Calculate Solar System Size

Solar System Size Calculator

Use this calculator to estimate the required solar panel array size and battery bank capacity for your specific energy needs, whether for an on-grid or off-grid system.

Enter your average daily electricity usage in kilowatt-hours. Check your utility bill for this value.

The average number of hours per day your location receives direct sunlight equivalent to 1000 W/m². This varies by geographic location and season.

Accounts for energy losses due to wiring, inverter inefficiency, temperature, dust, and shading. A typical value is 15-25%.

For off-grid systems, this is how many days your batteries can power your home without any sun. Enter 0 if you have an on-grid system without battery backup.

The maximum percentage of a battery's capacity that can be used without significantly shortening its lifespan. Typical values are 50% for lead-acid and 80-90% for lithium-ion.

The nominal voltage of your battery bank (e.g., 12V, 24V, 48V). Enter 0 if you have an on-grid system without battery backup.

Calculation Results:

Required Solar Panel Array Size:

Required Battery Bank Capacity:

Required Battery Bank Capacity:

Battery bank capacity not calculated as Desired Autonomy is set to 0.

Understanding Solar System Sizing

Designing an efficient and reliable solar power system requires careful planning, and one of the most critical steps is accurately sizing your system. This involves determining the right number of solar panels and the appropriate battery bank capacity to meet your energy demands. Whether you're looking to reduce your electricity bill with an on-grid system or achieve energy independence with an off-grid setup, understanding the factors that influence system size is key.

Why is Accurate Sizing Important?

• Cost-Effectiveness: An undersized system won't meet your needs, leading to reliance on grid power or frequent battery depletion. An oversized system costs more upfront than necessary.
• Reliability: For off-grid systems, correct sizing ensures you have power even during periods of low sunlight.
• System Longevity: Properly sized components, especially batteries, will last longer if not overstressed.
• Space Optimization: Knowing the required panel array size helps in planning installation space on your roof or property.

Key Factors in Solar System Sizing

1. Average Daily Energy Consumption (kWh/day)

This is the foundation of your solar system design. It represents the total amount of electricity your household or business uses in a typical day. You can usually find this information on your monthly utility bills, often listed as "kWh used" or "energy consumption." If you're building a new home or don't have historical data, you'll need to estimate by listing all your appliances, their wattage, and how many hours per day they operate.

Example: A typical household might consume 15-30 kWh per day, while a small cabin might use 5 kWh/day.

2. Average Daily Peak Sun Hours (hours/day)

Peak sun hours (PSH) are not the same as the total hours of daylight. PSH represent the equivalent number of hours per day when solar irradiance averages 1,000 watts per square meter (W/m²). This value accounts for the sun's angle, weather conditions, and seasonal variations. Locations closer to the equator generally have higher PSH, and summer months typically have more PSH than winter months.

You can find average PSH data for your specific location from resources like NASA's surface meteorology and solar energy data, or various solar atlases.

Example: A location might have 4.5 PSH in winter and 6.0 PSH in summer. Using the lowest average (e.g., winter) is crucial for reliable off-grid systems.

3. System Loss Factor (%)

No solar power system is 100% efficient. Energy is lost at various stages due to:

• Inverter Efficiency: Converts DC power from panels/batteries to AC power for appliances.
• Wiring Losses: Resistance in cables.
• Temperature Losses: Panels become less efficient at higher temperatures.
• Dust and Shading: Accumulation of dirt or partial shading reduces output.
• Module Mismatch: Slight variations in panel performance.

A typical system loss factor ranges from 15% to 25%. Using a higher loss factor provides a more conservative and reliable system design.

Example: A 20% loss factor means only 80% of the theoretical panel output reaches your appliances or batteries.

4. Desired Autonomy (days) – for Off-Grid Systems

This factor is critical for off-grid systems and refers to the number of consecutive days your battery bank can supply power without any input from the solar panels (e.g., during prolonged cloudy weather). For on-grid systems without battery backup, this value is 0.

Example: A desired autonomy of 2-3 days is common for off-grid homes to ensure power during inclement weather.

5. Battery Depth of Discharge (DoD) (%) – for Off-Grid Systems

The DoD is the percentage of a battery's capacity that has been discharged relative to its total capacity. Regularly discharging batteries too deeply can significantly shorten their lifespan. Different battery chemistries have different recommended DoD limits:

• Lead-Acid Batteries: Typically 50% DoD to maximize lifespan.
• Lithium-Ion Batteries: Can often be discharged to 80-90% DoD.

Example: If you have a 100 Ah lead-acid battery and a 50% DoD, you can only use 50 Ah of its capacity.

6. Battery Bank Voltage (V) – for Off-Grid Systems

This is the nominal voltage of your battery bank, which is determined by how your batteries are wired (series, parallel, or a combination). Common system voltages are 12V, 24V, and 48V. Higher voltages are generally more efficient for larger systems as they reduce current and thus wiring losses.

Example: A 48V system is common for larger off-grid homes, while 12V or 24V might be used for smaller cabins or RVs.

How the Calculator Works (Simplified)

The calculator uses these inputs to perform two main calculations:

1. Solar Panel Array Size: It determines the total daily energy you need, then divides that by the effective peak sun hours (after accounting for system losses) to find the required panel wattage.
2. Battery Bank Capacity: For off-grid systems, it calculates the total energy needed for your desired autonomy days and then adjusts this based on the battery's allowable depth of discharge to find the total battery capacity in Watt-hours (Wh) and Ampere-hours (Ah).

Tips for Accurate Sizing

• Audit Your Energy Use: For existing homes, review past utility bills. For new constructions, make a detailed list of all appliances and their estimated usage.
• Consider Future Needs: Plan for potential increases in energy consumption (e.g., new appliances, electric vehicle charging).
• Seasonal Variations: Always size your system based on the lowest average peak sun hours (usually winter) if you need consistent power year-round.
• Consult a Professional: For complex or critical systems, always consult with a qualified solar installer or engineer.

By using this calculator and understanding the underlying principles, you can take a significant step towards designing a solar power system that perfectly fits your energy requirements.