How do I calculate the size of my home's solar system? A complete step-by-step example.

Last updated: October 17, 2026

Calculating the size of your home's solar system starts with one number: how many kilowatt-hours (kWh) you consume per day. Divide this by the average kilowatt-hour output of solar panels in your city, then add a margin for losses. This will give you the required panel capacity in kilowatts, the number of panels, and the required area. In this article, we'll walk you through a complete example for a house in Riyadh, step by step.

First step: Know your actual consumption from the bill

Your electricity bill is your most accurate source of information. Don't rely on initial appliance estimates, as actual operating hours often differ significantly from assumptions. Log in to your Saudi Electricity Company account and calculate your kilowatt-hour consumption for the last 12 months.

Why 12 months? Because consumption in Saudi homes fluctuates significantly between summer and winter due to air conditioning. A July bill alone will give you a larger bill than you need, while a January bill alone will give you a much smaller bill.

Our example: A house in Riyadh

Let's assume the total electricity consumption for a household in a year is 18,000 kilowatt-hours. This is a hypothetical figure for illustration; replace it with your own.

• Monthly average = 18,000 ÷ 12 = 1,500 kWh.
• Daily average = 1,500 ÷ 30 = approximately 50 kilowatt-hours.

This figure (50 kilowatt-hours per day) is the basis for all subsequent calculations.

Step two: How many kilowatts are produced by the panels in your city?

One kilowatt of solar panels in Saudi Arabia produces approximately 4.5 to 6 kilowatt-hours per day on average. This figure varies depending on the city, installation angle, panel cleanliness, and temperature. Clear desert areas are generally closer to the upper limit, while locations with significant dust or partial shade are closer to the lower limit.

In our example, we use an average value: 5.5 kWh per kWh per day. If you're unsure of your specific situation, use 5 to be on the safe side.

Step three: Add the loss margin

The panels don't deliver all the power they produce to your devices. Some is lost due to heat, some to dust, some in the cables, and some in the inverter, which typically has an efficiency of 95–98%. The total of these losses under our conditions is usually 15–25%.

In the example, we assume a loss of 20%. This means that the actual output is 80% of the theoretical production. Therefore, we divide by (1 − 0.20) = 0.8.

Step four: Calculate the system's power in kilowatts.

Now let's apply the equation to our example:

1. Daily consumption ÷ Production per kilowatt = 50 ÷ 5.5 = 9.09 kilowatts.
2. Divide by the loss factor: 9.09 ÷ 0.8 = 11.36 kW.

Therefore, the house needs approximately 11.4 kilowatts of panels to cover its entire annual consumption.

How does the figure change with variations in production?

The following table shows the same house (50 kWh per day, losses 20%) with three output values, using 550W panels:

Note that the difference between the worst and best scenarios can be as much as 7 panels. Therefore, it's not advisable to take a generic figure from the internet without knowing your specific location.

Step five: Number of panels and surface area

Number of panels = System power ÷ Power of a single panel. Common panels today range between 550 and 650 watts.

• With 550W panels: 11.36 ÷ 0.55 = 20.7, rounding up to 21 panels, the actual power is 21 × 0.55 = 11.55 kW.
• With 650 watt panels: 11.36 ÷ 0.65 = 17.5, i.e., 18 panels, and the actual power is 18 × 0.65 = 11.7 kilowatts.

As for the area, we calculate it using an approximate formula: 5 to 6 m² per kilowatt, including walkways and maintenance distances. For an 11.55 kW system: 11.55 x 5 = approximately 58 m², and 11.55 x 6 = approximately 69 m². Therefore, you need a free, unshaded surface area of approximately 58–70 m².

If the roof area is less than that, you have two options: cover only part of your consumption, or use a solar carport to add space. You can estimate the appropriate structure via Solar Mounting Structure Calculator.

Step six: Check the result

It's a good idea to reverse the calculation to be sure. An 11.55 kW system produces approximately: 11.55 x 5.5 x 0.8 = 50.8 kWh per day. And in a year: 50.8 x 365 = approximately 18,549 kWh, which is close to a household's consumption (18,000). The result makes sense.

To estimate the impact on the bill: If we calculate the quantity produced at the first consumption bracket rate (18 halalas), we get 18,549 x 0.18 = approximately 3,339 riyals per year. This is based on the tariff announced at the time of writing; please refer to the Saudi Electricity Company for the current tariff. To learn when you will be able to recover the cost, read our article on Solar energy feasibility and payback period.

Do I cover all my consumption or only part of it?

It's not necessary for 100% to cover your consumption. Many homeowners start with half their consumption or less, for reasons such as space or budget. In our example, covering 50% means a system of approximately 11.36 ÷ 2 = 5.7 kW, or 11 panels of the 550-watt category. See the article for details on this size. 5 kW systemIf your consumption is close to our example, the entire system will be slightly more than 10 kilowatts.

What about the inverter?

In a grid-tied system, the inverter is usually close to the power rating of the panels, though the panels may have slightly higher ratings. In our example, a house with an inverter of around 10 kW and 11.55 kW panels would be suitable; the final decision depends on the voltage of the arrays and their connection.

What if the system is separate from the network?

In an off-grid system, two steps are added: calculating the battery capacity for overnight use, and calculating the maximum instantaneous load the inverter is operating at. Losses increase slightly due to charging and discharging. The complete method can be found in the article. Calculating battery capacity.

Common errors in calculating volume

• Relying solely on the summer bill results in a system larger than needed.
• Forgetting the losses, the system comes out about a fifth smaller.
• Confusing kilowatts (power) with kilowatt-hours (energy).
• The area is calculated based on the area of the panels only, without walkways or shaded spaces between rows.
• Ignore any expected increase in consumption, such as adding an extension or new air conditioners.

If you want to take a shortcut, enter your consumption in Solar Energy Calculator To get an initial estimate of size and savings, then review the result with an engineer before purchasing. For those who want a more in-depth understanding of the method, it explains... Solar System Design Course These steps are detailed below.

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