Sizing an off-grid solar system means matching three things: a panel array big enough to harvest your daily energy, a battery bank large enough to store it through cloudy days, and a charge controller rated for the current the array produces. This calculator works all three out from a short energy audit.
How it works
The array starts from your daily consumption and the sun you actually get:
required PV watts = daily Wh × 1.3 ÷ peak sun hours
The 1.3 factor oversizes the array by 30% to absorb wiring, dust, temperature and conversion losses, so the battery still recharges on an average day. We then divide by your panel wattage and round up to whole panels.
The battery bank must store more than you use, because you never fully drain it:
bank Ah = (daily Wh × days of autonomy) ÷ (depth of discharge ÷ 100) ÷ bank voltage
The charge controller (MPPT) must pass the array’s full output current, plus a code-required margin:
controller A = installed array W ÷ bank voltage × 1.25
Worked example
For 2,000 Wh/day, 4.5 peak sun hours, 400 W panels, a 24 V bank, 2 days autonomy and 50% depth of discharge:
- Required array:
2,000 × 1.3 ÷ 4.5 = 578 W→ round up to two 400 W panels (800 W installed) - Battery bank:
(2,000 × 2) ÷ 0.5 ÷ 24 ≈ 333 Ahat 24 V - Charge controller:
800 ÷ 24 × 1.25 ≈ 42 Aminimum → choose a 50 A MPPT controller
Choosing your system voltage
The bank voltage (12, 24, or 48 V) is one of the earliest design decisions and affects conductor sizing, charge controller availability, and inverter choice:
- 12 V — the simplest option, common in small campervans and boats. High current at modest power means thicker wiring over longer runs.
- 24 V — the most widely used voltage for mid-size systems (roughly 300–2,000 W). Halves the current compared to 12 V, allowing thinner conductors and more controller options.
- 48 V — standard for larger off-grid homes and serious van builds. Reduces current further, allows smaller wiring, and matches the input range of most modern lithium inverter-chargers.
For the worked example above at 24 V, the controller handles 42 A of output. The same system at 12 V would require an 84 A controller — a physically large and expensive unit. Stepping up to 48 V would drop the requirement to about 21 A, opening up a much wider range of controller options.
Lead-acid vs lithium battery sizing
Depth of discharge (DoD) is the most important difference:
- Lead-acid (AGM/gel/flooded): Regularly discharging deeper than 50% accelerates plate sulfation and shortens lifespan significantly. Most lead-acid banks are designed for 50% DoD, meaning a 333 Ah requirement needs a 667 Ah nominal bank.
- Lithium (LiFePO4): Designed for 80–100% DoD without comparable cycle life loss. The same 333 Ah usable capacity needs roughly 333–417 Ah of nominal lithium capacity. The bank is smaller and lighter, but the upfront cost per Ah is higher. Over a 3,000+ cycle lifetime the per-kWh cost often favors lithium.
Days of autonomy and winter sizing
Days of autonomy is how long the bank runs your loads without any charging. In climates with occasional multi-day overcast periods, 2–3 days is common. Sizing for 5 days of autonomy is not unusual in areas with long winter cloud spells, but it multiplies the battery cost directly. A common approach is to size the bank for 2–3 days and plan to run a small backup generator for extended low-sun stretches, which is usually more economical than a very large battery bank.
Also note: winter peak sun hours can be half or less of the annual average. For year-round off-grid living, size the array and battery bank to the winter sun figures for your location, not the annual average.
Notes
Round panels, batteries and the controller up to standard commercially available sizes. Confirm against panel and battery datasheets and your local winter sun-hour data. All calculations run locally in your browser.