Greenhouse Heating Cost Calculator

How this greenhouse heating cost calculator works

This calculator estimates how much it costs to heat a greenhouse by combining a simple heat loss formula with your local energy price and heater efficiency. It is designed for quick budgeting rather than exact energy billing, but it will put you in the right ballpark for winter operating costs.

Core idea

Heat constantly flows from warm air inside your greenhouse to colder outdoor air. To keep the inside temperature higher, your heater must replace the heat that escapes through the glazing, framing, and any air leaks. The larger the surface area and the bigger the temperature difference, the more energy you need.

For a basic estimate, the tool models hourly heat load using a typical rule of thumb for small to medium hobby or light commercial greenhouses:

Base heat demand (BTU/hour) ≈ Greenhouse area × Temperature rise × 1.2

• Greenhouse area: floor area in square feet (ft²).
• Temperature rise: how many °F warmer you want the inside compared to outside.
• 1.2 BTU/ft²·°F: a common approximate heat loss factor for a single‑layer hobby structure with modest air leaks.

Formula in more detail

The calculator follows these steps:

1. Estimate hourly heat loss in BTU.
2. Adjust for heater efficiency.
3. Convert BTU to kilowatt‑hours (kWh).
4. Multiply by heating hours per day and by your cost per kWh.

The main formula can be expressed as:

Hourly heat loss (BTU/h) ≈ Area × Temperature rise × 1.2

Because heaters are not 100% efficient, the fuel or electricity you buy must supply more heat than actually makes it into the greenhouse air. We account for this with the efficiency term:

Required input heat (BTU/h) = Hourly heat loss (BTU/h) ÷ (Efficiency ÷ 100)

To convert BTU to kWh we use the physical relationship that 1 kWh ≈ 3,412 BTU. Putting everything together in a single expression gives:

Where:

• C = daily heating cost in dollars.
• A = greenhouse area (ft²).
• ΔT = temperature rise (°F above outdoors).
• 1.2 = approximate BTU/ft²·°F factor.
• H = heating hours per day.
• P = energy price per kWh in dollars.
• η = heater efficiency in percent.

In words: the calculator estimates how much heat you lose, divides by efficiency to find how much heat the heater must supply, converts that to kWh, and then multiplies by your energy price to calculate cost.

Interpreting your results

When you click Calculate, the tool will typically show an estimated daily energy use in kWh and the corresponding daily and monthly cost. Use these outputs as a planning guide rather than a precise prediction.

Key points when interpreting the numbers:

• Daily cost shows how much you might spend on a typical cold day if conditions match your inputs.
• Monthly cost assumes similar conditions every day. In reality, milder days will cost less and cold snaps will cost more.
• Energy use in kWh is useful for comparing heater types or checking the sizing of your electrical service.
• Sensitivity to temperature rise: costs grow roughly in direct proportion to the temperature difference you request. Reducing your target by 5–10°F can noticeably cut the bill.
• Impact of efficiency: a more efficient heater lowers the required kWh input to maintain the same conditions, reducing costs.

If the calculated cost seems surprisingly high, double‑check:

• That you entered the floor area (length × width), not the perimeter.
• That the temperature rise is realistic for your climate (many greenhouses run 10–40°F above outside temperature).
• That your energy price per kWh matches your latest bill or supplier quote.

Worked example: small hobby greenhouse

Consider a 10 ft × 16 ft hobby greenhouse used to keep cool‑season crops growing through winter. The owner wants it to be about 20°F warmer than the outdoor temperature on cold nights.

Inputs:

• Area: 10 × 16 = 160 ft²
• Temperature rise: 20°F
• Energy cost: $0.15 per kWh (typical residential electricity rate in many regions)
• Heater efficiency: 80% (0.8 as a fraction)
• Heating hours per day: 12 hours (assuming nights and cold mornings)

Step 1: Estimate hourly heat loss.

Hourly heat loss ≈ 160 ft² × 20°F × 1.2 BTU/ft²·°F = 160 × 20 × 1.2 = 3,840 BTU/h

Step 2: Adjust for heater efficiency.

Required input heat ≈ 3,840 ÷ 0.8 = 4,800 BTU/h

Step 3: Convert to kWh.

kWh per hour ≈ 4,800 ÷ 3,412 ≈ 1.41 kWh/h

Step 4: Apply heating hours and energy price.

Daily energy use ≈ 1.41 kWh/h × 12 h = 16.9 kWh/day
Daily cost ≈ 16.9 × $0.15 ≈ $2.54 per day
Monthly cost (30 days) ≈ 30 × $2.54 ≈ $76.20

This example suggests that holding the greenhouse 20°F above outdoor temperatures for 12 hours per day would cost around $75 per month at $0.15/kWh, assuming nights are consistently cold. Warmer weeks will be cheaper; extreme cold spells will cost more.

You can also use the calculator to test what‑if changes:

• Lower the temperature rise from 20°F to 15°F and see how much the monthly cost drops.
• Increase efficiency from 80% to 90% to estimate savings from upgrading your heater or sealing leaks in ductwork.
• Change energy cost to compare different utility plans or alternative fuels converted to a kWh equivalent.

Comparing common greenhouse heater options

The calculator uses energy cost per kWh, but you can still compare different heater types by converting their fuel price to a kWh equivalent. The table below summarizes typical characteristics of popular greenhouse heating options.

To compare these using the calculator, convert your fuel price to an effective cost per kWh of heat:

• Find the fuel’s heat content (e.g., BTU per gallon of propane or per therm of natural gas).
• Multiply by the heater’s efficiency to find delivered BTU to the greenhouse.
• Divide by 3,412 to convert BTU to kWh.
• Divide the fuel price by the resulting kWh to get an approximate cost per kWh of usable heat, then enter that in the calculator.

Assumptions and limitations

This tool uses a simplified model that is helpful for planning and comparison, but it cannot capture every detail of real‑world greenhouse performance. Keep these assumptions and limitations in mind when using the results:

• Single approximate heat loss factor: The constant 1.2 BTU/ft²·°F assumes a relatively lightweight, single‑layer structure with moderate air leakage. Heavily insulated double‑glazed houses may lose less heat; thin plastic tunnels in windy sites may lose more.
• Floor area vs. total surface area: The calculator uses floor area as an indirect proxy for the full surface area (walls and roof). Two greenhouses with the same floor area but different shapes or heights can lose different amounts of heat.
• Steady‑state conditions: The formulas assume outdoor temperature, wind, and solar gain are fairly constant over the heating period. In reality, clouds, sun, and wind can change heat loss hour by hour.
• No explicit solar gain: Daytime sunlight can significantly reduce heating demand, especially in clear weather. The calculator does not subtract this free heat, so it may over‑estimate costs on sunny days and under‑estimate them on long, cloudy, windy spells.
• Uniform internal temperature: We assume the greenhouse air is well mixed by fans, so there are no large hot or cold pockets. Poor air circulation can increase real‑world heater run time.
• Heater efficiency as a single number: Actual efficiency varies with operating conditions, cycling, and maintenance. Use a realistic range (for many systems, 70–90%) rather than an ideal brochure value.
• Energy price stability: The same rate is applied to every kWh in the estimate. Tiered tariffs, demand charges, or seasonal rate changes are not modeled.
• Scope of use: The calculator is most suitable for small to medium hobby or light commercial greenhouses. Specialized glasshouses, large multi‑bay ranges, or tightly engineered facilities may need a detailed heat loss study.

Because of these simplifications, treat the calculator as a planning and comparison tool, not as a guarantee of future utility bills. It is ideal for checking the impact of design decisions (insulation, setpoint temperature, heater efficiency) and for estimating whether a proposed heating approach is roughly affordable.

Practical tips for reducing greenhouse heating costs

Once you understand how the calculator works, you can use it to explore strategies to lower your winter fuel use:

• Improve insulation: Upgrading from single to double glazing, adding inner plastic layers, or using thermal curtains at night effectively lowers the real‑world heat loss factor compared with the 1.2 BTU/ft²·°F default.
• Seal air leaks: Gaps around doors, vents, and frame connections can waste heat. Weather‑strip and seal obvious leaks to reduce heat loss without changing the calculator inputs.
• Adjust target temperature: Many crops tolerate a wider temperature range than you might expect. Lowering your setpoint by a few °F can significantly reduce required energy, which you can confirm by rerunning the calculation.
• Optimize heating schedule: Instead of running heat 24 hours per day, consider focusing on the coldest hours and allowing slight temperature drops at non‑critical times. Change the heating hours field to estimate savings.
• Upgrade heater efficiency: If the calculator shows high operating costs, compare your current heater efficiency with modern high‑efficiency models. Even a modest efficiency gain can cut fuel use over a long season.

By experimenting with different inputs and understanding the underlying assumptions, you can use this greenhouse heating cost calculator to support smarter design, crop planning, and budgeting decisions each winter season.