Greenhouse Heater Calculator

Calculate total BTU/hr heating requirements based on surface area, glazing U-value, and winter design temperatures.

Target Inside Temp (°F) Night minimum for plants

Lowest Winter Temp (°F) Local winter extreme low

Total Glazing Surface Area (Sq Ft) Roof area + all 4 walls combined

Glazing Material & U-Value

Wind Exposure & Infiltration Factor

Required Heater Capacity

19,500 BTU/h

5.71 kW Electric Equivalent

Temperature Differential (ΔT) 45 °F Inside vs Outside

Propane Burn Rate 0.21 gal/hr At 100% duty cycle

Safety Sizing Rule Heaters should be sized for the extreme design low, not the average nighttime temperature. Many growers install two smaller heaters instead of one large unit for redundancy.

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How Greenhouse Heat Loss Is Calculated

Unlike insulated residential homes, greenhouse envelopes are composed entirely of thin, translucent glazing materials. Conduction heat loss through glazing accounts for 75% to 85% of total greenhouse thermal demand.

Heat Loss (BTU/hr) = Surface Area (sq ft) × Glazing U-Value × (T_inside - T_outside) × Infiltration Factor

Upgrading from single-pane glass (U = 1.15) to 6mm twin-wall polycarbonate (U = 0.58) cuts seasonal heating costs and required heater capacity almost in half (by ~50%).

Frequently Asked Questions

What inside temperature should a winter greenhouse maintain?

Cold-hardy greens (spinach, kale, lettuce) thrive with minimum night temperatures of 38°F to 42°F. Warm-season tomatoes and peppers require at least 55°F to 60°F.

How do I convert BTU/hr to electric heater kilowatts?

Divide BTU/hr by 3,412. For example, a 17,000 BTU/hr demand requires approximately 5.0 kW of electric heating (often two 2,500W heaters).

What is a glazing U-value?

U-value measures overall heat transfer rate in BTU/(hr·sq ft·°F). Lower numbers indicate superior insulation. It is the reciprocal of the R-value (U = 1 / R).