Example Data Table
These sample rows show how insulation and temperature difference affect duty cycle and daily runtime.
| Setup | Area | U-value | Target / Outdoor | Heater | Duty | Runtime (h/day) |
|---|---|---|---|---|---|---|
| Small grow tent | 18 m² | 2.8 W/m²·°C | 20°C / 10°C | 1500 W, 95% | 35% | 8.4 |
| Poly greenhouse | 55 m² | 3.5 W/m²·°C | 18°C / 3°C | 3000 W, 90% | 92% | 22.1 |
| Insulated greenhouse | 55 m² | 1.6 W/m²·°C | 18°C / 3°C | 3000 W, 90% | 42% | 10.1 |
Formula Used
- Envelope area (dimensions):
A = 2(LW + LH + WH) × roof_factor - Temperature difference:
ΔT = max(0, T_target − T_outdoor) - Heat loss:
Q_loss = U × A × ΔT - Adjusted requirement:
Q_req = Q_loss × loss_factor - Useful heater output:
P_useful = P_input × η(or the label output if you select “Output”). - Duty cycle:
duty = clamp(Q_req / P_useful, 0..1) - Runtime per day:
hours = duty × heating_hours - Energy per day:
kWh = (P_input/1000) × hours
U and R describe insulation. U is the rate of heat transfer; R is resistance (R = 1/U in consistent units).
How to Use This Calculator
- Select your unit system and envelope method.
- Enter greenhouse dimensions (or total envelope area) and a roof/shape factor.
- Choose U-value or R-value, then enter your insulation figure.
- Set target and outdoor temperatures and heating hours per day.
- Enter heater rating, choose whether it is input or output, then set efficiency.
- Select fuel type, enter your local price, and press Calculate runtime.
For best results, use the coldest typical night temperature and a realistic loss factor for wind and leaks.
Runtime estimates for protected growing spaces
This tool estimates how long a greenhouse heater must run to maintain a chosen setpoint. It combines envelope area, insulation level, and the temperature gap between inside and outside. A loss factor adds realism for wind, door openings, and air leaks. Outputs include required heat, duty cycle, runtime hours per day, daily energy use, and a quick cost snapshot for smarter seasonal planning in everyday practice.
Envelope sizing and insulation inputs
Two paths compute envelope area: enter length, width, and height to approximate walls plus roof, or supply a known surface area from drawings. A roof or shape multiplier increases area for curved tunnels or pitched roofs. Better insulation lowers U-value, reducing heat loss. Draft sealing lowers the loss factor, often giving the fastest runtime improvement per hour spent weatherproofing in winter for most home growers.
Heater rating, efficiency, and cycling behavior
Heater power can be entered in watts or BTU per hour. If the rating is fuel input, the calculator converts it to useful heat using efficiency. If the rating is stated output, it back-calculates the required input energy. Duty cycle is capped at 100%, so results remain stable often even when the heater is undersized during the coldest nights and surprise frosts outside at dawn.
Energy and operating cost tracking
Cost estimates convert energy into common fuel units such as kilowatt-hours, propane gallons, and gas therms. The model uses typical energy contents to approximate units consumed per day. Enter your local price per unit to estimate daily and monthly spending. Because fuel quality, altitude, and heater tuning vary, treat cost numbers as planning ranges, not invoices and maintenance for billing, taxes, local surcharges, or fees.
Practical decisions for winter crop protection
Use this calculator when selecting heaters, comparing insulation upgrades, or scheduling nighttime protection for tender crops. Start with conservative outdoor temperatures and round up the loss factor if you expect frequent venting or strong winds. If runtime approaches the full heating window, consider adding thermal mass, double-layer film, or a backup heater to protect plants reliably and safely, even during outages and sudden storms overnight.
FAQs
What does duty cycle mean here?
Duty cycle is the fraction of time the heater must run within your selected heating window. A 50% duty cycle means the heater runs about half the time to match estimated heat loss at the chosen temperatures.
Why is there a loss factor?
Loss factor increases calculated heat demand to account for wind, air leakage, door use, and imperfect coverage. Tight structures may use 1.00–1.10, while drafty spaces or frequent access can justify 1.30–1.50.
Should I enter U-value or R-value?
Use whichever you know. U-value directly represents heat transfer rate. R-value represents thermal resistance. The calculator converts R to U internally, so the final runtime result is consistent when the values are equivalent.
My heater label shows BTU/hr. What should I select?
Choose BTU/hr as the rating unit, enter the label value, then choose whether it is input or output. If you are unsure, select input and use a realistic efficiency from the manual or typical equipment specs.
Why does runtime cap at 100% duty cycle?
When estimated heat demand exceeds useful heater output, the heater would run continuously during the heating window. Capping at 100% prevents misleading numbers and signals that insulation upgrades or more heater capacity may be needed.
How accurate are the fuel cost numbers?
They are planning estimates based on typical energy contents and your entered price. Real costs depend on fuel quality, burner tuning, cycling losses, and local billing. Use the results to compare scenarios rather than predict exact bills.
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