How many solar panels do I need? Calculate your exact requirements

Determining the right number of solar panels for your home ensures you generate enough clean energy to cover your electricity needs whilst avoiding costly oversizing. This comprehensive guide provides calculation methods, real-world examples, and practical considerations to size your system perfectly.

Quick calculation method

Simple formula:

Annual kWh ÷ (Peak sun hours × 365 × Panel watts ÷ 1000) = Number of panels

Example calculation:

• Annual usage: 12,000 kWh
• Peak sun hours: 5.5 hours/day
• Panel rating: 400W
• Calculation: 12,000 ÷ (5.5 × 365 × 0.4) = 15 panels

Add 10-20% buffer for system losses and future needs.

Step 1: Determine your energy usage

Find your annual electricity consumption

Check your electricity bills:

• Look for "kWh used" over 12 months
• Add up monthly usage for annual total
• Account for seasonal variations
• Consider recent usage trends

Typical UK household usage:

• 1-2 bedrooms: 2,000-3,500 kWh/year
• 3-4 bedrooms: 3,500-5,500 kWh/year
• 5+ bedrooms: 5,500-8,000+ kWh/year

Typical US household usage:

• Small home (1,000-1,500 sq ft): 6,000-9,000 kWh/year
• Medium home (1,500-2,500 sq ft): 9,000-15,000 kWh/year
• Large home (2,500+ sq ft): 15,000-20,000+ kWh/year

Account for future changes

Consider these factors:

• Electric vehicle charging (+3,000-4,000 kWh/year)
• Heat pump installation (+2,000-6,000 kWh/year)
• Home additions or family changes
• Energy efficiency improvements to reduce overall consumption
• Working from home trends increasing daytime usage

Step 2: Assess your solar resource

Peak sun hours by location

Definition: Peak sun hours represent equivalent hours of 1,000W/m² solar irradiance daily.

UK average peak sun hours:

• Scotland: 2.5-3.5 hours/day
• Northern England: 3.0-4.0 hours/day
• Southern England: 3.5-4.5 hours/day
• Wales: 3.0-4.0 hours/day

US peak sun hours by region:

• Northeast: 3.5-4.5 hours/day (see New York solar guide)
• Southeast: 4.5-5.5 hours/day (see Florida solar incentives)
• Midwest: 4.0-5.0 hours/day
• Southwest: 5.5-7.0 hours/day (see Arizona solar potential)
• West Coast: 4.5-6.5 hours/day (see California solar costs)

Roof orientation and tilt impact

Optimal conditions:

• South-facing orientation (180°)
• Tilt angle: 30-40° in most locations
• Minimal shading throughout the day

Production factors:

• South-facing: 100% production potential
• Southeast/Southwest: 90-95% potential
• East/West-facing: 80-85% potential
• North-facing: 60-70% potential (not recommended)

Step 3: Choose your panel specifications

Modern panel options

Residential panel ratings:

• Standard efficiency: 300-350W (19-20% efficiency)
• High efficiency: 350-400W (20-22% efficiency)
• Premium panels: 400-450W (22-23% efficiency)

Panel dimensions:

• Standard size: ~2m × 1m (6.5ft × 3.3ft)
• Roof space needed: ~20-25 sq ft per panel
• Weight: 18-22kg (40-50lbs) per panel

Efficiency considerations

Higher efficiency benefits:

• Fewer panels needed for same output
• Better for limited roof space
• Improved performance in partial shade
• Higher upfront cost but better space utilization

When to choose high-efficiency panels:

• Limited roof space available
• Complex roof geometry with obstacles
• Premium aesthetic preferences
• Maximum energy production goals

Step 4: Account for system losses

Real-world efficiency factors

Typical system losses (15-20% total):

• Inverter efficiency: 3-8% loss
• DC/AC wiring: 2-3% loss
• Soiling and debris: 2-5% loss
• Temperature effects: 2-5% loss
• Shading impacts: 0-10% loss
• Age-related degradation: 0.5% per year

System derating factor: Most installers use 0.8 (80% efficiency) to account for all real-world losses. Regular solar panel cleaning helps minimise soiling losses.

Adjusted calculation:

Required panels = (Annual kWh ÷ Peak sun hours ÷ 365 ÷ Panel kW) ÷ 0.8

Practical sizing examples

Example 1: UK suburban home

Home details:

• 4-bedroom house, South England
• Annual usage: 4,200 kWh
• Peak sun hours: 4.0/day
• Available roof space: 40 sq m

Calculation:

• System size needed: 4,200 ÷ (4.0 × 365) = 2.9kW
• With 400W panels: 2,900W ÷ 400W = 7.25 panels
• Recommended: 8-9 panels (3.2-3.6kW system)

Example 2: US family home

Home details:

• 2,200 sq ft house, Texas
• Annual usage: 14,400 kWh
• Peak sun hours: 5.3/day
• Large south-facing roof

Calculation:

• System size needed: 14,400 ÷ (5.3 × 365) = 7.4kW
• With 350W panels: 7,400W ÷ 350W = 21.1 panels
• Recommended: 22-24 panels (7.7-8.4kW system)

Example 3: Off-grid cabin

Home details:

• Remote cabin with battery storage
• Daily usage: 15 kWh (5,475 kWh/year)
• Peak sun hours: 4.5/day
• 3 days autonomy required

Calculation:

• System size for average day: 15 kWh ÷ 4.5 hours = 3.3kW
• Off-grid oversizing factor: 1.5x for weather
• Recommended: 5kW system (12-15 panels)

Roof space requirements

Calculating available area

Roof assessment checklist:

• Measure usable roof sections
• Account for chimneys, vents, skylights
• Maintain 3-foot setbacks from roof edges
• Consider fire access requirements
• Check structural load capacity

Space per panel:

• Panel area: ~2 sq m (21 sq ft)
• With spacing: ~2.5 sq m (27 sq ft)
• Row spacing: 1-2 metres for tilt systems

Working around constraints

Limited roof space solutions:

• High-efficiency panels (more power per sq ft)
• Ground-mount systems in garden
• Carport or pergola installations
• Split systems across multiple roof sections

Optimal panel arrangement:

• Portrait orientation often more efficient
• Minimize partial shading between rows
• Consider future roof maintenance access
• Plan for snow load in applicable climates

Financial considerations

System sizing for economics

Grid-tied system goals:

• Size for 80-100% of annual usage
• Avoid significant overproduction
• Consider net metering policies
• Factor in export rates vs import rates

Investment optimization:

• Larger systems have lower per-watt costs
• Diminishing returns on very large systems
• Consider available roof space utilization
• Plan for future energy needs

Financing impact on sizing

Cash purchase:

• Size for optimal long-term value
• Consider larger system for maximum savings
• Factor in available tax incentives

Solar loans or leases:

• Size to match monthly payment comfort
• Consider payment vs electricity bill offset
• Evaluate total cost of ownership

Installation considerations

Professional assessment

What installers evaluate:

• Electrical panel capacity and condition
• Roof structural integrity
• Shading analysis throughout the year
• Local permitting requirements
• Grid interconnection procedures

Site-specific factors:

• Microclimate conditions
• Snow load considerations
• Wind load requirements
• Historical weather patterns
• Future development impacts

Permitting and regulations

Common requirements:

• Building permits for roof installations
• Electrical permits for system connections
• Utility interconnection agreements
• HOA approvals where applicable
• Fire department access compliance

Maintenance and monitoring

System performance tracking

Key metrics to monitor:

• Daily/monthly energy production
• System efficiency compared to estimates
• Individual panel performance
• Inverter operation status
• Weather correlation analysis

Maintenance requirements:

• Annual visual inspections
• Cleaning as needed (2-4 times/year)
• Inverter monitoring and updates
• Electrical connection checks
• Performance verification

Troubleshooting underperformance

Common issues:

• Soiling or debris accumulation
• Shading from new obstacles
• Inverter malfunctions
• Wiring connection problems
• Panel degradation beyond expected rates

Future expansion planning

Designing for growth

Expansion considerations:

• Reserve roof space for additional panels
• Electrical panel capacity for larger system
• Inverter sizing for future capacity
• Conduit and wiring infrastructure
• Permit modification requirements

Battery storage integration:

• Plan electrical infrastructure for batteries
• Consider load centre modifications
• Evaluate backup power priorities
• Size for critical load coverage
• Future vehicle charging preparation

Next steps

1. Calculate your specific needs using our solar calculator
2. Get professional quotes from at least 3 certified installers
3. Compare system proposals for size, equipment, and costs
4. Review financing options including cash, loans, and incentives
5. Plan installation timeline accounting for permitting and weather

The right number of solar panels depends on your unique situation, but this guide provides the framework to make an informed decision. Most homeowners find that properly sized systems pay for themselves within 6-10 years whilst providing 25+ years of clean energy.

Related resources

Essential next steps:

• Solar Calculator - Get instant sizing estimates for your location
• State Solar Guides - Find local incentives and installer recommendations
• Solar Panel Efficiency Guide - Maximise your system's performance
• How to Clean Solar Panels - Maintain peak efficiency over time

Popular state guides:

• Solar in California - SGIP rebates and net metering policies
• Solar in Texas - Deregulated market opportunities
• Solar in Florida - Hurricane considerations and incentives

Product research:

• Best Solar Generators 2025 - Backup power options
• EcoFlow vs Bluetti vs Renogy - Portable power comparison

Use our solar calculator for personalised sizing recommendations, or explore state-specific guides for local incentives and installer recommendations.