Greenhouse Heating Calculator

Complete User Guide: Greenhouse Heating Calculator

What It Does

Calculate precise heating requirements for your greenhouse including:

• BTU/hour needed to maintain temperature
• Heater sizing recommendations
• Operating cost estimates (electric & propane)
• Surface area calculations for any greenhouse style

How to Use

Step 1: Select Greenhouse Style

• Gable/A-Frame: Traditional peaked roof
• Lean-To: Attached to building
• Hoop House: Curved polytunnel

Step 2: Enter Dimensions

• Length: Front to back (feet)
• Width: Side to side (feet)
• Ridge Height: Peak/apex height (feet)
• Eaves Height: Sidewall height (feet)

Click “Calculate Surface Area” to auto-calculate total sq ft

Step 3: Select Insulation

Choose heat loss factor:

• 1.1: Single glass (least efficient)
• 0.8: Double glass/polycarbonate
• 0.7: Polyethylene film
• 0.6: Bubble wrap lined
• 0.5: Insulated panels (most efficient)

Step 4: Set Temperatures

• Outside Temp: Coldest winter night (°F)
• Inside Temp: Target minimum (°F)

Step 5: Calculate

Get complete heating requirements, heater recommendations, and cost estimates

Understanding BTU Formula

textBTU/hour = Surface Area × Heat Loss Factor × Temperature Difference

Example:

• Surface area: 500 sq ft
• Heat loss: 1.1 (single glass)
• Outside: 20°F | Inside: 50°F
• Temp difference: 30°F
• BTU = 500 × 1.1 × 30 = 16,500 BTU/hour

Heat Loss Factors Explained

Lower number = better insulation = less heating needed

Improvement Impact:
Going from 1.1 to 0.6 = 45% heating cost savings!

Temperature Guidelines

By Plant Type:

• 40-45°F: Cold-hardy (lettuce, kale, spinach)
• 50-55°F: Cool-season (broccoli, pansies)
• 60-65°F: Warm-season (tomatoes, peppers)
• 70-75°F: Tropical (orchids, citrus)

Cost-Saving Strategy:

• Set night temp 5-10°F lower than day
• Every 1°F reduction = 5% cost savings
• Use programmable thermostat

Heater Sizing

Add 20% Safety Buffer:
If calculation shows 15,000 BTU needed, buy 18,000 BTU heater

Electric Heaters:

• 1,500W = ~5,000 BTU (small)
• 3,000W = ~10,000 BTU (medium)
• 5,000W = ~17,000 BTU (large)

Propane/Gas Heaters:

• 10,000-50,000 BTU common
• Better for larger greenhouses
• Need proper ventilation

Operating Cost Estimates

Electric (@$0.12/kWh):

text10,000 BTU = 3 kW
3 kW × 12 hrs × 60% duty × $0.12 = $2.16/night
$2.16 × 90 nights = $194/winter

Propane (@$2.50/gallon):

text10,000 BTU heater
1 gallon propane = 91,000 BTU
0.11 gal/hr × 12 hrs × 60% duty × $2.50 = $2.00/night
$2.00 × 90 nights = $180/winter

Duty Cycle: Heater typically runs 50-70% of time (not continuous)

Energy-Saving Strategies

Insulation Improvements:

• Bubble wrap lining: -30% costs
• Thermal curtains: -20% costs
• Seal air leaks: -15% costs
• Combined: Save 50%+

Temperature Management:

• Lower night temp 10°F: -50% costs
• Zone heating: Heat only needed areas
• Use thermal mass: Water barrels store heat

Passive Solar:

• Maximize south-facing exposure
• Black water barrels absorb/release heat
• Thermal blankets at night

Common Mistakes

❌ Undersizing heater – Can’t maintain temp on coldest nights
❌ No backup heater – Single point of failure
❌ Poor insulation – Heating costs 2-3× higher
❌ No thermostat – Wastes energy, risks overheating
❌ Ignoring wind – Windbreaks save 20-30%

Safety Considerations

For Propane/Gas Heaters:

• ✅ Proper ventilation required
• ✅ CO detector mandatory
• ✅ Annual inspection/service
• ✅ Keep 3 ft clearance from plants

For Electric Heaters:

• ✅ GFCI outlets required
• ✅ Proper amp circuit
• ✅ Ground fault protection
• ✅ Weather-resistant fixtures

For All Heaters:

• ✅ Temperature alarm system
• ✅ Backup power (generator)
• ✅ Fire extinguisher nearby
• ✅ Emergency contact list

Practical Examples

Example 1: Small Hobby Greenhouse

textSize: 6' × 8' gable
Surface area: ~200 sq ft
Insulation: Bubble wrap (0.6)
Outside: 25°F | Inside: 50°F
Temp diff: 25°F

BTU = 200 × 0.6 × 25 = 3,000 BTU
Heater: 3,500-4,000 BTU
Cost: ~$100-150/winter (electric)

Example 2: Medium Production Greenhouse

textSize: 12' × 20' gable
Surface area: ~800 sq ft
Insulation: Double glass (0.8)
Outside: 20°F | Inside: 60°F
Temp diff: 40°F

BTU = 800 × 0.8 × 40 = 25,600 BTU
Heater: 30,000 BTU recommended
Cost: ~$500-700/winter (propane)

Example 3: Large Commercial Greenhouse

textSize: 30' × 96' hoop house
Surface area: ~4,500 sq ft
Insulation: Poly film (0.7)
Outside: 15°F | Inside: 55°F
Temp diff: 40°F

BTU = 4,500 × 0.7 × 40 = 126,000 BTU
Heater: 150,000 BTU (multiple units)
Cost: ~$2,500-3,500/winter (propane)

Maintenance Schedule

Pre-Season (Fall):

• Inspect/clean heater
• Test thermostat accuracy
• Seal air leaks
• Install insulation
• Check electrical connections

During Season (Winter):

• Monitor daily temperatures
• Check fuel levels weekly
• Clean vents monthly
• Test backup systems

Post-Season (Spring):

• Service heater
• Remove/store seasonal insulation
• Document performance
• Plan improvements

This complete calculator and guide provides everything needed for efficient greenhouse heating planning and cost management!

Angelina Everly

With over 15 years of hands-on experience in controlled-environment agriculture, Angelina leads our lab audits. Her focus is on bridging the gap between high-end agricultural tech and the home grower, ensuring every recommendation is backed by real-world data and yield performance.