Solar Heat Gain Calculator Guide

What is Solar Heat Gain?

Solar heat gain is the increase in temperature caused by sunlight passing through windows and heating your home's interior. On a sunny 85°F day, south-facing windows can add 8,000-15,000 BTU/hour of heat load—equivalent to having 10-15 people standing in your home generating body heat.

This is why two identical 1,500 sq ft homes need different AC sizes: one with 20 windows (high solar gain) needs 30,000 BTU while another with 8 windows needs only 22,000 BTU. Use our BTU calculator to account for window area and orientation in your load calculation.

SHGC (Solar Heat Gain Coefficient)

What is SHGC?

SHGC is a rating from 0 to 1.0 showing how much solar heat passes through windows:

• 0.25 SHGC: Blocks 75% of solar heat (best for hot climates)
• 0.40 SHGC: Blocks 60% (standard low-E windows)
• 0.60 SHGC: Blocks 40% (standard clear glass)
• 0.85 SHGC: Blocks 15% (single-pane, no coating)

Lower SHGC = less cooling load. Replacing 0.80 SHGC windows with 0.30 SHGC windows cuts solar heat gain by 62%, reducing AC capacity requirements by 15-25%.

SHGC by Window Type

Find SHGC rating on window label (NFRC sticker) or in window specifications from manufacturer.

Calculating Solar Heat Gain

Formula:

BTU/hr = Window Area (sq ft) × SHGC × Solar Intensity (BTU/hr-sqft) × Orientation Factor

Solar Intensity by Climate:

• Hot climates (Zones 1-2): 250 BTU/hr-sqft (average over cooling season)
• Mixed climates (Zones 3-4): 220 BTU/hr-sqft
• Cool climates (Zones 5+): 200 BTU/hr-sqft

Orientation Factors:

• South: 1.8-2.2 (most sun exposure in summer)
• West: 1.5-1.8 (afternoon sun + higher ambient temps)
• East: 1.2-1.4 (morning sun, cooler air temps)
• North: 0.6-0.8 (minimal direct sun)

Example Calculation: Living Room

• Location: Phoenix (Zone 2), south-facing
• Windows: Two 4' × 5' windows = 40 sq ft total
• Type: Double-pane clear (0.70 SHGC)
• Calculation: 40 sq ft × 0.70 SHGC × 250 BTU/hr-sqft × 2.0 (south) = 14,000 BTU/hr

That's equivalent to running a 14,000 BTU space heater in your living room on a sunny day! Your AC must remove this heat plus heat from walls, ceiling, people, and appliances.

After Window Upgrade:

• New windows: Triple-pane low-E (0.25 SHGC)
• New calculation: 40 sq ft × 0.25 SHGC × 250 × 2.0 = 5,000 BTU/hr
• Reduction: 14,000 - 5,000 = 9,000 BTU/hr saved (64% reduction)

For a whole house, this can reduce total cooling load by 15-30%, allowing you to downsize from 3 tons to 2.5 tons = $800-1,200 savings on AC equipment.

Impact by Window Orientation

South-Facing Windows (Biggest Problem)

• Summer sun angle: 70-75° above horizon at noon (sun shines directly through)
• Duration: 8-10 hours/day of direct sun
• Heat gain: 2-3x higher than north windows
• Solutions:
  • Install awnings or overhangs (3-4 ft projection blocks 70-90% of summer sun)
  • Use low-SHGC windows (0.25-0.35)
  • Plant deciduous trees 15-20 ft from house (shade in summer, allow sun in winter)
  • Interior cellular shades (block 40-60% of heat)

West-Facing Windows (Second Worst)

• Problem: Afternoon sun hits when outdoor temps peak (3-6 PM = hottest part of day)
• Heat gain: 1.5-2x higher than north windows
• Solutions:
  • Exterior solar screens (block 65-90% of heat, reduce visibility 10-15%)
  • Reflective window film (block 50-70% heat, somewhat darkens view)
  • Vertical shade structures (pergolas with louvers)
  • Minimize west window area if building new/renovating

East-Facing Windows (Moderate)

• Morning sun: Hits when outdoor temps are 10-15°F cooler than afternoon
• Heat gain: 30-50% less than west windows
• Solutions: Low-E windows usually sufficient; awnings if extreme

North-Facing Windows (Minimal Impact)

• No direct sun: Only diffuse/reflected light
• Heat gain: 70-80% less than south windows
• Strategy: Maximize north windows for natural light without cooling penalty

Window Upgrade Payback Calculation

Scenario: 2,000 Sq Ft Home, Phoenix

Current Windows:

• Total: 200 sq ft of windows (10% of floor area—typical)
• Type: Double-pane clear (0.70 SHGC)
• Distribution: 60 sq ft south, 50 sq ft west, 50 sq ft east, 40 sq ft north
• Solar heat gain:
  • South: 60 × 0.70 × 250 × 2.0 = 21,000 BTU/hr
  • West: 50 × 0.70 × 250 × 1.6 = 14,000 BTU/hr
  • East: 50 × 0.70 × 250 × 1.3 = 11,375 BTU/hr
  • North: 40 × 0.70 × 250 × 0.7 = 4,900 BTU/hr
  • Total: 51,275 BTU/hr

After Upgrade to Low-E (0.30 SHGC):

• New solar gain: 51,275 × (0.30 / 0.70) = 21,975 BTU/hr
• Reduction: 51,275 - 21,975 = 29,300 BTU/hr saved

Financial Impact:

• AC downsizing: Reduce from 3.5 tons (42k BTU) to 2.5 tons (30k BTU) after accounting for total load reduction
• Equipment savings: $1,200 (smaller AC)
• Annual energy savings: 29,300 BTU/hr × 1,800 hrs/yr ÷ 16 SEER2 ÷ 1,000 × $0.13/kWh = $424/year
• Window upgrade cost: 200 sq ft × $45/sq ft (low-E double-pane) = $9,000
• Net investment: $9,000 - $1,200 = $7,800
• Payback: $7,800 ÷ $424 = 18.4 years

Long payback, BUT: Windows also reduce heating costs (lower U-factor), improve comfort (less radiant heat), increase resale value, and last 20-30 years. Total payback considering heating savings: 10-14 years.

Cost-Effective Solar Heat Reduction Strategies

1. Exterior Shading (Best ROI: 2-5 years)

• Awnings: $300-800 per window, block 65-75% of heat
• Solar screens: $50-150 per window (DIY) or $150-300 installed, block 70-90%
• Shade trees: $100-500 per tree, block 70-90% after 5-10 years growth
• Pergolas/lattice: $1,500-4,000, shade multiple windows

Why exterior shading wins: Blocks heat BEFORE it enters home, preventing glass from heating up and radiating indoors. Interior shades only block 30-50% because glass still absorbs heat.

2. Window Films (ROI: 4-8 years)

• Cost: $6-12/sq ft installed
• Effectiveness: Block 50-70% of solar heat
• Pros: Cheap, fast install, no structural changes
• Cons: Reduces visible light (darker view), may void window warranty, degrades over 10-15 years

Good short-term solution while saving for window replacement.

3. Interior Cellular Shades (ROI: 3-6 years)

• Cost: $50-200 per window
• Effectiveness: Block 40-60% of heat (less than exterior shading)
• Pros: Cheap, easy DIY, doubles as privacy/blackout
• Cons: Must remember to close during sunny hours (compliance issue)

4. Low-E Window Replacement (ROI: 10-18 years)

• Cost: $400-900 per window installed
• Effectiveness: Block 60-75% of heat vs old single-pane
• Pros: Permanent solution, improves comfort/resale, reduces heating costs too
• Cons: High upfront cost, long payback on cooling alone

When to replace windows: If existing windows are 20+ years old, single-pane, drafty, or fogged (seal failure), replacement makes sense. Otherwise, start with cheaper shading solutions.

Solar Heat Gain in Different Climates

Hot Climates (Zones 1-2): Minimize SHGC

• Target: 0.25-0.30 SHGC on all windows
• Priority: South and west windows first
• Payback: 6-12 years (high cooling demand justifies investment)
• Bonus: Exterior shading (awnings, trees) compounds savings

Mixed Climates (Zones 3-4): Balance SHGC & U-Factor

• Target: 0.30-0.40 SHGC (block summer heat but allow some winter solar warming)
• Strategy: Low SHGC on south/west, moderate SHGC on north/east
• Payback: 10-16 years

Cold Climates (Zones 5+): Maximize Solar Gain in Winter

• Target: 0.40-0.50 SHGC (want winter solar heat to offset heating costs)
• Strategy: Prioritize low U-factor (insulation) over low SHGC
• Cooling impact: Minimal—short cooling season means solar gain less critical

Use our climate zone guide to determine optimal SHGC for your region.

Window Area Rules of Thumb

Optimal Window-to-Floor Ratio by Climate:

If your home exceeds these ratios, you'll need larger AC capacity or aggressive shading to compensate. Calculate impact with our BTU calculator by inputting actual window square footage.

Tools & Resources

• BTU Calculator: Includes window area and orientation in load calculation
• Climate Zone Guide: Regional solar intensity and SHGC recommendations
• Insulation Guide: Combine with window upgrades for maximum load reduction
• Climate Load Adjuster: Fine-tune BTU requirements for your specific conditions