How to Size an Off-Grid Solar System: Panels, Batteries & Backup

Modified on Thu, 18 Sep at 12:03 AM

Designing an off-grid solar system requires careful planning to ensure you have enough energy to power your appliances reliably. Sizing your panels, batteries, and backup correctly helps avoid power shortages and extends the life of your equipment. This guide walks you through the step-by-step process.

1. Calculate Your Daily Energy Usage

The first step is to work out how much power you need each day.

List all appliances and devices you plan to run (lights, fridge, water pump, TV, laptop, etc.).
Check their wattage (W) and estimate how many hours per day each will run.
Multiply wattage × hours = daily watt-hours (Wh).
Add up the totals for all appliances.

? Example:

LED light (10W × 5 hrs) = 50 Wh
Fridge (120W × 8 hrs) = 960 Wh
Laptop (60W × 3 hrs) = 180 Wh
Total = 1,190 Wh (≈ 1.2 kWh per day)

2. Consider Peak Sun Hours

Solar panels only produce power when sunlight is available. “Peak sun hours” varies by location and season.

In Australia, expect 3–6 hours of usable peak sun daily (more in summer, less in winter).
To size panels:
$hours\text{Solar panel watts required} = \frac{\text{Daily load (Wh)}}{\text{Peak sun hours}}$

? Example: 1,200 Wh ÷ 4 sun hours = 300W of solar panels needed. Add a 30–50% margin for cloudy days and system losses → 400–450W recommended.

3. Size the Battery Bank

Your batteries store energy for night-time and cloudy days. Consider:

Days of autonomy: How many days you want to run without sun (usually 2–3 days).
Depth of discharge (DoD): LiFePO₄ (lithium) can use up to 80–90% of capacity; AGM/lead acid only 50%.

Formula:

$(V)×DoD\text{Battery capacity (Ah)} = \frac{\text{Daily load (Wh)} \times \text{Days of autonomy}}{\text{System voltage (V)} \times \text{DoD}}$

? Example:

Daily load = 1,200 Wh
Days autonomy = 2
Voltage = 12V
LiFePO₄ DoD = 0.8

= (1,200 × 2) ÷ (12 × 0.8) = 250Ah battery bank

4. Choose Your Inverter Size

The inverter converts DC power from your battery into AC power for appliances.

Must handle your peak load (all appliances running at once).
Add a 20–30% safety margin.
Example: If your max load is 800W, choose at least a 1,000W inverter.

5. Plan for Backup Power

Even with the right design, off-grid systems need backup:

Generator: Provides emergency charging during long cloudy weeks.
Hybrid inverter/charger: Automatically switches between solar, battery, and generator/grid.

Backup ensures you’re never left without power.

6. Extra Sizing Considerations

System voltage: 12V for small systems, 24V/48V for larger setups to reduce cable size and losses.
Cable & fuses: Must be sized to handle the current safely.
Future expansion: Oversize slightly if you expect to add more appliances later.

Summary

To size an off-grid solar system:

Calculate your daily energy usage (Wh).
Divide by peak sun hours to size your solar panels.
Multiply daily load × days of autonomy ÷ system voltage to size your battery bank.
Select an inverter based on peak load + margin.
Include backup (generator or hybrid inverter) for reliability.

With correct sizing, your off-grid solar system will deliver reliable power year-round, giving you independence and peace of mind.