The architecture
Two 100W panels charge a 100Ah LFP battery. A thermostat drives a 12V exhaust fan above the high setpoint. A second thermostat can power low-watt seedling cable/mat under insulated trays when root temperature falls. Sensors log temperature/humidity and can send alerts.
Load and sizing math
Ventilation is cheap. Heating is expensive. Keep the heated mass small and insulated.
| Load | Watts | Hours/day | Daily Wh | Notes |
|---|---|---|---|---|
| Vent fan | 25 | 3 | 75 | Sunny-day average |
| Root-zone heat | 60 | 4 | 240 | Frost-night duty |
| Sensors/controller | 3 | 24 | 72 | Continuous |
| LED service light | 10 | 0.5 | 5 | Occasional |
| Illustrative total | 392 Wh/day | Before system margin | ||
About 417Wh/day on a frost-heavy cycle. A 100Ah battery gives reasonable overnight reserve; 200W PV is enough for shoulder-season use but prolonged cloudy cold may require grid/generator or no heating.
Full parts list
This is a shopping architecture, not a demand that you buy one exact brand. Match voltage, current and connectors before ordering. Budget ranges reflect the class of hardware rather than a live quote.
200W solar array
Target spec: 2×100W
Sized for control plus limited heat.
Budget: $100–$260 each
100Ah LiFePO4 battery
Target spec: 12.8V
Energy reserve for night heat.
Budget: $150–$300 each
20A MPPT controller
Target spec: LFP profile
Efficient shoulder-season charging.
Budget: $60–$140 each
12V greenhouse fan
Target spec: 20–40W, shutter or duct fan
Dumps heat automatically.
Budget: $35–$100 each
Dual thermostat controller
Target spec: 12V relay outputs
Separate high-temp fan and low-temp heat control.
Budget: $20–$60 each
Low-voltage heat cable/mat
Target spec: 40–80W root-zone heater
Heats trays/soil instead of entire air volume.
Budget: $30–$90 each
Wiring map
Treat heating and fan as independent thermostat-controlled branches with manual override switches.
[200W PV]-->[MPPT]-->[100Ah LFP]-->[FUSE BLOCK]
| |
High-temp Low-temp
thermostat thermostat
| |
FAN HEAT MAT
Step-by-step build
Insulate before electrifying
Seal gaps and insulate north/back surfaces so the heat load stays small.
Place sensors at plant level
A sensor against the glazing can read wildly different temperatures than seedlings experience.
Mount solar and battery
Keep battery shaded/protected while panels get full shoulder-season sun.
Wire fan thermostat
Set a conservative high threshold and verify louvers/shutters open freely.
Wire heat thermostat
Put the probe in the root zone and set only the minimum temperature crops require.
Add manual overrides
A switch lets you force ventilation during maintenance or disable heat during warm spells.
Run a sunny-day overheat test
Watch how quickly the fan pulls the frame back below setpoint.
Run an overnight frost test
Log minimum root temperature and battery SOC; adjust insulation before adding more battery.
Commission it before you trust it
The project is not finished when the LED comes on. Run it through a controlled test so the first real failure is not in the field.
- Frame sealed
- Sensor at plant height
- Fan airflow unobstructed
- Heat mat isolated from water
- Thermostats independently fused
- Battery low-temp charging protected
- Overheat test passed
- Frost-night SOC logged
- Manual override labeled
Mistakes to avoid
Undersizing the battery because the panel is large
Panels make energy only when conditions cooperate. The battery has to carry the load through night, cloud and short high-demand events. Size energy storage from watt-hours, not panel watts.
Ignoring standby loads
Controllers, routers, cameras, inverters and cellular hardware can consume power 24/7. On small projects, a 5W always-on device is 120Wh per day—sometimes more than the 'main' load.
Using one fuse for the entire project
Fuse each branch according to its conductor and load. The fuse protects wire, not the gadget. A downstream short should not require an oversized main fuse to clear.
Mounting the panel where the project is instead of where the sun is
The load can sit in shade. The panel cannot. Use a cable run, pole or remote mount to put the collector in the best solar window.
Skipping a low-voltage disconnect
Deep-discharge protection is cheap insurance. Many charge controllers or smart battery systems can disconnect noncritical loads before the battery is abused.
Upgrade path
Add automatic vent opener too
A wax/piston vent provides passive fail-safe cooling if the electrical system fails.
Add thermal mass
Water jugs or masonry can reduce night temperature swings with zero watts.
Add remote alert
A low-power LoRa/cellular sensor can warn if temperature crosses a crop-specific threshold.
Frequently asked questions
Can this heat a cold frame all winter?
Not economically in cold climates; it is for frost protection and root-zone assistance.
Why 100Ah battery?
Night heating can consume hundreds of watt-hours; the larger battery provides margin.
Can I use a passive vent only?
Yes, and it is a good fail-safe; the fan adds control on still hot days.
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