Greenhouse Heating Calculator
Heater size to hold your target temperature.
Enter your values
Enter the surface area in m².
Choose one of: Single glass, Twin-wall polycarbonate, Triple-wall, Polythene film.
Enter the target inside temp in °C.
Enter the coldest outside temp in °C.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Heater size needed
- 4.38 kW
- In BTU/h
- 14,961
- Temperature lift
- 18 °C
- Nightly cost at $0.25/kWh
- $13.15
Quick answer
Greenhouse heat demand depends on surface area, glazing U-value and the inside-outside temperature difference.
Watts = area × U × ΔT
At a glance
| What it does | Heater size to hold your target temperature. |
|---|---|
| Category | Science |
| Inputs needed | Surface area, Glazing, Target inside temp, Coldest outside temp |
| Main output | Heater size needed |
| Formula | Watts = area × U × ΔT |
| Cost | Free — no sign-up, no download |
How to use the Greenhouse Heating Calculator
- 1Calculate the total glazed surface area.
- 2Pick your glazing type.
- 3Enter target and coldest outside temperatures.
Inputs explained
- Surface area(m²)
- Enter the surface area in m².
- Glazing
- Choose one of: Single glass, Twin-wall polycarbonate, Triple-wall, Polythene film.
- Target inside temp(°C)
- Enter the target inside temp in °C.
- Coldest outside temp(°C)
- Enter the coldest outside temp in °C.
Worked example
Using the values the calculator loads with:
Inputs
- Surface area42 m²
- GlazingSingle glass
- Target inside temp15 °C
- Coldest outside temp-3 °C
Results
- Heater size needed4.38 kW
- In BTU/h14,961
- Temperature lift18 °C
- Nightly cost at $0.25/kWh$13.15
Frequently asked questions
Should I add bubble wrap?
Yes — internal bubble insulation can cut heat demand by a third.
Does soil store heat?
Yes, thermal mass such as water barrels smooths overnight swings.
What do I need to enter into the Greenhouse Heating Calculator?
Just 4 values: surface area, glazing, target inside temp, coldest outside temp. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Greenhouse Heating Calculator work out the answer?
Greenhouse heat demand depends on surface area, glazing U-value and the inside-outside temperature difference. It applies the formula Watts = area × U × ΔT and shows the working so you can check each step by hand.
Is the Greenhouse Heating Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.