Off-grid solar system sizing: calculate your power needs

Creating a properly sized off-grid solar system ensures reliable power whilst avoiding the expense of oversizing. This comprehensive guide walks through the calculation process, component selection, and design considerations for sustainable off-grid living.

Understanding off-grid solar basics

Key components:

• Solar panels: Convert sunlight to DC electricity
• Charge controller: Regulates power flow to batteries
• Batteries: Store energy for night-time and cloudy days
• Inverter: Converts DC to AC for household appliances
• Monitoring system: Tracks performance and battery status

Off-grid vs grid-tie differences:

• No utility grid connection for backup power
• Battery storage essential for continuous power
• System must handle peak loads independently
• Generator backup often recommended
• Higher initial cost but complete energy independence

Step 1: Calculate your energy consumption

Daily energy audit

Essential loads (priority 1):

• Lighting: LED fixtures, security lighting
• Refrigeration: Fridge, freezer
• Communication: Phone charging, internet
• Water systems: Pumps, pressure tanks
• Medical equipment: CPAP, refrigerated medicines

Comfort loads (priority 2):

• Entertainment: TV, sound systems
• Computing: Laptops, desktop computers
• Kitchen appliances: Microwave, coffee maker
• Laundry: Washing machine (efficient models)
• Climate control: Fans, small heaters

Heavy loads (priority 3):

• Air conditioning/heating
• Electric vehicle charging
• Workshop tools and equipment
• Hot water heating
• Electric cooking ranges

Energy calculation worksheet

For each appliance, calculate:

Daily energy (Wh) = Power (W) × Hours used per day

Example calculation:

• LED lights (100W): 100W × 6 hours = 600Wh
• Efficient fridge (150W): 150W × 8 hours = 1,200Wh
• Laptop (60W): 60W × 4 hours = 240Wh
• Daily total: 2,040Wh (2.04kWh)

Sizing factors:

• Add 20% safety margin for calculation uncertainties
• Include inverter efficiency losses (10-15%)
• Account for battery charging inefficiencies (10-20%)
• Consider seasonal usage variations

Step 2: Assess your solar resource

Sun hours calculation

Peak sun hours definition: Peak sun hours represent the equivalent hours per day of 1,000W/m² solar irradiance at your location.

Seasonal variations:

• Summer: Longer days, higher sun angles
• Winter: Shorter days, lower sun angles
• Spring/Autumn: Moderate conditions
• Design for worst-case: Size system for winter months

Solar resource tools:

• NREL PVWatts calculator for US locations
• Global Solar Atlas for international data
• Local weather station historical data
• Professional solar assessment tools

Example peak sun hours:

• Arizona: 6.5 hours (summer), 4.5 hours (winter)
• Germany: 4.5 hours (summer), 1.5 hours (winter)
• Australia (Sydney): 5.8 hours (summer), 3.2 hours (winter)
• UK: 4.2 hours (summer), 1.1 hours (winter)

Tilt and orientation optimization

Optimal tilt angle:

• Fixed mounting: Latitude ± 15° for year-round optimization
• Winter optimization: Latitude + 15° for off-grid systems
• Seasonal adjustment: Manual adjustment spring and autumn
• Tracking systems: 25-35% increase for horizontal single-axis

Shading assessment:

• Morning shading more critical for off-grid systems
• Use Solar Pathfinder or similar tools
• Consider seasonal vegetation changes
• Plan for future tree growth

Step 3: Size your solar array

Solar panel calculation

Basic formula:

Required solar capacity (W) = Daily energy needs (Wh) ÷ Peak sun hours ÷ System efficiency

System efficiency factors:

• Panel derating: 85% (temperature, dust, aging)
• MPPT efficiency: 95-98%
• Wiring losses: 3-5%
• Battery charging efficiency: 85-95%
• Overall system efficiency: 70-80%

Example calculation:

Daily needs: 5,000Wh
Peak sun hours: 4.5 (winter)
System efficiency: 75%

Required capacity = 5,000 ÷ 4.5 ÷ 0.75 = 1,481W
Recommended size: 1,800W (safety margin)

Panel selection considerations

Monocrystalline advantages:

• Higher efficiency (20-22%)
• Better low-light performance
• Longer lifespan (25+ years)
• Smaller footprint for equivalent power

Panel sizing strategy:

• Larger panels reduce installation costs
• Smaller panels easier to handle and transport
• Consider voltage requirements for system design
• Match panel specifications for series strings

Step 4: Battery system design

Battery capacity calculation

Energy storage needs:

Battery capacity (Wh) = Daily energy needs × Days of autonomy ÷ Depth of discharge

Days of autonomy:

• Minimum: 2-3 days for essential loads
• Comfort: 5-7 days for full power
• Remote locations: 7-14 days
• Seasonal considerations: Extra capacity for winter

Depth of discharge (DoD) limits:

• Lead-acid: 50% DoD (80% for cycling applications)
• AGM: 80% DoD
• Lithium (LiFePO₄): 90-95% DoD
• Gel: 80% DoD

Example calculation:

Daily needs: 5,000Wh
Days of autonomy: 3 days
Depth of discharge: 80% (lithium)

Required capacity = 5,000 × 3 ÷ 0.8 = 18,750Wh (18.75kWh)

Battery technology comparison

Lead-acid (flooded):

• Pros: Low cost, proven technology, recyclable
• Cons: Maintenance required, shorter lifespan, ventilation needed
• Lifespan: 5-8 years, 1,500-3,000 cycles
• Cost per kWh: £200-£400

AGM (Absorbed Glass Mat):

• Pros: Maintenance-free, better cycling, no ventilation
• Cons: Higher cost than flooded, moderate lifespan
• Lifespan: 7-10 years, 1,000-2,000 cycles
• Cost per kWh: £400-£600

Lithium (LiFePO₄):

• Pros: Long lifespan, high efficiency, lightweight, fast charging
• Cons: Higher upfront cost, requires BMS
• Lifespan: 15-20 years, 6,000-10,000 cycles
• Cost per kWh: £600-£1,200

Battery configuration

Voltage selection:

• 12V: Small systems up to 1kW
• 24V: Medium systems 1-3kW
• 48V: Large systems over 3kW (most efficient)

Series vs parallel:

• Series: Increases voltage, maintains capacity
• Parallel: Increases capacity, maintains voltage
• Series-parallel: Achieves both higher voltage and capacity

Step 5: Charge controller sizing

MPPT vs PWM controllers

PWM (Pulse Width Modulation):

• Efficiency: 70-80%
• Cost: Lower initial cost
• Best for: Small systems with matched panel voltage
• Limitations: Panel voltage must closely match battery voltage

MPPT (Maximum Power Point Tracking):

• Efficiency: 95-98%
• Cost: Higher initial cost
• Best for: Most off-grid systems
• Advantages: Optimizes power harvest, flexible panel configurations

Controller sizing calculation

Current rating:

Required controller current = Solar array current × 1.25 safety factor

Voltage rating:

• Must handle maximum open-circuit voltage of solar array
• Include temperature coefficient for cold weather
• Allow headroom for series string expansion

Example:

Solar array: 8 × 300W panels in series
Panel Voc: 40V each
String Voc: 320V (×1.25 for temperature) = 400V
Panel Isc: 9A each
Required controller: 80A, 400V+ rating

Step 6: Inverter selection

Inverter sizing

Continuous power rating: Size for 125% of maximum continuous load to allow for efficiency losses and surge capacity.

Surge capacity: Many appliances require 2-5× starting current:

• Motors: Pumps, fans, compressors
• Transformers: Microwaves, power supplies
• Electronics: Computers, TV power supplies

Example sizing:

Continuous loads: 2,000W
Largest motor start: 1,500W (3× surge = 4,500W)
Required inverter: 3,000W continuous, 9,000W surge

Inverter types

Modified sine wave:

• Pros: Lower cost
• Cons: Efficiency losses, equipment incompatibility
• Use: Basic applications, resistive loads only

Pure sine wave:

• Pros: Clean power, universal compatibility
• Cons: Higher cost
• Use: Recommended for all modern off-grid systems

Inverter-charger combinations:

• Integrated design: Battery charging from generator
• Transfer switch: Automatic generator backup
• Grid-tie capability: Future connection option

Step 7: System design and integration

Wiring and electrical design

DC system considerations:

• Minimize wire runs to reduce voltage drop
• Use appropriate wire gauge for current and distance
• Include proper fusing and disconnects
• Ground fault protection for safety

AC distribution:

• Standard household electrical panels
• GFCI protection for outlets
• Proper grounding and bonding
• Code compliance for safety inspections

Monitoring and control

Battery monitoring:

• State of charge indicators
• Voltage and current monitoring
• Temperature compensation
• Low voltage disconnect protection

System monitoring:

• Solar production tracking
• Load monitoring and analysis
• Historical data logging
• Remote monitoring capabilities

Step 8: Backup power integration

Generator backup

Generator sizing:

• Size for essential loads plus battery charging
• Consider altitude and temperature derating
• Fuel type: diesel, propane, gasoline considerations
• Automatic start/stop capability

Integration requirements:

• Transfer switch for safe operation
• Battery charger compatibility
• Exhaust and ventilation planning
• Fuel storage and safety

Load management

Automatic load shedding:

• Priority-based load control
• Battery voltage triggers
• Time-based load scheduling
• Manual override capabilities

Energy conservation strategies:

• High-efficiency appliances
• LED lighting throughout
• Proper insulation and sealing
• Smart power management

Common sizing mistakes to avoid

Undersizing errors

Insufficient battery capacity:

• Not accounting for winter conditions
• Underestimating backup power needs
• Ignoring depth of discharge limits
• Skipping safety margins

Inadequate solar array:

• Using summer sun hours for winter loads
• Not accounting for system inefficiencies
• Ignoring seasonal load variations
• Underestimating shading impacts

Oversizing problems

Excessive solar capacity:

• Wasted energy in summer months
• Higher system costs with minimal benefit
• Battery overcharging concerns
• Charge controller oversizing

Battery bank too large:

• Increased cost and complexity
• Longer charging times
• Reduced efficiency
• Space and weight considerations

Maintenance and expansion planning

Maintenance requirements

Regular tasks:

• Battery voltage and electrolyte checks (lead-acid)
• Solar panel cleaning and inspection
• Connection tightness verification
• System performance monitoring

Annual maintenance:

• Deep system inspection
• Battery capacity testing
• Inverter and controller calibration
• Generator servicing

Future expansion planning

Scalable design principles:

• Modular battery bank design
• Electrical panel capacity planning
• Conduit and wiring infrastructure
• Controller and inverter upgrade paths

Technology refresh:

• Plan for 10-15 year equipment lifecycles
• Monitor efficiency improvements
• Consider partial system upgrades
• Maintain documentation for modifications

Proper off-grid solar sizing ensures reliable, cost-effective energy independence. Take time to accurately assess your needs, understand local conditions, and design for future requirements. A well-sized system provides decades of reliable service whilst minimizing both initial costs and operational complexity.