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

Angelina Everly leads the editorial desk at Live Green Gardens, blending practical plant care, hands-on product testing, and approachable outdoor styling. She focuses on step-by-step how-tos, buyer’s guides, and small-space makeovers that work in real life and real budgets. When she’s not comparing pruning shears or setting up a drip kit, you’ll find her creating cozy corners with planters, solar lights, and pollinator-friendly picks—always with clear pros/cons and safety notes so you can buy once and garden happy.