Off‑Grid System Sizing Made Simple – How to Match Batteries and Solar Panels

Designing an off‑grid system comes down to three steps: figure out your daily energy consumption, size the battery bank, and then calculate the solar array. Start by listing every appliance you plan to run, multiply its wattage by the hours used per day, and add everything up to get your total daily watt‑hours (Wh). For example, a 100W light running 5 hours consumes 500Wh. Next, determine battery capacity based on two factors: how many cloudy days you want to survive (autonomy) and the depth of discharge (DoD) of your chosen battery chemistry. The formula is Battery Capacity (kWh) = Daily Consumption (kWh) × Autonomy Days ÷ DoD. Take a daily need of 5kWh and 3 days of autonomy: with LiFePO₄ (DoD = 80%) you need 5×3÷0.8 = 18.75kWh; with lead‑acid (DoD = 50%) you’d need 30kWh – almost double the physical size. For system voltage, a good rule of thumb is 12V for systems under 1000W, 24V for 1000–3000W, and 48V above 3000W.

Now size the solar panels – always use your location’s winter Peak Sun Hours (PSH), because summer numbers will leave you powerless in winter. The formula is Array Power (W) = Daily Wh ÷ Winter PSH ÷ 0.75, where 0.75 accounts for wiring losses, controller efficiency, and temperature derating. If you need 5000Wh/day and your winter PSH is only 4 hours, you’ll need 5000 ÷ 4 ÷ 0.75 ≈ 1667W of panels (e.g., six 300W modules). When wiring panels in series/parallel, make sure the total open‑circuit voltage stays below your inverter’s maximum input, and that the MPPT voltage falls within the inverter’s operating range. Finally, the inverter’s continuous power rating must exceed the total wattage of all loads running simultaneously – especially for inductive loads like air conditioners, fridges, or pumps, whose starting surge can be 3–5 times their rated power. I always oversize by at least 1.25× the peak load. With this approach, I built a 48V system with 18.75kWh LiFePO₄ and ~1700W of panels for my van, and it comfortably gets me through three overcast days. Anyone else run into weird issues while sizing their system? Drop your stories below!