Solar Energy

How to Size the Best Off-Grid Solar System For the Home? (Step-by-Step)

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In this article, I will try to explain to you how to make the most accurate system design starting from the working principle of off-grid solar systems. The system design I will explain is prepared in a simple and understandable language so as not to overwhelm you with technical details. First, we will talk about the parameters you need to consider when calculating the system elements. Then you will see how to apply them with an example.

What is an Off-Grid Solar System?

An off-grid solar system is a system in which the electricity generated by solar panels is stored in batteries without being connected to the electricity grid and this stored electricity is converted into AC energy with solar inverters and used for powering devices.

Off-grid systems consist of four main elements: solar panels, charge controllers, batteries, and solar inverters. I have already mentioned these system elements in general in the other article. I am telling you the system design assuming that you already know the working logic of these elements. So if you do not know their working principles, you should read the article called “The Ultimate Guide to Solar Photovoltaic System

Calculation of Off-Grid Solar System Components

First of all, I want to mention you the calculation criterias for the off-grid solar system components such as solar panels, batteries, and inverters.

1) Solar Panel Calculation

Solar Panel Calculation

Solar panel capacity calculation varies according to the following parameters.

  • Total daily energy consumption
  • Average daily sunshine duration according to the winter months in the region where the solar system will be installed (Always calculate according to the lowest sunshine duration.)
  • The output voltage of the battery pack
  • Inverter power and operating voltage

These parameters are important in calculating the power and number of solar panels. This plays a vital role in off-grid solar system design.

2) Solar Charge Controller Calculation

Charge Controller

As I mentioned in the previous article, off-grid charge controllers regulate the electricity coming from the panel and are used for charging the batteries. They also protect the panels from reverse current (from the batteries to the solar panels) and the batteries from overcharging. The following parameters are important in calculating the charge controller capacity.

  • Total voltage and the current value of the solar panel group,
  • The total voltage value of the battery pack

These parameters are used to determine the capacity of the charge controller.

3) Battery Pack Calculation

The battery is the heart of off-grid systems and its quality is crucial for the system. Its capacity also needs to be calculated very well. Batteries must be long-lasting, resistant to high temperatures, have high cycle efficiency, and have minimum gas leakage. The following parameters are important in calculating the number and capacity of batteries.

  • The capacity should be determined by taking into account the efficiency loss due to temperature, battery efficiency, and DC/AC conversion losses.
  • The operating voltage of the selected inverter is also important in determining the exact number of batteries.
  • The cloudiness factor should be taken into account.
  • A long-lasting off-grid system depends on a good design of the battery pack and the use of quality batteries.

4) Off-Grid Solar Inverter Calculation

Off-grid solar inverter selection is determined according to the total instantaneous power drawn by the devices to be used at the same time. An inverter with 1.5 times the instantaneous power draw should be used. The choice of full sine or modified inverter is also important when choosing an off-grid inverter. It will be healthier for your devices to use full sine models in devices such as large refrigerators, washing machines, and water motors.

Furthermore, smart-type inverters include a charge controller. The capacities of these charge controllers are 12V/24V 50A or 48V/50A.

Note: When calculating the off-grid solar inverter capacity, the mooring currents of motorized devices (the high currents drawn by motorized devices during initial operation) should be taken into consideration. Therefore, the user should not use all motorized devices at the same time unless it is mandatory. If the user wants to use them at the same time, inverter selection should be made by taking into account the anchor currents. For example, the instantaneous power draw of a 1 hp (0.75kW) water motor initially increases 3-3.5 times more than the label value. This factor should be taken into consideration in off-grid solar system design.

Designing the Off-Grid Solar System

A sample city is our location for installing the off-grid solar system. So the lowest sunshine duration in December for this sample city is 3 hours. In this case, the user will not run all of the specified appliances at the same time and will not use high-powered motorized appliances on days when there is no sun.

DevicesPower Daily Usage (Hours)Total Daily Electricity Consumption
LED TV150 W12 h1,8 kWh
A+ Refrigerator1200 W24 h1,2 kWh
A+ Washing Machine1200 W2 h2,4 kWh
A+ Dishwasher1200 W1 h1,2 kWh
Lighting (10 Pieces 12W LED Bulb)120 W12 h1,44 kWh

Total: 8.04 kWh/day We choose this value as 9 kWh/day for a more comfortable system.

Calculating Solar Panel Capacity

The lowest sunshine duration and solar radiation value in the sample city conditions belong to December. This value is 3 hours for our sample city.

Solar Panel Capacity: 9 kWh/3h=3 kW, so the panel capacity is such that it produces 3 kW per hour.

Since the battery capacity will be high due to the cloudiness factor and considering the efficiency loss of solar panels and the change in solar radiation during the day, it would be healthier to determine the number of panels by dividing the panel power by a correction factor. In other words, we put some safety margin in the system.

The most important reason for using such a correction factor is that solar panels need to charge the batteries faster during the day. In this way, the battery life can be longer.

Assuming a 450 W half-cut monocrystalline solar panel is selected;

Number of Panels: 3000W/0.6 = 5,000 W / 450 W = 11.11 We round this value to 12, so 12 pieces of 450 W solar panels should be used to meet the electricity needs of this system. When calculating the system, make sure that the total number of solar panels is an even number. Because after a certain system size, it is necessary to connect the panels in two rows.

Calculating Charge Controller Capacity

Charge Control Capacity: The total panel output will be approximately 80V/66A for full power where the solar panels will be connected in two rows of 6. So the charge controller capacity should be a minimum of 48V/80A. Of course, the capacity of charge controller in the smart inverter is usually limited to a maximum of 48V/50A. Therefore, a 30A charge controller should also be used externally for this system.

Calculating Battery Capacity

Battery Capacity: Since the daily energy consumption is 9 kW, this value is multiplied by the cloudiness factor (the number of consecutive cloudy days) and divided by the battery charging efficiency and the temperature correction factor to prevent efficiency loss due to high temperature. In our system, we take the efficiency of the selected battery as 80% and the temperature correction factor as 0.8. The cloudiness factor is 2 days for our sample city.

9 kW * 2 days / 0.8 * 0.8 = 28,125 kW battery power to be installed

Battery: Let’s choose a 12V 200 Ah gel battery for this system.

Note: Lithium-ion batteries are lighter, more efficient, and take up less space than gel batteries. If there is no budget problem, a lithium-ion battery should be preferred.

Battery power=12*200=2400Wh

Number of Batteries: 28,125 kWh / 2,4 kWh = 11,25 If we round this value up, 12 pieces of 12V 200Ah batteries should be used.

Calculating Inverter Capacity

Inverter Capacity: Let’s assume that TV+Satellite Receiver, Refrigerator, Washing Machine, dishwasher, and lighting are working at the same time. Simultaneous operation of all these devices creates 2900 W instantaneous power consumption. This value is multiplied by 1.5 to find the inverter capacity.

2900*1.5= 4350W, so 4000W/5000VA inverter is suitable for this system.

Off-Grid Solar System Cost

Let’s take a look at how much this system cost to us.

Components Quantity Price per piece Total Price
450 W Solar Panel12250 $2400 $
12V 200Ah Gel Battery12300 $3600 $
4000W/5000VA Off-Grid Solar İnverter + 48V 50A Charge Controller1500 $500 $
External 48V/30A MPPT Charge Control Device125 $25 $
Mounting Equipment300 $
Installing and transportation1.000 $

Total: $7,825

Such a system varies between $7,000 and $8,500 turnkey in the market, depending on labor and product quality. Let’s not forget that in off-grid solar systems, installation and workmanship are at least as important as system design and product quality. It should be done by technicians and engineers trained in the field.

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