Winter Farm Costs: Heat at 36–90 kWh/m² a Month

Quick answer

Winter changes a small farm's bill more than any other season. Hold the reservoir at 18–22 °C for lettuce and microgreens (20–24 °C for basil), heat the water before you heat the air, and budget heating at the farm sizer's 36 kWh per m² of floor per month indoors or 90 kWh under glass — then check that figure against your own space with heat loss = U × A × ΔT. A greenhouse also needs light: the sizer's 6 hours a day of top-up adds 5.4 mol/m²/day, enough for lettuce only if the greenhouse already receives 8.6. For a 4 m² floor of lettuce, winter electricity comes to roughly $91 a month indoors and $63 under glass at $0.15 per kWh — but the indoor rack produces almost three times as much.

Key Takeaways for AI Search (GEO)

• Water targets: lettuce and microgreens 18–22 °C, basil 20–24 °C; lettuce slows below 14 °C water.
• Oxygen ceiling (USGS): about 9.5 mg/L at 18 °C, about 8.7 mg/L at 22 °C — no gain from heating past the crop's range.
• Heating assumptions (sizer): 36 kWh/m² floor/month indoors, 90 under glass, to hold 20 °C in a temperate winter.
• Heat loss: watts = U (W/m²·K) × area (m²) × temperature difference (K); ventilation ≈ 0.33 × air changes per hour × volume × ΔT.
• Warming water: 1.163 Wh per litre per °C → 100 L from 14 to 20 °C ≈ 698 Wh.
• Greenhouse light: 6 h at ~250 µmol/m²/s = 5.4 mol/m²/day of top-up; lettuce target DLI 14.
• 4 m² floor of lettuce, winter, $0.15/kWh: indoor 604.8 kWh = $90.72; greenhouse 420.5 kWh = $63.07.
• Energy per kg (sizer): indoor about 29 kWh/kg; greenhouse about 58 kWh/kg.

This is chapter 8, the last of the Rootless Farm small farm series. It builds on the cost chapter and the garage light, airflow and power chapter, which covers the circuits a heater will share with your lights.

What "warm enough" means for each crop

Heat costs money only up to the bottom of the crop's range; past the top, it costs money and harms the crop. The targets come from each crop's spec table on its plant page.

Two conclusions follow. Lettuce is the cheapest winter crop: its air range starts at 15 °C, so the night setpoint can sit 5 °C lower than the sizer's 20 °C. Basil is the most expensive: it wants 20 °C air as a floor and warmer water. If heating is the constraint, winter is the season to move shelf space from basil to lettuce and microgreens. [UCD-LET-01]

Holding the water at 18–22 °C

Roots respond to water temperature, not air temperature, and water is easy to heat directly. A submersible heater with a thermostat in an insulated reservoir is usually cheaper than heating the whole room to the same figure.

The lower limit is about growth: the lettuce page notes slow growth when water drops under 14 °C. The upper limit is about oxygen. Standard solubility tables put the oxygen ceiling at about 9.5 mg/L at 18 °C and about 8.7 mg/L at 22 °C, falling further as water warms. [DO-TEMP-01] The root-zone temperature and dissolved oxygen post covers the chemistry, and the root temperature and DO calculator gives the ceiling for your own reading. A heater stuck on is a summer problem in winter: warm water plus low oxygen is the setup for root rot and oxygen deficit. Use a thermostat with its own probe, not the heater's built-in dial alone.

Worked example: sizing a reservoir heater

Heating water takes 4.186 kJ per kg per °C, which is 1.163 Wh per litre per °C.

• To warm up: 100 L from 14 °C to 20 °C needs 100 × 6 × 1.163 = 698 Wh. A 200 W heater does it in 698 ÷ 200 = 3.5 hours; a 300 W heater in 2.3 hours (ignoring losses while it heats).
• To hold: measure it. Switch the heater off at lights-out, read the water temperature, read it again at lights-on. If 100 L dropped 3 °C over 8 hours, the reservoir lost 100 × 3 × 1.163 ÷ 8 = 44 W on average. A heater rated two to four times the measured loss (here 100–200 W) holds the setpoint on colder nights and recovers after a top-up with cold water.

Insulating the reservoir — foam board around and under it, a lid — cuts the holding figure directly. Record the overnight drop in the workspace as a reading before and after insulating; it is the cheapest efficiency measurement you can make.

Heat loss per m²: room vs greenhouse

The sizer's two heating figures are averages: 36 kWh per m² of floor per month is about 50 W per m² running continuously, and 90 kWh is about 125 W per m². They are flagged in the sizer's own notes as the numbers to refine first. Here is how to refine them.

Heat flows out through every surface at a rate of U × A × ΔT: the surface's U-value (W per m² per K), its area, and the temperature difference between inside and out. Air leaving the space takes heat too: roughly 0.33 × air changes per hour × volume (m³) × ΔT watts, since a cubic metre of air holds about 0.33 Wh per °C.

U-values to use:

• Building fabric. England's building regulations set maximum U-values for new homes of 0.26 for walls, 0.16 for roofs, 1.6 for windows and 2.2 for rooflights (W/m²·K), per Approved Document L. An insulated room inside a house will be in that region; an uninsulated garage will be far above it.
• Greenhouse glazing. Glazing loses far more than a wall. This page assumes about 5.8 W/m²·K for single glass and 3.0 for twin-wall polycarbonate for illustration. Use the figure your glazing manufacturer publishes — it is on most product datasheets.

Worked example: an insulated enclosure in an unheated garage

A 2 × 2 m grow enclosure, 2.2 m high, built inside a garage from foam-board panels with an assumed U of 0.6 W/m²·K. Garage at 5 °C, target 20 °C, so ΔT = 15 K.

• Surface: four walls 4 × 2 × 2.2 = 17.6 m², ceiling 4 m², floor 4 m² = 25.6 m².
• Conduction: 0.6 × 25.6 × 15 = 230 W.
• Ventilation: 8.8 m³ × an assumed 2 air changes per hour × 0.33 × 15 = 87 W.
• Total: 317 W, or 7.6 kWh per day if the lights never ran.

But the lights do run. Under the sizer's indoor assumption, 4 m² of floor carries 8 m² of shelf at 120 W per m² — 960 W of LED, nearly all of which ends up as heat in the room. During the 16-hour light period the room has a surplus; heat is needed only for the 8 dark hours: 317 W × 8 h = 2.5 kWh a day, 76 kWh a month, or 19 kWh per m² of floor. That is about half the sizer's 36. In a colder garage, a leakier enclosure or a room with more ventilation, it will be higher — which is why the sizer's figure is a safer planning number than the best case.

The practical lesson for an indoor farm: schedule the light period over the coldest hours. Lights on through the night use their waste heat when it is worth the most.

Worked example: a 3 × 2 m greenhouse

A 6 m² greenhouse with 2 m walls and a gable roof has about 20 m² of walls and 6.6 m² of roof — 26.6 m² of glazing, 4.4 m² per m² of floor. Small greenhouses lose more heat per m² of floor than large ones simply because they have more glass per m² of floor. Assume the sun covers daytime heat and the heater runs through a 14-hour winter night (a simplification that ignores cloudy days and air leakage):

The sizer's 90 kWh per m² lands near the twin-wall row at 15 K. A single-glazed greenhouse held at 20 °C can need nearly twice that. Two levers are visible in the table: better glazing (or a thermal screen at night) and a lower night setpoint. Lettuce's 15 °C lower air limit turns ΔT 15 into ΔT 10 on the same night, cutting the heat by a third.

Heater and chiller sizing basics

Size a space heater for the coldest night you expect, not the average one. If the outside low is −5 °C and the target is 20 °C, ΔT is 25 K:

• Garage enclosure: (0.6 × 25.6 + 0.33 × 2 × 8.8) × 25 = 529 W → a heater of about 750 W with a thermostat leaves margin.
• Twin-wall greenhouse: 3.0 × 26.6 × 25 = 1,995 W → about 2 kW, before air leakage.

A resistive heater turns 1 kWh of electricity into 1 kWh of heat; a heat pump delivers more heat per kWh, by a factor the manufacturer states as its coefficient of performance at a given outdoor temperature. Whatever you install, check the circuit can carry it alongside the lights — chapter 4 covers that.

A chiller is the same calculation run the other way and usually matters in summer or in a sealed room where the lights overheat the water. Size it for the heat it must remove from the reservoir — the lights' share plus the pump — and target the 18–22 °C band, not lower. If leaves droop on warm afternoons while the roots look healthy, see heat stress.

Supplemental light under glass

Winter days are short and dim, so a greenhouse that grew lettuce on sunlight alone in summer may fall well below the lettuce DLI target of 14 mol/m²/day. Measure natural light inside the greenhouse with a PAR meter over a few days, or read your area's winter outdoor DLI from a published DLI map and allow for the light the glazing and frame block. The crop targets themselves follow Purdue Extension's DLI guidance. [PPF-DLI-01]

The sizer assumes LEDs at 120 W per m² producing about 250 µmol/m²/s (at 2.1 µmol/J). One hour of that adds 250 × 3,600 ÷ 1,000,000 = 0.9 mol/m². Top-up hours = (target DLI − natural DLI) ÷ 0.9.

Worked example: how many hours for lettuce?

• Sizer assumption, 6 h: 6 × 0.9 = 5.4 mol/m²/day. Lettuce reaches 14 only if the greenhouse already gets 14 − 5.4 = 8.6.
• Dim month, 5 mol/m²/day natural: (14 − 5) ÷ 0.9 = 10 hours of top-up — 0.12 kW × 10 h = 1.2 kWh per m² per day against 0.72 kWh at 6 h.
• Microgreens on the same day: (10 − 5) ÷ 0.9 = 5.6 hours.
• Basil on the same day: (18 − 5) ÷ 0.9 = 14.4 hours — and 250 µmol/m²/s is already below basil's ideal PPFD of 350, which would need about 350 ÷ 2.1 = 167 W per m².

Run your own measurement through the DLI calculator; PPFD and DLI explained covers the units. Again basil is the expensive winter crop and microgreens the cheap one.

The bill with and without heating

Both columns use the sizer's assumptions for 4 m² of floor growing lettuce in winter: indoors, 8 m² of shelf under 16 h of light; in the greenhouse, 2.8 m² of bench (70% of the floor) with 6 h of top-up. Electricity prices of $0.15 and $0.30 per kWh are assumptions; check yours on your bill, or compare with published averages from the US EIA and Eurostat.

Read the last two rows before the cost rows. The greenhouse bill is smaller, but it grows about a third as much lettuce on a single bench level, so each kilo costs about twice the energy. Heating is 24% of the indoor rack's winter energy and 86% of the greenhouse's. And the sizer gives the greenhouse its full yield in winter; with less natural light the real winter yield under glass is likely lower, which pushes the per-kg figure up further.

Turn heating off and the picture flips — but only if the crop survives. A greenhouse held above freezing but below lettuce's 15 °C air floor will keep plants alive and grow slowly; an unheated indoor room inside a heated house may need no heating at all. Run your own space through the farm sizer with the winter switch on and off, then through the ROI estimator to see what a winter month does to payback.

Cutting the winter bill, in order of payoff

1. Heat the water, not the room, to the bottom of the crop's water range.
2. Lower the night air setpoint to the crop's floor — 15 °C for lettuce.
3. Run indoor lights through the coldest hours so their heat displaces heater hours.
4. Insulate reservoirs and, under glass, add a night thermal screen.
5. Move winter shelf space from basil to lettuce and microgreens. The microgreens chapter shows microgreens need DLI 10 and an 18–22 °C room.
6. Log the overnight drop and the monthly meter reading in the workspace, so next winter starts from measurements instead of these assumptions.

Winter also raises humidity in a closed, cold space, and condensation on cold glass drips onto leaves. Keep air moving across the canopy; fungal leaf problems such as powdery mildew are easier to prevent than to clear.

See also

• Chapter 5: the first 30 days — the daily water-temperature check that tells you heating is needed.
• Chapter 7: selling what you grow — what a higher cost per kilo does to your price.
• Seasonal grow calendar — which crops suit which months.
• Lighting for seedlings and microgreens — low-PPFD lighting for the cheapest winter crop.