Introduction: Why Is Accurate Calculation Important?

Welcome to the Solar Systems Designer blog. Many people think that installing home solar power is just about buying some panels and batteries from the market, connecting them, and you're done! But in reality, it's a matter of "precision engineering." If the system is too small, you'll suffer from power outages and your batteries will degrade quickly. If it's too large, you'll have wasted your money.

In this article, we'll walk through it step by step, combining simple explanations for everyday users with the precision of international engineering standards (such as IEC and NEC) so that technicians and engineers can benefit as well.

1. Theory Section: Key Concepts Before We Begin

Before we pick up the calculator, we need to agree on 3 essential terms used in solar power installations:

First: Electrical Loads (Watts and Watt-hours)

Any device in your home draws power, which we call power capacity (measured in Watts). When you run that device for a certain period, it becomes energy consumption (measured in Watt-hours or Wh). So, if you have a 10-watt light bulb and run it for 5 hours, the consumption is: 10 × 5 = 50 watt-hours.

Second: Peak Sun Hours (PSH)

The sun rises from morning to evening, but not all of that time gives us full energy. As engineers, we rely on something called "Peak Sun Hours." This is the number of hours during which solar irradiance reaches 1,000 watts per square meter. This number varies by country (in Arab countries, it typically ranges from 4.5 to 6 hours daily).

Third: System Losses

According to the international standard IEC 62548 for solar array design, it's impossible to get 100% of the panel energy due to: ambient heat, dust, wiring losses, and inverter efficiency. That's why we always calculate based on a loss factor (Derating Factor) ranging between 20% and 30%.

Quick Comparison Between Battery Types

Before we continue, let's look at the differences between the available battery types on the market:

Battery Type	Allowed DoD	Lifespan	Efficiency	Relative Price
Lithium LiFePO4	80-90%	10-15 years	95%	💰💰💰
Gel	50%	3-5 years	85%	💰💰
Lead-Acid	50%	2-3 years	80%	💰

2. Practical Section: Step-by-Step Calculation

Now comes the decisive moment! Grab a pen and paper, and let's calculate a system for a small home as a practical example.

Step 1: Calculate Daily Consumption

We need to know how much energy the home uses per day. Let's look at this simple table:

Device	Quantity	Device Power (Watts)	Daily Operating Hours	Daily Consumption (Wh)
LED Lights	10	10	8	800
TV and screen	1	100	6	600
Refrigerator	1	200	12 (actual runtime)	2400
Total	-	400 Watts (continuous load)	-	3800 Wh/day

So, we need to produce 3,800 Wh daily.

Step 2: Choose the Inverter Size

The inverter's job is to convert the DC current from the panels and batteries to AC current for your home. To determine its size, you add up the "continuous power" of all devices that will run at the same time.

In the example above, the total is 400 watts. But! Refrigerators and motors draw a very high inrush current (Surge Current) that can reach 3 times their running current. That's why the engineering rule says: multiply the continuous load by 1.5 or 2 to cover the starting current and provide a safety margin per the American code NEC Article 690.

Proper size: 400 × 2 = 800 watts. It's best to choose a commonly available inverter of 1,000 watts (1 kW) or slightly more.

Step 3: Calculate the Number of Solar Panels

To calculate the panel energy needed to cover our consumption (3,800 Wh) and compensate for losses (assuming 75% system efficiency), we apply this formula:

Total panel energy = (Daily consumption ÷ System efficiency) ÷ Peak Sun Hours

• Consumption = 3,800
• Efficiency = 0.75
• Peak Sun Hours (in your country, assumed) = 5 hours

So: (3,800 ÷ 0.75) ÷ 5 = 1,013 watts of panels.

If you're buying panels rated at 540 watts each, certified per IEC 61215, you'll need: 1,013 ÷ 540 = 1.87. That means we'll install two (2) panels.

Step 4: Battery Sizing

Batteries are your storage. To determine the required size, you need to decide how many days you want electricity to keep running without sun (Autonomy Days), and the allowed Depth of Discharge (DoD).

Lithium batteries (LiFePO4): allow safe discharging of up to 80% or 90% with a long lifespan.

The formula: Battery capacity (Wh) = (Daily consumption × Number of storage days) ÷ (DoD × Inverter efficiency)

Let's assume we want one day of storage (1), a DoD of 80% for lithium, and an inverter efficiency of 95%:

Capacity = (3,800 × 1) ÷ (0.80 × 0.95) = 5,000 watt-hours (5 kWh).

If your system is 48 volts, the capacity in amp-hours = 5,000 ÷ 48 = approximately 104 Ah. (A 48V 100Ah lithium battery would be perfect.)

Advanced Example: Large Home in a Hot Climate

Let's work through an example for a large home in the Gulf (6 Peak Sun Hours, but with high heat):

• Daily consumption: 12,000 Wh (large home with air conditioners)
• Peak Sun Hours: 6 hours (but we compensate 20% for heat = 4.8 effective hours)
• System efficiency: 75%

Panel calculation: (12,000 ÷ 0.75) ÷ 4.8 = 3,333 watts

Using 540-watt panels: 3,333 ÷ 540 = 6.17 → 7 panels

Battery calculation (2 days of backup): (12,000 × 2) ÷ (0.80 × 0.95) = 31,578 Wh ≈ 32 kWh

48V system: 32,000 ÷ 48 = 667 Ah (meaning 4 batteries of 48V 200Ah in parallel)

3. Conclusion

Simply put, to design your system correctly, you need to:

• Calculate your daily consumption in watt-hours (Wh) accurately — don't leave anything out.
• Choose an inverter that covers your loads with extra margin for inrush current and safety.
• Compensate for system losses by increasing the number of panels by 25–30%.
• Choose lithium batteries with a capacity that covers your nighttime consumption, taking Depth of Discharge (DoD) into account.

Frequently Asked Questions (FAQ)

Can I run an air conditioner on solar power?

Absolutely, but air conditioners consume a very high amount of energy. You need to make sure your inverter can handle the AC's inrush current, and significantly increase the number of panels and batteries. It's recommended to use inverter-type ACs because their starting current is very low and their energy efficiency is excellent.

How much roof space is needed to install solar panels?

A modern solar panel (e.g., 540 watts) requires approximately 2.5 square meters of space. Multiply the number of panels you calculated by 2.5, and add 20% for walkways and maintenance to determine the net area required.

What happens to the system in winter when it's cloudy?

Solar panels produce energy from "light," not "heat," but their output certainly drops during heavy cloud cover. That's why we factor in winter Peak Sun Hours into our calculations and slightly oversize the system to compensate for this shortfall.