How many solar panels do I need for off-grid?

Divide your daily watt-hour use by peak sun hours, then multiply by about 1.3 to cover wiring, heat and conversion losses. Divide that wattage by your panel rating and round up. For 2000 Wh/day at 4.5 sun hours you need roughly 578 W, or two 400 W panels.

What does depth of discharge mean for battery sizing?

Depth of discharge is how far you safely drain the bank. Lead-acid is usually limited to 50%, lithium to 80–100%. A lower limit means you must install more capacity to deliver the same usable energy, so the bank amp-hours rise accordingly.

How is the charge controller amperage calculated?

For an MPPT controller the output current equals array watts divided by battery voltage. We add a 1.25 continuous-current safety margin per electrical code, so a 800 W array on a 24 V bank needs at least a 42 A controller.

Why does the array get oversized by 30%?

Real systems lose energy to wiring resistance, dust, high panel temperature, and charge-controller and inverter inefficiency. Oversizing the panel wattage by about 30% ensures the batteries still fully recharge on an average day.

Does days of autonomy matter?

Yes. Days of autonomy is how long the bank runs your loads with no sun. Two days is common; cloudier climates use three or more. More autonomy multiplies the required battery capacity directly.

Off-Grid Solar System Calculator

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

Example

For 2000 Wh/day, 4.5 sun hours, 400 W panels, a 24 V bank, 2 days autonomy and 50% depth of discharge: required PV is about 578 W, so two 400 W panels (800 W installed). The bank needs (2000 × 2) / 0.5 / 24 ≈ 333 Ah at 24 V, and the controller needs at least 800 / 24 × 1.25 ≈ 42 A.

Notes

Round panels, batteries and the controller up to standard sizes you can actually buy. These are planning figures — confirm against panel and battery datasheets and your local sun-hour data, and remember winter sun hours are far lower than the annual average. All calculations run locally in your browser.