UnitConv

Greenhouse Heater Sizing & Heating Cost Calculator

Work out the heater output your greenhouse needs and its heating cost. Enter dimensions or surface area, the cladding, and the indoor target and outdoor design-low temperatures to get the required output (W, kW, BTU/h) and the electricity cost from the heat loss.

m
m
m
°C
°C
%
Required heater output
3,260 W
3.26 kW · 11,125 BTU/h

Includes a 10% safety margin

Heat loss
2,964
W
Temp difference (ΔT)
20
K
Surface area
49.4
m²

Cladding comparison

Required output by cladding under the same conditions. See at a glance which cover saves the most energy.

Single glass6,303 W
Single poly (film)6,521 W
Double poly (air gap)4,347 W
Twin-wall polycarbonate3,260 W
Triple-wall polycarbonate2,174 W
Double glazing3,043 W

Heating cost

h
days
/kWh
%
Per day
7.82
26.1 kWh
Season total
939
3,130 kWh

Sources: ASHRAE greenhouse heating guidance and published U-values from cladding manufacturers. Actual needs vary with wind, ventilation, and solar gain.

About this calculator

A greenhouse has a large surface in contact with the outdoors, so heat escapes quickly on cold nights and through winter. To protect your plants you need a heater large enough to replace that heat loss. This tool calculates heat loss from the surface area, the cladding's heat-transfer coefficient (U-value), and the indoor-outdoor temperature difference, then adds a safety margin and shows the required heater output in W, kW, and BTU/h. It also compares the output needed for different claddings and estimates the heating cost.

How to use

1. Choose the input method: "From dimensions" or "Enter surface area." With dimensions, enter width, length, and height and the surface area is computed for you. 2. Pick the cladding (single glass through triple-wall polycarbonate). Each has a different U-value. 3. Enter the indoor target temperature and the outdoor design low. 4. Set a safety margin (for wind, ventilation, and headroom) — typically 10-25%. 5. The required heater output is shown. The cladding comparison bars reveal how energy-efficient each cover is. 6. Enter heating hours, days, electricity rate, and efficiency to estimate the per-day and season-total cost.

Formula

Heat loss: Q (W) = U × A × ΔT U: cladding heat-transfer coefficient (W/m²·K) A: surface area (m²) ΔT: indoor target − outdoor design low (K) Required output = Q × (1 + safety margin) BTU/h conversion: BTU/h = W × 3.412 From dimensions, A is the full heat-losing envelope: four walls plus roof and floor.

Reading the results

The required heater output is the continuous power needed to hold the indoor temperature on the coldest night. When choosing real equipment, pick a heater rated at or above this value. The shorter a cladding's comparison bar, the less heat it loses and the lower its heating cost. Upgrading to double poly or polycarbonate costs more upfront but can save a lot on electricity over time.

FAQ

How do I find the required heater output? ▾

Calculate heat loss as Q = U × A × ΔT from the surface area, the cladding U-value, and the indoor-outdoor temperature difference, then add a safety margin. This tool does it automatically in W, kW, and BTU/h.

What is the U-value (heat-transfer coefficient)? ▾

The U-value is the heat lost per square metre of cladding for each 1 K of temperature difference (W/m²·K). A lower U-value means better insulation and a smaller required heater.

What changes when I switch cladding? ▾

The cladding's U-value changes, which changes heat loss substantially. Moving from single film to twin-wall polycarbonate or double glazing can cut the required output and heating cost by roughly 30-60%.

What is the difference between BTU and W? ▾

Both measure heat output. They convert as 1 W = 3.412 BTU/h. W and kW are common in most of the world, while BTU/h is common in North America. This tool shows both.

How can I lower heating costs? ▾

Choose better-insulating cladding, add a night thermal curtain or inner liner, seal drafts, lower the target temperature by 1-2 °C, and heat only around the plants. Use the cladding comparison and cost estimate here to gauge the savings.