Welcome! Many of us are now seriously considering switching to solar energy to get relief from electricity bills and annoying power outages. But the first question that always comes to mind is: "How many panels do I need? And what size battery and inverter should I get?"

In this article, we'll answer that question in detail, and we'll learn how to calculate the size of a solar system using the correct yet simplified engineering approach — step by step, without getting lost in complex terminology.

1. Theory Section: What Is a Solar System and How Does It Work?

Before we pick up the calculator, we need to understand what an off-grid solar system is fundamentally made of. Simply put, it's about producing, storing, and converting energy:

• Solar Panels: Their job is to convert sunlight into direct current (DC) electricity.
• Batteries: The tank where we store electricity to use at night or on cloudy days.
• Charge Controller: The guardian that regulates the electricity coming from the panels to the batteries to prevent damage from overcharging.
• Inverter: The maestro that converts DC current from the panels and batteries into AC current that powers your home appliances.

Key Terms You Need to Know Before Calculating

To be junior engineers, we need to distinguish between two terms we hear a lot:

• Watt: The device's power consumption at any given moment. (e.g., a 10-watt bulb).
• Watt-hour (Wh): The amount of energy a device consumes over a period of time. (e.g., a 10-watt bulb running for 5 hours = 50 watt-hours). This is the number that matters for determining the system size!
• Peak Sun Hours (PSH): Not all daylight hours give us the same solar intensity. We only count the hours when solar irradiance reaches 1,000 watts per square meter. (In the Arab region, this ranges from 4.5 to 6 hours daily).

2. Practical Section: Step-by-Step Calculation

Now it's time for hands-on work. Let's imagine we have a small home and we need to calculate a solar system that fully covers it.

Step 1: Calculate Daily Load Consumption (Wh)

The first thing you need to do is make an inventory of all the devices you want to run, how many watts they draw, and how many hours they'll run per day. Let's create this table as an example:

Device	Quantity	Power (Watts)	Daily Operating Hours	Total Energy (Wh)
LED Lights	5	10W	6 hours	300 Wh
TV and receiver	1	100W	5 hours	500 Wh
Small refrigerator	1	150W	10 hours (compressor runtime)	1500 Wh
Ceiling fan	2	70W	5 hours	700 Wh
Total Daily Consumption:				3000 Wh (or 3 kilowatt-hours)

Step 2: Calculate the Required Solar Panels

To calculate the panel capacity needed to generate 3,000 Wh, we need to add a system loss factor (wiring, dust, panel heat). Engineering-wise, per IEC 61215 standards, we multiply the consumption by 1.3 to compensate for losses (30%).

• Energy required from panels = 3,000 × 1.3 = 3,900 Wh.
• Let's assume the Peak Sun Hours (PSH) in your area are 5 hours.
• Total panel capacity required = 3,900 ÷ 5 = 780 watts.

If the panels available on the market are rated at 400 watts each:
Number of panels = 780 ÷ 400 = 1.95 panels. (That means we need two 400-watt solar panels).

Step 3: Calculate the Inverter Size

The inverter must be able to run all devices if they're all turned on at the same time (this is called the Peak Load). We also need to add a 20% increase as a safety factor so the inverter doesn't run at full capacity and overheat (per IEC 62109 standards).

• Total device power = (5×10) + 100 + 150 + (2×70) = 440 watts.
• But watch out! The refrigerator draws a surge current that can reach 3 times its rated power when it starts. So we'll calculate the refrigerator at 450 watts during startup.
• Adjusted peak load = 50 + 100 + 450 + 140 = 740 watts.
• Inverter size = 740 × 1.2 (safety factor) = 888 watts.

The closest inverter available on the market would be 1,000 watts (1 kW).

Step 4: Calculate the Batteries (Power Bank)

Battery sizing depends on how long you want electricity to keep running without sun (cloudy days or nighttime). Typically, we calculate for one day without sun (1 Day of Autonomy).

The formula: (Daily consumption × Storage days) ÷ (System voltage × Depth of Discharge DoD).

• Consumption = 3,000 Wh.
• The Depth of Discharge (DoD) for Gel Battery or Lead-Acid batteries should not exceed 50% if you want them to last, while lithium batteries can go up to 80–90%. (We'll calculate using lithium at 80%.) For more on the differences between types, check out our article on lithium vs. gel batteries.
• The system voltage for the inverter we chose is typically 12V or 24V. (Let's assume a 24V system).

Battery capacity (amp-hours) = 3,000 ÷ (24 × 0.80) = 156.25 Ah.
So you'd need a 24-volt lithium battery with a capacity of approximately 150 or 200 Ah.

Quick Reference Summary

In short, system design goes through 4 simple stages:

1. Calculate how much you consume per day (watt-hours).
2. Add 30% for losses and divide by sun hours to determine panel capacity.
3. Add up the power of all your devices and add 20% to determine the inverter size.
4. Use the daily consumption along with the system voltage to determine the battery capacity in amp-hours.

Frequently Asked Questions (FAQ)

Can I run a split AC on solar power?
Absolutely, but air conditioners draw a very high amount of power (especially at startup if it's not an Inverter AC). Running an AC means you'll need more panels and larger batteries, and the system cost will increase significantly.

What is the expected lifespan of system components?
If installed to proper standards: panels last 20 to 25 years, inverters 5 to 10 years, and lithium batteries (LiFePO4) around 10 years (while gel batteries last 2 to 4 years).

Is it necessary to install protections and grounding (earthing)?
Absolutely! The absence of grounding or protective breakers (DC Breakers & SPDs) exposes your entire system to fire due to lightning strikes or electrical shorts. This is a red line in engineering safety.