Off Grid Solar System Calculator
Size a complete off-grid solar system from daily energy use, battery days, and sun hours. Enter values for instant results with step-by-step formulas.
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer
Off Grid Solar System Calculator
Calculator
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Formula: Array kW = (Daily kWh / Loss Factor) / Peak Sun Hours
Worked example — 20 panels (8 kW), 20 batteries (48 kWh), ~$17,800 estimated total system cost
Formula
Array kW = (Daily kWh / Loss Factor) / Peak Sun Hours
The solar array must produce enough energy in available sun hours to cover daily usage after accounting for system losses (typically 25%). Battery bank is sized as Daily Usage x Autonomy Days / Depth of Discharge, ensuring power during cloudy periods.
Worked Examples
Example 1: Remote Cabin Off-Grid System
Problem:Size an off-grid system for a cabin using 30 kWh/day, 5 peak sun hours, 3 days autonomy, 50% DOD, 48V system, 400W panels, 200Ah batteries.
Solution:Adjusted daily need: 30/0.75 = 40 kWh Array size: 40/5 = 8 kW => 20 x 400W panels Battery bank: 30 x 3 / 0.50 = 180 kWh Bank Ah: 180,000/48 = 3,750 Ah => 19 x 200Ah batteries Series: 48V/12V = 4 batteries per string Parallel strings: ceil(19/4) = 5 => 20 batteries total Actual bank: 5 x 200Ah x 48V = 48 kWh, usable = 24 kWh
Result:20 panels (8 kW), 20 batteries (48 kWh), ~$17,800 estimated total system cost
Example 2: Small RV Solar Setup
Problem:Size a system for an RV using 5 kWh/day, 4 sun hours, 2 days autonomy, 80% DOD (lithium), 12V, 200W panels, 100Ah lithium batteries.
Solution:Adjusted daily: 5/0.75 = 6.67 kWh Array: 6.67/4 = 1.67 kW => 9 x 200W panels (1.8 kW) Battery bank: 5 x 2 / 0.80 = 12.5 kWh Bank Ah: 12,500/12 = 1,042 Ah => 11 x 100Ah batteries 1 battery per string (12V), 11 parallel strings => 11 batteries Actual: 1,100Ah x 12V = 13.2 kWh, usable = 10.56 kWh
Result:9 panels (1.8 kW), 11 batteries (13.2 kWh), estimated $6,100 total cost
Frequently Asked Questions
What is an off-grid solar system and who needs one?
An off-grid solar system is a self-contained power generation and storage setup that operates independently from the electrical utility grid. It consists of solar panels for energy generation, batteries for energy storage, a charge controller to manage battery charging, and an inverter to convert DC battery power to AC household power. Off-grid systems are essential for remote locations where utility power is unavailable or prohibitively expensive to connect, such as rural cabins, remote homes, boats, RVs, and wilderness locations. They are also chosen by people seeking complete energy independence, disaster preparedness, or those in areas with unreliable grid power. The key difference from grid-tied solar is the battery bank, which must store enough energy to power the home through nights and cloudy periods.
How do you calculate the solar panel array size for off-grid?
Sizing the solar array starts with your daily energy consumption in kilowatt-hours. First, divide the daily consumption by the system loss factor (typically 0.75-0.80) to account for inverter efficiency, wiring losses, battery charging losses, and temperature derating. Then divide this adjusted daily demand by the average peak sun hours for your location to get the required array capacity in kilowatts. For example, if you use 30 kWh/day with 5 peak sun hours: adjusted demand = 30/0.75 = 40 kWh, required array = 40/5 = 8 kW. Always round up to the nearest panel count. It is wise to add 10-20% extra capacity to account for panel degradation, unexpected loads, and seasonal variations in solar irradiance.
How do you size a battery bank for off-grid solar?
Battery bank sizing depends on three key factors: daily energy usage, desired days of autonomy (how many cloudy days the batteries can power the home without solar input), and the battery depth of discharge (DOD). The formula is: Total Battery kWh = Daily Usage (kWh) x Autonomy Days / DOD. For example, with 30 kWh daily usage, 3 days of autonomy, and 50% DOD for lead-acid batteries: 30 x 3 / 0.50 = 180 kWh total battery capacity. Lithium batteries can use 80-90% DOD, significantly reducing the number of batteries needed. Convert kWh to amp-hours by dividing by system voltage: 180,000 Wh / 48V = 3,750 Ah. Then divide by individual battery capacity to determine the number of parallel strings needed.
What is depth of discharge and why does it matter?
Depth of discharge (DOD) is the percentage of a battery capacity that is used before recharging. A 50% DOD means you only use half the battery capacity before recharging. DOD directly affects battery lifespan: lead-acid batteries cycled to 50% DOD typically last 1,500-2,000 cycles, but only 500-800 cycles at 80% DOD. Lithium iron phosphate (LiFePO4) batteries can handle 80-90% DOD for 3,000-5,000 cycles. Using a lower DOD extends battery life but requires more batteries (and higher cost) to store the same usable energy. The DOD setting is a trade-off between upfront battery investment and battery replacement frequency. Most off-grid system designers use 50% DOD for lead-acid and 80% DOD for lithium batteries.
What system voltage should I choose for off-grid solar?
Common off-grid system voltages are 12V, 24V, and 48V, with higher voltages being more efficient for larger systems. A 12V system is suitable for small loads under 2 kW, such as RVs, boats, and tiny cabins. A 24V system works well for medium loads of 2-5 kW, typical of small off-grid homes. A 48V system is recommended for larger homes with loads exceeding 5 kW. Higher voltage means lower current for the same power, which reduces wire gauge requirements, lowers resistive losses, and allows longer cable runs between components. Most modern off-grid inverters and charge controllers support 48V, and it has become the standard for residential off-grid installations. The wire cost savings alone often justify choosing 48V over lower voltages.
What are peak sun hours and how do I find mine?
Peak sun hours (PSH) represent the number of hours per day that solar irradiance averages 1,000 watts per square meter, which is the standard test condition for solar panel ratings. A location receiving 6 peak sun hours does not mean 6 hours of sunshine; it means the total solar energy received equals what you would get from 6 hours at full 1,000 W/m2 intensity. PSH varies significantly by location, season, and weather. The US Southwest averages 6-7 PSH, the Midwest 4-5 PSH, and the Pacific Northwest 3-4 PSH. For off-grid design, use the worst-month average (typically December or January) to ensure adequate power year-round. The NREL PVWatts tool and Global Solar Atlas provide PSH data for any location worldwide.
What role does the charge controller play in off-grid systems?
The charge controller sits between the solar panels and the battery bank, regulating the charging process to protect batteries from overcharging and optimize energy harvest. There are two types: PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking). MPPT controllers are preferred for off-grid systems because they convert the higher voltage from the solar array to the optimal battery charging voltage, capturing 15-30% more energy than PWM controllers. MPPT controllers also allow higher-voltage panel strings, reducing wiring costs. The charge controller is sized based on the maximum current from the solar array: divide the total array wattage by the system voltage, then multiply by 1.25 for a safety margin. Multiple charge controllers can be paralleled for larger systems.
How long do off-grid solar batteries last and what affects their lifespan?
Battery lifespan depends on the chemistry, depth of discharge, temperature, and charging practices. Lead-acid batteries (flooded or AGM) typically last 5-8 years with proper maintenance, providing 1,500-3,000 cycles at 50% DOD. Lithium iron phosphate (LiFePO4) batteries last 10-15 years with 3,000-6,000 cycles at 80% DOD. Temperature extremes reduce lifespan: every 10 degrees Celsius above 25 degrees roughly halves lead-acid battery life. Proper charging is critical: consistently undercharging or overcharging accelerates degradation. Equalization charges (periodic overcharging) are needed for flooded lead-acid batteries to prevent sulfation. Lithium batteries require less maintenance but need a Battery Management System (BMS) to balance cells and prevent damage. Budget for at least one battery replacement over a 25-year system lifetime.
What is LCOE and how does off-grid solar compare to generator power?
Levelized Cost of Energy (LCOE) spreads the total lifetime cost of a power system over all the energy it produces, giving a per-kWh cost for comparison. For off-grid solar, LCOE includes panels, batteries (with replacements), inverters, charge controllers, installation, and maintenance over 25 years. Typical off-grid solar LCOE ranges from $0.20-$0.50/kWh depending on system size and battery costs. By comparison, gasoline generators cost $0.40-$0.80/kWh when you factor in fuel, maintenance, and replacement every 5-10 years. Diesel generators are slightly better at $0.35-$0.60/kWh but require fuel delivery to remote locations. Off-grid solar has higher upfront costs but dramatically lower operating costs, and the gap continues to narrow as battery prices fall by approximately 10-15% annually.
Can I expand an off-grid solar system later if my needs grow?
Yes, off-grid systems can be expanded, but planning for expansion from the start saves significant money. Key considerations include choosing a charge controller with higher amperage capacity than initially needed, selecting an inverter that can be paralleled or stacked for more power, using a system voltage (48V recommended) that accommodates growth, and installing conduit and wiring infrastructure sized for future expansion. Adding solar panels is straightforward if the charge controller has spare capacity. Adding batteries requires matching the existing battery type, voltage, and ideally age (mixing old and new batteries reduces overall performance). The most cost-effective approach is to initially oversize the inverter and charge controller while installing only the panels and batteries you need now, adding more as your budget and needs grow.
References
Reviewed for accuracy by Daniel Agrici, Founder & Lead Developer · Editorial policy
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