Off-Grid Design Reference
50 worked problems · Quick formulas · Design rules · Real-world examples. Built for students, DIYers & solar technicians.
Simplified planning formulas for educational and preliminary sizing. Actual equipment engineering must account for temperature coefficients, continuous current ratings, and applicable electrical codes (e.g., NEC Article 690).
P = Rated appliance watts · T = Daily operating hours
PSH = Worst-month peak sun hours · η = System efficiency (0.75–0.85)
DOD = Usable Depth of Discharge (0.50 lead-acid · 0.85–0.90 lithium)
V = System DC nominal voltage (12V / 24V / 48V)
1.25 = Standard NEC continuous current safety factor (verify string Voc limit)
Simple payback in years (excludes ongoing battery replacement costs)
Standard Test Conditions (STC) irradiance = 1,000 W/m² @ 25°C
η_inv = Inverter efficiency factor (typically 0.88–0.93)
Based on EPA / IEA average electricity grid emission factors (~0.42 kg CO₂/kWh)
Always apply 15–25% derating for wiring, inverter, and temperature losses. Raw panel wattage ≠ usable power.
Lead-acid batteries degrade rapidly below 50% DOD. Always size for 50% max discharge to extend lifespan 3×.
Motors and pumps draw 3–6× rated watts at startup. Your inverter must handle peak surge, not just running load.
PSH varies by location, season, and tilt angle. Using your city's best-case PSH will leave you short in winter.
Adding new batteries to an aged bank forces new ones to work at old capacity levels, drastically reducing efficiency.
Always add 25% safety margin to your calculated controller current. Overheating controllers are a top failure cause.
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