How to Size an Off-Grid Photovoltaic System

This guide provides the basics for sizing a simple photovoltaic solar system, useful for those who wish to install an off-grid system.

“Estimate the Consumption”

To correctly size the system, it is necessary to calculate the energy consumption of the appliances that will use the produced energy.

Let's take a camper as an example with the following consumption:

• 2 x 15W Low-energy bulbs (12VDC)
• 1 x 60W Water pump (12VDC)
• 1 x 48W Refrigerator (12VDC)
• 1 x 50W TV (240VAC)
• 1 x 600W Microwave (240VAC) → Actual consumption: 900W

“1. Calculation of Loads for AC and DC Voltages”

Let's calculate the total consumption for the different voltages:

🔹 **DC Loads**

• Lighting: 2 bulbs at 15W → 60Wh/day
• Pump: 60W used for 15 min → 15Wh/day
• Refrigerator: 48W for 8 hours → 384Wh/day
• Total DC Loads: 459Wh/day

🔹 **AC Loads**

• TV: 50W for 2 hours → 100Wh/day
• Microwave: 900W for 15 min → 225Wh/day
• Total AC Loads: 325Wh/day
• Considering an inverter efficiency of 85% → 382Wh/day

Total daily consumption: 841Wh/day

“2. Calculation of Required Photovoltaic Power”

Estimating an average of 5 hours of sun per day:

Required power: (841Wh / 5 hours) * 1.4 = 235W

Note: In winter, consider 4-4.5 hours of sun to ensure adequate production.

“3. Choosing Solar Panels or DIY Kits”

To achieve at least 235W of power, you can opt for:

• 2 x 123W solar panels in parallel → 246W total

“4. Charge Controllers”

The short circuit current of the panels will be about **16.2A**. To choose the controller:

Controller current = 16.2A x 1.25 = 20.25A

A **30A** controller is recommended, allowing for the addition of future panels.

“5. Inverter”

To choose the inverter, consider the maximum simultaneous load (microwave + TV):

**950W required → recommended a 1200W pure sine wave inverter**.

If the budget is limited, you can opt for a modified sine wave inverter, but only if the appliances do not contain motors.

“6. Batteries”

To ensure **3 days of autonomy**, the battery sizing at **12V** will be:

Required Ah = (841Wh * 3/12V) / 0.7 * 1.1 = 330Ah

Note: The factor **1.1** is due to battery efficiency, which is usually 90%.

“Conclusion”

By following this guide, it is possible to correctly size an off-grid photovoltaic system. MR WATT offers DIY photovoltaic kits and components for custom installations.