The off-grid solar battery sizing calculator determines the battery bank capacity and solar panel wattage you need for residential off-grid power. Unlike van/RV calculators, this tool is designed for homesteads, cabins, and full-time off-grid living at higher energy usage levels.
System Sizing Calculator
US average home: 30 kWh/day. Efficient cabin: 1-3 kWh/day.
System Summary
How to Size an Off-Grid Solar Battery System
Off-grid solar sizing involves two separate calculations: how much battery storage you need for days without sun, and how many solar panels you need to recharge those batteries and meet daily demand on average days.
Step 1: Measure Your Daily Energy Usage
Don't estimate — measure. Use a Kill-A-Watt meter on each appliance, or check your power bill. An energy-efficient off-grid cabin might use 1-3 kWh/day. A family homestead with refrigerator, washing machine, and lights uses 5-10 kWh/day. A full-sized house with air conditioning may need 20-30 kWh/day, making off-grid solar expensive.
Step 2: Size Battery Bank for Autonomy
Formula: daily kWh × days of autonomy ÷ depth of discharge = bank kWh needed. Then convert to amp-hours: bank Ah = (bank kWh × 1000) ÷ system voltage. For 5 kWh/day × 3 days ÷ 0.8 DoD = 18.75 kWh bank. At 48V: 18,750Wh ÷ 48V = 391Ah. Round up to the nearest standard configuration.
Step 3: Size Solar Panels for Daily Recharging
Formula: daily kWh ÷ peak sun hours × 1.25 (system losses) = minimum panel wattage. For 5 kWh/day at 5 sun hours: 5 ÷ 5 × 1.25 = 1.25 kW (1,250W). Oversizing panels by 20-30% is cheap insurance against winter production drops and panel degradation over time.
FAQ
Is this off-grid solar battery sizing calculator free?
Yes, completely free with no signup or account required. All calculations run locally in your browser.
What is 'days of autonomy' in a solar system?
Days of autonomy is the number of cloudy or low-sun days your battery bank can power your home without solar input. Most off-grid homesteads target 2-4 days. In cloudy climates (Pacific Northwest, UK), 4-7 days is more common. More autonomy means a larger, more expensive battery bank.
What is battery depth of discharge (DoD) and why does it matter?
DoD is the maximum percentage you should discharge your batteries before recharging. Lead-acid batteries should only discharge to 50% for long life. Lithium batteries handle 80-90% DoD. Using more than the rated DoD significantly shortens battery lifespan — a 50% DoD lead-acid battery bank sized for 200Ah usable only stores 400Ah total.
What system voltage should I choose for off-grid solar?
12V is fine for small systems under 1kWh/day. 24V suits 1-5kWh/day systems. 48V is standard for homesteads using 3-15kWh/day — higher voltage means lower current, allowing thinner wires and less energy loss. Most residential off-grid systems use 48V for efficiency.
How many solar panels do I need for an off-grid system?
A rough guide: divide your daily kWh by your average peak sun hours (4-6 hours for most of the US), then multiply by 1.25 for system losses. For 5 kWh/day at 5 peak sun hours: 5/5 × 1.25 = 1.25kW of panels (5 × 250W panels). Add 20-30% more capacity for cloudy seasons if you can't oversize batteries.
What's the difference between lead-acid and lithium batteries for off-grid?
Lead-acid batteries (AGM/gel/flooded) cost less upfront but weigh more, need more maintenance, and tolerate less deep discharging. Lithium (LiFePO4) costs 2-3x more initially but lasts 3-5x longer, tolerates 80-90% DoD, and weighs 60% less. The total cost of ownership over 10 years is often similar or lower for lithium.