Off-Grid Design Reference
50 worked problems · Quick formulas · Design rules · Real-world examples. Built for students, DIYers & solar technicians.
The core equations every off-grid designer needs at their fingertips.
P = Watt rating · T = Hours used per day
PSH = Peak Sun Hours · η = System efficiency (0.75–0.85)
DOD = Depth of Discharge (0.5 lead-acid · 0.9 lithium)
V = System voltage (12V / 24V / 48V)
Also applies to: Charge Controller sizing
Result in months. Divide by 12 for years.
Standard irradiance = 1,000 W/m²
η_inv = Inverter efficiency (typically 0.90–0.95)
Global avg emission factor: 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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