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A solar fan setup needs to pass three separate checks: enough panel power while the fan is running, enough daily solar energy for the hours you want, and—if you need operation beyond useful sun—enough verified battery reserve. This calculator screens those three constraints from your inputs. It does not size greenhouse airflow, predict temperature, or design an electrical system.
Calculate solar fan feasibility
First establish required airflow with the greenhouse ventilation calculator and verify candidate fan data with the fan sizing guide. Then enter exact current manufacturer values where available. A product headline, housing dimension, or unverified marketplace claim is not enough. For the broader airflow-to-energy planning sequence and backup questions, see the solar greenhouse fan planning guide.
Solar fan energy check
Defaults are examples, not recommendations. Replace every value with a documented value or a deliberately conservative planning assumption.
Stop before buying: these outputs cannot verify controller compatibility, motor starting, battery current capability, wiring, overcurrent protection, listing, wet-location suitability, mounting, structural work, code compliance, installed CFM, or greenhouse temperature. Follow exact manufacturer instructions and use qualified electrical help where required.
How to read the result
| Result | Meaning | Next action |
|---|---|---|
| SHORTFALL | Entered panel fails simultaneous power, daily energy, or both. | Do not treat setup as matched. Verify better inputs or change system. |
| ARITHMETICALLY PLAUSIBLE | Direct-solar inputs pass basic power and daily-energy arithmetic. | Verify hourly/seasonal production and exact controller-fan compatibility. |
| VERIFY STORAGE | Solar arithmetic passes and reserve hours were requested. | Use battery estimate only as preliminary nameplate Wh; obtain chemistry-specific design. |
| CONDITIONAL | Reserved for incomplete or future bounded cases. | Treat unknown data as unknown, not zero. |
Worked example
A hypothetical fan uses 40 W and is stated at 800 CFM. Desired operation is 8 hours/day. A 120 W panel, 5 equivalent-full-sun hours, and 75% retained output produce this screening arithmetic:
- Fan load:
40 W × 8 h = 320 Wh/day. - Panel energy:
120 W × 5 h × 0.75 = 450 Wh/day. - Energy-based panel minimum:
320 ÷ (5 × 0.75) = 85.3 W. - Simultaneous-power minimum:
40 ÷ 0.75 = 53.3 W. - Governing screening minimum: the larger value,
85.3 W. - Four reserve hours at 80% usable fraction and 90% delivery efficiency:
(40 × 4) ÷ (0.80 × 0.90) = 222.2 Whpreliminary battery nameplate energy.
The example passes arithmetic, not engineering review. It does not establish that the panel supplies 120 W at the hottest hour, that the fan delivers 800 CFM after shutters and restrictions, or that a 222 Wh battery is suitable for the current, chemistry, temperature, cycle life, controller, and protection requirements.
Why daily energy can still mislead
Equivalent-full-sun hours convert variable daily irradiance into an energy estimate. They do not mean the panel produces nameplate watts continuously for that many clock hours. Clouds, shade, panel temperature, orientation, soiling, wiring, mismatch, controller behavior, and other losses change output. NLR describes PVWatts results as estimates with assumptions and uncertainty, not exact reproduction of every installed system.
This matters in a greenhouse because peak ventilation need is time-specific. A daily surplus after averaging does not prove the fan can start, remain stable, or deliver required airflow during the hottest interval. For the broader heat-response sequence—including shade, air exchange, intake restrictions, circulation, and evaporative limits—use the summer greenhouse cooling guide.
Inputs to verify before buying
- Required airflow: calculate separately. Do not choose a fan only because its panel and motor watts match.
- Fan watts: confirm normal operating power and any start/control requirements from exact documentation.
- Fan airflow: distinguish stated free-air CFM from delivered airflow after shutters, screens, intake restriction, and static pressure.
- Solar resource: use a conservative location-and-season estimate. Consider NLR PVWatts for location-aware PV analysis rather than guessing from daylight hours.
- Loss assumption: document what your retained-output percentage includes. Do not silently assume 100%.
- Controller compatibility: verify voltage, current, panel range, motor type, start behavior, and manufacturer-approved combinations.
- Battery system: verify chemistry, BMS, usable range, charge controller, current capability, temperature limits, protection, enclosure, and manufacturer instructions.
- Installation: verify wet-location suitability, mounting, wind and structural loads, wiring, disconnects, overcurrent protection, local requirements, and qualified-help needs.
What calculator does not tell you
- Whether solar ventilation is sufficient for greenhouse heat load.
- Expected greenhouse temperature reduction.
- Hourly production or worst-month performance.
- Fan startup, speed control, or performance under changing irradiance.
- Installed airflow, intake sizing, or static-pressure performance.
- Electrical, battery, structural, fire, waterproofing, or code compliance.
- Which product is “best.” For grid-powered system fit and exact-model documentation boundaries, see the greenhouse exhaust fan selection guide.
Method and formulas
| Output | Formula |
|---|---|
| Daily fan load | fan W × run hours |
| Estimated panel energy | panel W × equivalent-full-sun hours × retained-output fraction |
| Energy-based panel minimum | daily fan Wh ÷ (sun hours × retained-output fraction) |
| Simultaneous-power minimum | fan W ÷ retained-output fraction |
| Governing screening minimum | larger of energy-based and simultaneous-power minimum |
| Battery nameplate estimate | (fan W × reserve hours) ÷ (usable fraction × delivery efficiency) |
| Stated airflow per watt | stated CFM ÷ fan W |
These formulas are deterministic arithmetic using values you provide. My Greenhouse Lab used AI-assisted research and drafting plus fixed calculation tests; no fan, solar panel, battery, controller, greenhouse, or installation was physically tested for this page.
Sources
- NLR PVWatts Calculator Version 8 — production-estimate scope and uncertainty.
- NLR PVWatts V8 API documentation — current model and solar-resource overview.
- NREL PVWatts Version 5 Manual — documented system-loss categories and model background.