How to Design an Off-Grid Solar System: A Step-by-Step Sizing Guide

Against the backdrop of rising energy costs, building an off-grid energy storage system has become a preferred solution for many households to achieve energy independence. However, the most challenging part for most users is system sizing.

This guide will walk you through designing and configuring an efficient, reliable off-grid energy storage system step by step. We will cover load assessment, inverter sizing, solar panel configuration, and battery bank calculations—helping you easily achieve self-sufficiency.

Step 1: Calculate Total Daily Energy Consumption (Load Assessment)

The core of an off-grid system is precise alignment with your actual energy needs. To determine this, you must calculate the total daily consumption in kilowatt-hours (kWh).

Below is a detailed calculation based on common household appliances. Note: You should adjust these values based on your actual device ratings.

Common Household Appliance Power Ratings Table

Appliance	Quantity	Power Rating (kW)	Running Time (Hrs/Day)	Daily Consumption (kWh)
Air Conditioner	2	2.75 kW	5	27.5 kWh
Induction Cooker	2	2.00 kW	2	8.0 kWh
Refrigerator	2	0.15 kW	24	7.2 kWh
Lighting	10	0.05 kW	10	5.0 kWh
Washing Machine	2	0.40 kW	2	1.6 kWh
TV	3	0.15 kW	2	0.9 kWh
TOTAL	-	-	-	50.2 kWh / Day

Total Daily Consumption: 50.2 kWh. This figure is the baseline for sizing your battery bank and solar array.

Step 2: Sizing the Off-Grid Inverter for Peak Power Loads

Your inverter must be powerful enough to handle the Peak Load—the maximum power surge when all devices are turned on simultaneously. If the inverter is too small, the system will trip during peak usage hours.

Calculating Total Peak Power

Sum the maximum power rating of all appliances to find the required inverter capacity:

• Air Conditioner: 2.75kW × 2 = 5.5kW
• Induction Cooker: 2kW × 2 = 4kW
• Washing Machine: 0.4kW × 2 = 0.8kW
• Lighting: 0.05kW × 10 = 0.5kW
• TV: 0.15kW × 3 = 0.45kW
• Refrigerator: 0.15kW × 2 = 0.3kW

Total Peak Power Demand: 5.5 + 4 + 0.8 + 0.5 + 0.45 + 0.3 ≈ 11.55 kW

Recommended Solution: We suggest selecting a 12kW Inverter (e.g., Seplos 12kW Hybrid Inverter). This ensures a stable power supply with a safety margin for future expansion.

Step 3: Determining Solar Panel Quantity & Voltage Configuration

Your solar panel array must meet two criteria: generating enough daily energy (kWh) and staying within the inverter's MPPT voltage range (Voltage Open Circuit).

Matching PV Input to Inverter MPPT Voltage

For a high-efficiency system, high-voltage series connections are often used. Assuming an inverter limit of 650V:

• Single Panel Specs: 550W Power, 31.8V Operating Voltage.
• Calculation Formula: Total Voltage = Single Panel Voltage × Number of Panels.
• Verification: 31.8V × 20 panels = 636V.

Result: 636V is < 650V, which is within the safe and efficient operating range.

Recommended Configuration: 20 × 550W Solar Panels.

• Total Capacity: 11kW.
• Daily Generation: Approx. 50-60kWh (depending on sunlight hours), fully covering the 50.2kWh daily demand.

Step 4: Calculating Storage Battery Capacity & Depth of Discharge (DoD)

Battery capacity calculation must account for the Depth of Discharge (DoD) to ensure battery longevity. You cannot drain a battery to 0% every day without damaging it.

The Formula for Battery Sizing

For a Lithium Iron Phosphate (LiFePO4) system, we typically use an 80% DoD calculation:

Formula: Required Capacity = Daily Consumption ÷ DoD (0.8)

Calculation:

50.2 kWh ÷ 0.8 ≈ 62.75 kWh

Recommended Solution:
We recommend 2 × Seplos 560Ah Vertical Battery Packs.

Each pack provides ~32kWh.

Total Capacity: 64kWh.

Benefit: This offers slightly more than the required 62.75kWh, ensuring you have backup power even on days with heavy usage.

Complete Off-Grid System Configuration Summary

Here is the final list of components for your 50kWh daily use system:

• Batteries: Seplos 560Ah Battery Pack × 2 (Total 64kWh Storage)
• Inverter: Seplos 12kW Hybrid Inverter
• Solar Panels: 550W Monocrystalline Panels × 20 (Total 11kW Array)

Daily Generation: ~55kWh (under adequate sunlight)

Optimization Tip: If your budget allows, adding a second inverter and another string of 20 panels can enhance charging speed and stability, especially during cloudy days.

Why Choose Seplos for Your Off-Grid Solution?

• High Compatibility: Seplos inverters, panels, and batteries are designed to "talk" to each other via BMS (Battery Management System), maximizing efficiency.
• Safety & Reliability: Featuring intelligent temperature control and overload protection for long-term safety.
• Scalability: The modular design supports flexible expansion. You can easily add more battery modules as your family grows or energy needs increase.

FAQ about Off-Grid System Design

Q: Can a 12kW inverter run my whole house including air conditioners?
A: Yes. As calculated in Step 2, the peak load of this household is approx 11.5kW. A 12kW inverter can handle this load, allowing you to run ACs, induction cookers, and other appliances simultaneously.

Q: Why do I need 64kWh of battery for 50kWh of usage?
A: This accounts for Depth of Discharge (DoD). To extend the lifespan of your battery, it is recommended not to fully deplete it. Using 64kWh ensures you have 50kWh of usable energy while keeping 20% in reserve.

Q: Is this system expandable?
A: Absolutely. The Seplos system is modular. You can start with this configuration and add more solar panels or battery packs later if your energy demand increases.

Take Action Now, Achieve Energy Freedom!
Through precise calculations and scientific configuration, this system meets daily power needs in off-grid areas. For customized plans or product inquiries, contact our energy experts for one-on-one support!