = Monthly kWh × Electricity Rate Cycle Cost (60 days) = Total Daily kWh × 60 × Electricity Rate System kW = Total Daily kWh ÷ Peak Sun Hours ÷ 0.80 Panels = System kW × 1000 ÷ 400W (round up) Off-Grid Battery kWh = Total Load kW × Night Hours ÷ 0.80 DoD Annual Savings = Total Daily kWh × 365 × Electricity Rate

The solar sizing is based on daily average kWh averaged over the year. This means in summer you'll produce excess solar that credits your bill (net metering) and in winter you'll draw more grid power. The 60-day grow cycle cost is a key metric for licensed grows or any operation tracking cost per harvest.

Example

Indoor vegetable garden — 4 × LED 600W in Los Angeles

Maria runs a dedicated indoor growing room for tomatoes and herbs in Los Angeles with 4 × LED 600W fixtures on an 18-hour vegetative photoperiod, a 250W inline fan, and no AC (LED runs cool). She pays $0.25/kWh — LA rates are high.

Lights4 × LED 600W = 2,400W
Photoperiod18 hrs/day (vegetative)
Ventilation250W inline fan
LocationLos Angeles, CA (5.6 PSH)
Rate$0.25/kWh

Result

Total daily kWh47.7 kWh/day
Monthly electricity cost~$363/mo
Cost per 60-day cycle~$715/cycle
Panels needed19 × 400W panels (7.6 kW)
Annual savings~$4,350/yr
System cost~$22,300
Payback~5.1 yrs

At $0.25/kWh, Maria's grow room costs over $4,300/year in electricity — a major operating expense. A 7.6kW rooftop solar system pays for itself in 5 years and runs essentially free for the following 20 years. High-rate states like California and Hawaii make indoor grow solar economics especially compelling.

FAQ

A single 600W LED running 18 hours a day uses 10.8 kWh/day or 328 kWh/month — about $43/month at $0.13/kWh or $82/month at California rates ($0.25). Four of these fixtures cost $170-$330/month just for the lights, not counting fans or AC. This is why indoor growers are among the most motivated solar adopters — the payback periods are short when electricity bills are high.
Technically yes, practically difficult. The challenge is the 18-hour photoperiod: solar only produces for 4-7 hours/day. The remaining 11-14 hours must come from batteries. A 4 × 600W setup running 18 hours needs ~52 kWh/day total — requiring at least 30-50 kWh of battery storage (5-10 × Tesla Powerwall equivalent). At $900/kWh installed, that's $27,000-45,000 in batteries alone, plus panels. For flowering (12 hrs), the numbers are more manageable. For most indoor growers, grid-tied solar to reduce bills is far more practical.
LED is dramatically better for solar integration for three reasons: (1) Efficiency — modern LEDs produce 2.5-3.5 µmol/J vs HPS at 1.7-2.0 µmol/J, meaning 40-50% less electricity for equivalent yield. (2) Less heat — LEDs convert 60-70% of electricity to light vs ~35% for HPS, eliminating or reducing AC costs. (3) Smaller solar system — fewer watts needed means fewer panels and lower system cost. A 600W LED replacing a 1000W HPS reduces your solar system by 2-3 panels. The higher upfront LED cost is recovered through electricity savings and smaller solar system within 2-3 years.
Yes, with net metering. Your grow lights typically run overnight (when rates are lower) and during the day. Your solar panels produce electricity during the day — even if your lights aren't on, this production offsets what you drew from the grid overnight via net metering credits. The utility essentially uses your meter like a bank: export during the day, draw down the credit at night. As long as your annual production roughly matches your annual consumption, your bill drops to near zero (plus fixed charges). This is why grid-tied solar for indoor grows has the best economics.
Solar for indoor grows makes sense at any rate above $0.10/kWh, but the economics improve dramatically as rates rise. At $0.13/kWh (US average), payback on a 7.6kW system is 8-10 years. At $0.25/kWh (California, Hawaii, Northeast), payback drops to 4-6 years. At $0.35/kWh (some Massachusetts and Hawaii utilities), payback can be as low as 3-4 years. If your state has a net metering policy that credits excess solar at retail rates (full retail net metering), the economics are even better. Check your utility's net metering policy before sizing your system.

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