How to Calculate Your Solar Panel Requirements

Proper sizing ensures your solar system meets your needs without overspending. Follow this comprehensive calculation guide.

Step 1: Calculate Daily Energy Consumption

Method 1: From Electricity Bills

Example: 600 kWh/month ÷ 30 = 20 kWh/day

Method 2: From Appliances

List all appliances and calculate:

Formula: Watts × Hours Used × Days = Watt-hours (Wh)

Common Appliances:

Total Example: 6,300 Wh = 6.3 kWh/day

Step 2: Account for System Losses

Solar systems aren't 100% efficient:

Loss Factors:

Total system efficiency: ~70-85%

Formula: Daily consumption ÷ System efficiency

Example: 20 kWh ÷ 0.75 = 26.7 kWh needed

Step 3: Determine Peak Sun Hours

Kenya's peak sun hours vary by location:

Nairobi: 5.5 hours

Mombasa: 5.8 hours

Kisumu: 5.3 hours

Nakuru: 5.4 hours

Eldoret: 5.2 hours

Peak Sun Hour = Time when sun provides 1000W/m²

Step 4: Calculate Required Panel Wattage

Formula: Daily kWh needed ÷ Peak sun hours

Example: 26.7 kWh ÷ 5.5 hours = 4.85 kW system

Round up for safety: 5 kW system

Step 5: Determine Number of Panels

Common Panel Sizes in Kenya:

Formula: System size (W) ÷ Panel wattage

Example: 5,000W ÷ 330W = 15.15 panels

Round up: 16 panels × 330W = 5,280W (5.28 kW)

Step 6: Calculate Roof Space Required

Panel Dimensions (typical 330W):

Formula: Number of panels × Panel area × 1.3 (for spacing)

Example: 16 panels × 1.94m² × 1.3 = 40.35 m²

Minimum roof space needed: 40 m²

Step 7: Battery Sizing (If Off-Grid/Hybrid)

Determine Backup Days Needed:

Formula: Daily consumption × Backup days ÷ Battery DoD

Example (Lead Acid at 50% DoD):

Example (Lithium at 80% DoD):

Convert to Battery Ah:

Formula: (kWh × 1000) ÷ System voltage

For 48V system:

Battery Configuration:

Step 8: Inverter Sizing

Peak Power Requirement:

List simultaneous loads:

Peak: 600W + 80W + 100W + 50W + 1000W = 1,830W

Add 25% safety margin: 1,830W × 1.25 = 2,288W

Choose: 3kW inverter (next standard size)

Inverter Types:

Pure Sine Wave (Recommended):

Modified Sine Wave:

Step 9: Charge Controller Sizing

For PWM Controllers:

Formula: (Panel watts ÷ Battery voltage) × 1.25

Example: (5,280W ÷ 48V) × 1.25 = 137.5A

Choose: 140A or 150A PWM controller

For MPPT Controllers (Better):

Formula: (Panel watts × 1.25) ÷ Battery voltage

Example: (5,280W × 1.25) ÷ 48V = 137.5A

Choose: 150A MPPT controller

MPPT Advantages:

Real-World Examples

Small Home (4 kWh/day)

System Sizing:

Medium Home (15 kWh/day)

System Sizing:

Small Business (40 kWh/day)

System Sizing:

Large Business (200 kWh/day)

System Sizing:

Common Sizing Mistakes

1. Underestimating Consumption

Problem: System too small

Solution: Add 20% buffer

2. Ignoring Future Growth

Problem: Can't expand easily

Solution: Design for 5-year needs

3. Wrong Peak Sun Hours

Problem: System underperforms

Solution: Use local data, not generic values

4. Inadequate Battery Backup

Problem: Power cuts during cloudy days

Solution: Size for 2-3 days autonomy

5. Oversizing Unnecessarily

Problem: Wasted money

Solution: Size to actual needs, not wants

Sizing Checklist

Professional Sizing Services

What We Provide:

Service Cost: Free with installation

Value: Ensures optimal system size

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