Basic Parameters

SI Unit: 1 kWh = 3.6 MJ = 3.6 × 10⁶ J
Equivalent sun hours (ESH) - insolation expressed as hours at 1000 W/m² STC
Rated at Standard Test Conditions (STC): 1000 W/m², 25°C cell temp, AM1.5 spectrum

Advanced Settings

Includes temperature de-rating (±0.4%/°C above 25°C), soiling, mismatch, and wiring losses
European efficiency weighted average (10% at 10% load, 20% at 20%, etc.)
Consider motor starting currents (5-7× rated power) and surge capacity
Cycle life vs DoD trade-off: Flooded LA: 50%, AGM: 70-80%, LiFePO₄: 80-95%
Annual optimal: latitude; Winter: latitude +15°; Summer: latitude -15°

Results

Adjust parameters in the Basic or Advanced tabs, then click "Calculate" to see results.
Energy Balance

Solar Energy Guide

Solar panels convert sunlight into electricity through photovoltaic cells. Key factors affecting performance:

  • Wattage: The power output under ideal conditions (measured in watts)
  • Efficiency: Percentage of sunlight converted to electricity (typically 15-22%)
  • Peak Sun Hours: Equivalent hours of full sunlight your location receives daily

To better understand how your energy consumption translates into system requirements, you might also explore our electric power consumption calculator for a detailed load analysis.

A complete solar power system typically includes:

  • Solar Panels: Generate DC electricity from sunlight
  • Inverter: Converts DC to AC for home use. Properly sizing this component is critical, and our inverter sizing calculator can help you find the right specifications.
  • Batteries (optional): Store excess energy for later use
  • Charge Controller: Regulates battery charging
  • Mounting System: Secures panels to roof or ground

Proper sizing is crucial for efficiency and cost-effectiveness:

  1. Calculate your daily energy consumption (kWh)
  2. Determine your peak sunlight hours
  3. Account for system losses (typically 15-25%)
  4. Add buffer for future expansion (10-20%)
  5. For off-grid, size batteries for 2-3 days of autonomy

  • Panels should face true south (northern hemisphere) or true north (southern hemisphere)
  • Tilt angle should equal your latitude for year-round performance
  • Avoid shading from trees, chimneys, or other obstructions
  • Leave space for air circulation behind panels
  • Consider local building codes and permits

Keep your system running efficiently:

  • Clean panels 2-4 times per year (more in dusty areas)
  • Check connections and wiring annually
  • Monitor system performance regularly
  • For battery systems, check water levels (if applicable) and terminals
  • Have a professional inspection every 3-5 years

Technical Reference

Core Photovoltaic Sizing Equations
Array Power Requirement:
Parray = Eload / (Hsun × ηsystem)
where: Parray = array power (kW), Eload = daily energy (kWh), Hsun = peak sun hours, ηsystem = system efficiency
Panel Count Calculation:
Npanels = ⌈Parray × 1000 / Ppanel⌉
where: Npanels = number of panels, Ppanel = panel wattage (W), ⌈⌉ = ceiling function
Battery Bank Sizing (Off-grid):
CkWh = (Eload × Dautonomy) / (DOD × ηbatt)
CAh = (CkWh × 1000) / Vsystem
where: CkWh = capacity in kWh, Dautonomy = days autonomy, DOD = depth of discharge, ηbatt = battery efficiency (≈0.85), Vsystem = system voltage
IEEE/UL Compliance Note: Inverter sizing follows NEC 690.8 and UL 1741 requirements, with 125% continuous load derating factor for safety margins.

SI Units in Photovoltaic Engineering
  • Watt (W): Power unit (1 W = 1 J/s = 1 V·A)
  • Kilowatt-hour (kWh): Energy unit (1 kWh = 3.6 MJ)
  • Ampere-hour (Ah): Battery capacity (C = I × t)
  • Volt (V): Electrical potential difference
  • Peak Sun Hour: Equivalent hours at 1000 W/m² irradiance
Industry Standards Reference
  • IEC 61215: Crystalline silicon terrestrial PV modules
  • IEC 61730: PV module safety qualification
  • UL 1703: Flat-plate PV modules and panels
  • NEC Article 690: Solar photovoltaic systems
  • IEEE 1547: Interconnecting distributed resources with electric power systems

Engineering Applications
  • Residential System Design: Sizing rooftop arrays with shading analysis. For more precise calculations on energy delivery, you might find our transformer efficiency calculator useful for related power conversion stages.
  • Microgrid Planning: Islandable systems with battery storage
  • Load Profile Matching: Aligning generation with consumption patterns
  • Feasibility Studies: Pre-installation energy yield estimates
  • Educational Demonstrations: Teaching photovoltaic principles
Common Design Scenarios
  • Grid-Tied Residential: 5-10 kW systems with net metering
  • Off-Grid Cabin: 1-3 kW systems with battery autonomy
  • Agricultural Pumping: Direct-drive DC systems
  • Telecom Backup: Critical loads with extended autonomy

Calculation Assumptions
  • Ideal cosine response for tilt angle adjustments
  • Uniform insolation throughout peak sun hours
  • Constant temperature coefficient of -0.4%/°C above STC
  • Battery round-trip efficiency of 85% (lead-acid)
  • Linear derating with system age (0.5%/year typical)
Accuracy Considerations
Professional Design Required: This calculator provides estimates only. Actual system design requires:
  • Detailed site assessment (shading, orientation)
  • Local meteorological data (TMY files)
  • Electrical load analysis (power factor, harmonics)
  • Voltage drop calculations for wire sizing – our cable loss calculator can assist with this.
  • Local code compliance verification
Typical Error Ranges
  • Energy yield: ±10-15% (weather variability)
  • Battery life: ±20% (temperature, cycling patterns)
  • Roof space: ±15% (obstructions, layout constraints)

Electrical Safety Warnings
  • PV systems produce hazardous DC voltages (up to 1000V) even when grid power is off
  • Battery banks can deliver extremely high fault currents (>10,000A)
  • Arc flash hazards exist in combiner boxes and inverters. Understanding these risks can be aided by our arc flash calculator for hazard assessment.
  • Roof work requires fall protection and structural assessment
Professional Practice Guidelines
  • Always use licensed electricians for final connections
  • Follow NEC Article 690 requirements for disconnects and labeling
  • Verify roof load capacity (4 psf minimum for panels + racking)
  • Use rapid shutdown compliant equipment (NEC 690.12)
  • Obtain proper permits and inspections
Educational Purpose Statement

This tool is designed for educational use, preliminary feasibility studies, and learning photovoltaic system sizing principles. It is not a substitute for professional engineering design, which must account for site-specific conditions, local codes, and safety requirements.

Frequently Asked Questions (Electrical Engineering)

Peak sun hours normalize variable irradiance to an equivalent duration at standard test conditions (1000 W/m²). This accounts for:

  • Solar angle changes throughout the day (cosine effect)
  • Atmospheric attenuation (air mass, aerosols)
  • Weather patterns and cloud cover
  • Seasonal variations in solar path

The calculation uses the integral of irradiance over the day divided by 1000 W/m², providing a consistent engineering parameter for system sizing.

Photovoltaic cells exhibit negative temperature coefficients:

  • Silicon panels: -0.4% to -0.5% per °C above 25°C STC
  • Thin-film: -0.2% to -0.3% per °C above 25°C
  • NOCT: Nominal Operating Cell Temperature (typically 45°C) reflects real-world conditions

For every 25°C above STC, crystalline silicon panels lose approximately 10% of rated power. This is accounted for in the system efficiency factor through temperature de-rating calculations.

These represent fundamentally different electrical quantities:

  • Kilowatt (kW): Instantaneous power (rate of energy transfer)
    Example: A 300W panel produces 0.3 kW under STC
  • Kilowatt-hour (kWh): Energy (power × time)
    Example: That same panel producing 0.3 kW for 5 hours yields 1.5 kWh

E(kWh) = P(kW) × t(h)
Energy = Power × Time

System sizing requires converting between these units: panels are rated in kW (power), but consumption is measured in kWh (energy).

Inverter oversizing (DC:AC ratio) accounts for several engineering factors:

  1. Clipping avoidance: Panels rarely produce nameplate power due to temperature, soiling, and aging
  2. Future expansion: Allows adding panels without inverter replacement
  3. Performance optimization: Inverters operate most efficiently at 30-80% load
  4. Code compliance: NEC 690.8 requires derating for continuous operation

Typical DC:AC ratios range from 1.1:1 to 1.3:1. The 125% factor in this calculator follows NEC requirements for continuous load circuits.

The battery calculations provide first-order approximations with these limitations:

  • Simplified chemistry: Assumes ideal Peukert behavior (n=1)
  • Temperature effects: Does not account for capacity reduction at low temperatures
  • Charge acceptance: Assumes batteries can accept full charge current
  • Cycle life: Does not predict battery lifespan under specific DoD patterns

For lead-acid batteries, actual usable capacity is approximately:
Cusable = Crated × DOD × ηroundtrip × ftemp × fage

Where ftemp ≈ 0.7 at 0°C and fage degrades to 0.8 after 500 cycles.

Tool Integrity & Trust Information
  • Client-Side Calculation: All computations occur in your browser; no data transmission to servers
  • Open Methodology: Formulas based on IEEE and IEC standards for photovoltaic system design
  • Educational Focus: Designed for learning and preliminary design, not installation guidance
  • No Commercial Bias: Calculations use engineering principles without product promotion
  • Technical Review: Formulas verified against industry standards (September 2025)
This photovoltaic calculator is maintained by electrical engineering professionals for educational use. For actual installation, consult licensed professionals who can perform site-specific analysis and ensure code compliance.