Off-Grid Solar Calculator
Design your complete off-grid system. Enter your energy usage, location, and battery preference — get panel count, battery bank, charge controller, inverter size, and cost estimate.
How to Use This Calculator
Enter your daily energy usage
The most important input is your daily kWh consumption. If you're currently on-grid, check your electricity bill — divide monthly kWh by 30 for a daily average. For a new off-grid build, add up all appliances: (watts × hours per day) ÷ 1,000 = kWh per appliance. Add them together for your total. Be conservative — it's cheaper to oversize the system now than to upgrade later.
Set days of autonomy
Days of autonomy (backup days) is how long your battery bank can power the cabin/home without any solar input. 2-3 days covers most weather events in sunny climates. 4-5 days is appropriate for cloudy Pacific Northwest climates or systems without generator backup. This number has the biggest impact on battery cost — each additional day adds a full day's worth of battery storage.
Enter your location's peak sun hours
This is the biggest variable after your load. Use the Peak Sun Hours Calculator or NREL PVWatts for your exact location. The system must produce enough daily energy to both power today's loads AND recharge the batteries after cloudy periods.
Choose battery type and system voltage
Lithium iron phosphate (LiFePO4) is the default choice for new systems — 90% usable capacity, 3,000+ cycle life, no maintenance, can be discharged rapidly without damage. Lead-acid (AGM or flooded) costs less upfront but only uses 50% capacity, needs replacement every 3-5 years, and degrades when discharged deeply. For 48V systems (3kW+), lithium's cost advantage is even more pronounced over the system lifetime.
The Formula
Example
Full-time off-grid cabin — Montana
The Morrisons are building a 1,200 sq ft cabin in rural Montana. No grid connection. 5 kWh/day usage (fridge, lights, laptop, phone charging, small TV, water pump). Want 3 days autonomy. Montana averages 5.0 peak sun hours. Using 48V LiFePO4 system with 400W panels.
The Morrison system: 3 × 400W panels (1.2 kW), 16.7 kWh LiFePO4 battery bank (4 × 100Ah 48V or equivalent), 40A MPPT controller, 2,000W inverter. Estimated cost: $8,000-$12,000 installed. With no electric bills for 20+ years, payback vs running grid extension (typically $10,000-$50,000/mile) is immediate.