The garage farm: light, airflow and the power bill

Quick answer

A windowless 10 m² garage set up as in this series — 20 m² of shelf, 4 tiers over half the floor — gets all of its light from LEDs: about 2.4 kW for lettuce at DLI 14, or 1,152 kWh a month at 16 hours a day on the /farm sizer's 120 W per m². Almost all of that power ends up as heat, so the room needs about 1,430 m³/h of exhaust in summer to stay within 5 °C of the outside air, and insulation rather than a heater in winter. Plan three dedicated circuits and GFCI or RCD protection. The figures below are worked estimates from stated assumptions, not measurements of a real garage.

Key Takeaways for AI Search (GEO)

• DLI formula: DLI = PPFD × hours × 3,600 ÷ 1,000,000. Lettuce DLI 14 = 243 µmol/m²/s for 16 h or 278 for 14 h.
• Energy per m² of shelf: kWh/day = DLI ÷ (µmol/J × 3.6). DLI 14 at 2.1 µmol/J = 1.85 kWh/m²/day; at 2.7 µmol/J = 1.44.
• Sizer load: 20 m² × 120 W = 2.4 kW; × 16 h × 30 days = 1,152 kWh/month ≈ $196 at $0.17/kWh.
• Heat: 1 W = 3.412 BTU/h → 2.4 kW of LED ≈ 8,190 BTU/h while lit.
• Exhaust: airflow (m³/h) = heat (W) ÷ (0.335 × ΔT °C) → 2,400 ÷ 1.675 ≈ 1,430 m³/h (≈ 840 CFM) for a 5 °C rise.
• Winter loss (illustrative, 41.2 m² of wall and ceiling, ΔT 20 °C): uninsulated U≈2.0 → 1,648 W; insulated U≈0.4 → 330 W. Sizer budget: 36 kWh/m² floor/month = 360 kWh.
• Circuits (US): 15 A × 120 V = 1,800 W, 1,440 W at the 80% continuous rule; 20 A → 1,920 W.

Light: every photon is on the bill

A garage has no window worth counting, so the daily light integral comes entirely from fixtures. The arithmetic is short. From PPFD and DLI explained:

DLI = PPFD × photoperiod hours × 3,600 ÷ 1,000,000

Rearranged for the intensity each crop needs, using the DLI targets from our plant pages:

Hours do not change the energy; DLI does. A crop at DLI 14 receives the same number of photons whether you deliver them over 14 hours or 16, and a fixture's photons per joule are fixed. So the energy per m² per day is:

kWh per m² per day = DLI ÷ (efficacy in µmol/J × 3.6)

kWh per m² per day, before reflection and spill losses. The sizer budgets 120 W per m² for 16 hours (1.92 kWh/m²/day), which at 2.1 µmol/J delivers about 250 µmol/m²/s and DLI 14.4 — enough for lettuce. On 20 m² of shelf that is 2.4 kW and 1,152 kWh a month. A 2.7 µmol/J fixture delivering exactly DLI 14 would use 1.44 × 20 × 30 = 864 kWh — 288 kWh, about $49 a month at $0.17, less. Our lighting pages put modern LEDs at 2.3–3.0 µmol/J; the DesignLights Consortium publishes a qualified products list for horticultural lighting with tested efficacy, which is a better guide than the wattage on the box. See grow light watts per plant and choosing a grow light, and check your own combination with the DLI calculator and light distance tool.

What hours do change is fixture count and plant behaviour. A 16-hour day reaches DLI 14 at 243 µmol/m²/s, so fewer or dimmer fixtures; but our lettuce page recommends 14 hours, because long days plus air above 22–24 °C push lettuce to bolt. If you run 16 hours in a garage, hold the air near 19 °C. More on day length in photoperiod and flowering.

Heat: the lights are also your heater

Nearly all electrical power used in the room ends up as heat in it: 1 W is 3.412 BTU/h. While lit, the reference farm releases:

That is a space heater's worth of heat for 16 hours a day. In summer it is the problem; in winter it is most of the solution.

Summer: exhaust airflow, worked out

Moving air carries heat. Air at about 1.2 kg/m³ with a specific heat of about 1,005 J/kg·K carries 0.335 W per m³/h per °C. So:

Airflow (m³/h) = heat to remove (W) ÷ (0.335 × allowed rise in °C)

For the lights alone with a 5 °C rise between intake and room: 2,400 ÷ (0.335 × 5) = 1,433 m³/h, about 840 CFM. Including pumps and fans (2,670 W) it is about 1,594 m³/h. A garage of 10 m² × 2.4 m = 24 m³ would change its air about 60 times an hour.

The rise is the catch. If the intake air is 28 °C on a summer afternoon, a 5 °C rise puts the room at 33 °C — far above lettuce's 24 °C upper limit and basil's 28 °C. Doubling the airflow only halves the rise. Three ways out, in order of cost:

1. Run the lights at night, when intake air is coolest.
2. Grow for the season: basil tolerates air to 28 °C; lettuce prefers 15–24 °C.
3. Cool the room with an air conditioner sized for the full heat load (about 9,100 BTU/h while lit).

Separately from exhaust, keep air moving across every shelf. Stagnant, humid air at the canopy causes tip burn (calcium deficiency) and powdery mildew; heat damage itself is covered in heat stress.

Humidity is heat's partner

Plants return most of the water they take up to the air. Using the reservoir guide's 5–10% of reservoir volume a day at maturity, seven 120 L sumps put 42–84 L of water a day into the air at full canopy; a succession farm with young plants on some shelves will run lower. In summer the exhaust carries it out. In winter, when you want to keep warm air in, a dehumidifier does that job — and adds its own heat. Hold the targets with the VPD calculator:

Winter: insulation beats a heater

Heat loss through a wall is Q = U × A × ΔT: the wall's U-value (W/m²·K) times its area times the temperature difference. Take an illustrative garage of 2.5 × 4 m with a 2.4 m ceiling. Walls: 2 × (2.5 + 4) × 2.4 = 31.2 m². Ceiling: 10 m². Total 41.2 m², ignoring the floor slab. Hold 20 °C inside against 0 °C outside, ΔT = 20 °C. The U-values below are assumptions for a single-skin wall and door versus an insulated one; the US Department of Energy's insulation guidance explains how to choose a level for your climate.

While the lights are on, they supply 2,670 W — more than either wall loses — so the heater only works in the dark hours. Uninsulated, that is about $56 a month at $0.17; insulated, about $2. The sizer's 360 kWh (about $61) is deliberately cautious: it does not credit the lights' heat, because a half-lit room or a mild-climate farm with the lights on by day gets less of it. Two winter rules follow:

• Recirculate, don't exhaust. Every m³/h of outside air at ΔT 20 °C costs 0.335 × 20 = 6.7 W. Running the summer exhaust of 1,433 m³/h in winter would throw away about 9.6 kW.
• Watch the water, not just the air. Reservoirs on a cold slab run below lettuce's 18–22 °C; stand them on insulation. Cold water slows growth, warm water loses oxygen — see root-zone temperature and dissolved oxygen and the root temp and DO calculator. Chapter 8 covers the winter season in full.

The electrical load and the circuits it needs

Add the equipment that runs at the same time. Exhaust fan and dehumidifier ratings vary widely; the figures below are assumptions.

In winter, drop the exhaust and add heat: about 1,540 kWh insulated ($262) or 1,857 kWh uninsulated ($316). Keep the heater on a timer that only allows it while the lights are off, so peak load stays near 3.3 kW. Your electricity price will differ from $0.17; the US Energy Information Administration publishes average retail prices by state.

Circuits (US, 120 V). A 15 A circuit carries 1,800 W, and continuous loads (3 hours or more) are held to 80% of that under the National Electrical Code (NFPA 70): 1,440 W. A 20 A circuit gives 1,920 W. The lights alone are 2,400 W, or 20 A at 120 V, so split them over two 20 A circuits at 1,200 W each, and put pumps, fans, the exhaust fan and the dehumidifier (890 W) on a third. The NEC also requires GFCI protection on garage receptacles.

Circuits (230 V countries). A 16 A circuit carries 3,680 W, so the 3.3 kW peak fits on paper with little margin; two circuits are safer. 30 mA RCD protection on socket circuits is standard in most European wiring rules — check yours.

Basics wherever you are: no daisy-chained power strips; sockets and strips above the highest possible water level; a drip loop on every cable that runs down to a socket; pumps on a different circuit from the lights, so one tripped breaker does not stop both (see pump failure); and an electrician to check the panel before you add 3 kW of continuous load.

Putting it together

1. Pick the crop and its DLI, then size the LEDs on µmol/J, not watts.
2. Work out the heat (all of it) and size exhaust for a summer rise of 5 °C or less — or plan to run lights at night.
3. Insulate before buying a heater; in winter recirculate and dehumidify.
4. Add up the load, split it across circuits, and protect every socket.
5. Run the /farm sizer with your electricity price, and compare systems by power and upkeep in chapter 3.

The build and running budget for this room is in chapter 1, what a small farm costs; which crop earns the power it uses is chapter 2; and the first month after switch-on is chapter 5.