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How to Size an Off-Grid Solar System: A Step-By-Step Guide

Off-grid solar systems generate electricity only from solar panels and store it in batteries for use when the sun is not shining. Its purpose is to provide electricity to homes or facilities that are not connected to the power grid. It also offers independence from utility companies and the ability to generate your own power.

The main components of an off-grid solar system include solar panels, charge controllers, a battery bank, inverters, etc. The solar array converts sunlight into electricity, the charge controller regulates the flow of electricity to the batteries, the batteries store the electricity for later use, and the inverter converts the stored DC electricity into usable AC electricity that we use at home.

If you want to install an off-grid system, you may wonder how to size an off-grid system. Scroll down as we will discuss everything related to sizing your off-grid system in the following lines.

How to size an off-grid solar system?

Solar panels generate electricity. The more the number of panels, the more power it generates. But how big should your system be? That entails taking into account how much energy you would need.

Step 1: Determine Your Energy Needs

This step involves calculating your energy requirements throughout the year, including average daily energy usage for your home or facility. You will also have to include your usage patterns to ensure that your off-grid solar system is powerful enough to meet your requirements.

Identify the appliances and devices you will use

Start by identifying all the appliances and devices you want to power through your off-grid solar system. This will include items such as lights, refrigerators, well-pumps, air conditioning and heating system, and any other electrical equipment you may use. To determine the energy consumption of each item, you can refer to the product specifications—the wattage is usually mentioned in the data sheets. If you do not have product data sheets, we recommend using a power meter to measure the usage—a power meter gives more accurate and real-time consumption data.

Calculate your average daily usage

Once you have identified the appliances and devices you will be using, you can use this information to determine your average daily energy usage. This can be done by adding up the energy consumption of each item and then dividing it by the number of days in a year. This will give you an estimate of how much energy you will need on a daily basis.

The final output will be in watts (W) or kilowatts (kW). For the sake of simplicity, let’s say you have 15 lights, each one 10-watt and used for 8 hours a day. Add a refrigerator of 400 watts, used for 24 hours a day. Similarly, add the energy consumption of all the devices, and you will get a final output like this:

Total energy consumption per day: (15 x 10 x 8) + (400 x 24) = 1200 + 9600 = 10,800 watts or 10.8 kW

It’s also important to consider usage patterns, as they will affect energy consumption. For example, your energy usage may increase during certain days, such as Christmas or summer. In order to size your off-grid solar system correctly, you need to have an accurate estimate of your energy needs for the whole year.

Do not miss efficiency losses

You will use an inverter to convert DC electricity to AC electricity. But they cannot function at 100% efficiency—they lose some power during the operation. You will also have to account for the power lost or consumed by the inverter itself. Add that to the total daily energy usage. We also recommend adding a certain amount more than your requirement for the sake of an emergency.

With this information, you can proceed to the next step, which is calculating the peak sun hours.

You may like to read: 7 Things to Know About Off-Grid Solar Power Systems

Step 2: Calculate Peak Sun Hours

The number of peak sun hours depends upon where you live. After calculating your daily usage, check how much solar radiation your home receives on a typical day. We have provided a map of the annual direct normal solar irradiance below, thanks to The National Renewable Energy Laboratory (NREL). But only annual peak sun hours would not suffice. It would be best if you calculated the same for every month, especially in winter, as the performance of solar panels drops due to the low number of peak sun hours. You can find monthly solar irradiance here.

When you calculate your peak sun hours, you can then assess the size of your off-grid solar system. Based on the map above, note how much radiation your state receives annually but also monthly and move on to the next step.

Step 3: Size your off-grid solar system

It is time to calculate the size of your system. For the sake of convenience, we will keep your monthly consumption as 1000 kWh—almost the average American monthly consumption. Also, let’s suppose you are a resident of Florida, where households receive, on average, five peak sun hours a day.

(You can put your own consumption instead of 1000 kWh and your peak sun hours based on the above data.)  

First, calculate the amount of energy produced by one solar panel.

Solar panel capacity x peak sun hours = energy produced by one solar panels a day

Let’s say you install 400-watt solar panels. That means energy produced by one solar panel would be 400 x 5 = 2kW. The energy produced by one solar panel a month would be 60 kW. Now, divide your monthly consumption by 60 kW, and we would need a rough number of solar panels.

Number of solar panels needed: 1000/60 = 16.6 ~ 17 solar panels

So, the system you need would be 6.8 kW (17 panels x 400 watts = 6.8 kW). But this does not factor in real-world losses usually caused by inefficiencies, module soiling, aging, and voltage drop, which may amount to around 15%. When you add that, your required system size will increase to at least 7.7 kW.

Lastly, this system is only for your daily energy needs when the sun shines above. But the off-grid system also features batteries, and since off-grid systems are self-sustaining, you will have to make arrangements for charging your batteries. This might translate to doubling your system’s size—in some cases, even more, depending upon the size of your energy storage system. Why? Because solar panels would give you peak power only for around 6-7 hours. You will meet your remaining energy demands from the battery bank.

Note that the information contained here is for general guidance and doesn’t paint an exact picture of your system’s requirements. Every homeowner is different, with different goals, needs, and location. We can suggest the accurate system’s size only after taking into account many factors peculiar to your household, your goals, and where you live.

You may like to read: How Much Does an Off-Grid Solar System Cost?

Step 3: Choosing the right inverter size

Now that we know the size of our system, let’s move to the next step: sizing your solar inverter.

When calculating the size of a solar inverter for an off-grid system, it is important to consider the total wattage of all the appliances and devices that the system will power. You will already have calculated that in the first step. But note that the inverter should be sized to handle the peak power requirements of all connected loads.

Ideally, the inverter should match your peak load requirement. But we suggest going for a bit higher size, as inverters are not 100% efficient. There are also losses, which you will have to account for. Secondly, inverters are designed to work at an optimal temperature, usually 25°C. If the temperatures increase, the performance of the inverter decreases, sometimes by more than 15%. A 5kW inverter designed for 25°C will give an output of 4 kW at 40°C. Keeping these factors in mind, always choose a bigger size.

For example, if your peak demand is 5 kWh, go for an inverter that can supply 6 kWh under optimal conditions. It is also important to factor in future expansion and potential increases in power consumption.

Next Earth Solar Tip:

Some brands provide inverters power ratings in kVA, which can confuse you. The general conversion ratio used for kVA to kW is kVA x 0.8 = kW. For instance, if it is a 5kVA inverter, in watts, it equates to roughly a 4kW inverter power rating.

Step 4: Choosing the battery configuration

This is the last but most crucial step. Selecting the appropriate battery will ensure that your system can provide reliable power throughout the day and night.

To choose the correct size, you will have the following:

  • Daily peak consumption (which you will already have calculated in the first step).
  • Days of autonomy (these are the number of days for batteries to provide power when the sun may not shine. We will keep it 5, but you can increase or decrease it.)
  • Depth of discharge (this is the extent to which you can safely discharge a battery. For lead acid batteries, it is 50%; for lithium-ion, it could be 80% or even more.)

Let’s assume you consume 5 kWh per day and want your batteries to last for five days without recharging.

Battery size needed: (Daily consumption x days of autonomy)/DoD

The size will turn out to be (5 x 5)/0.5= 50kWh battery size. We took lead acid batteries here, with 50% depth of discharge (DoD). If you want to install lithium-ion, we will keep the DoD at 0.8. Let’s calculate the size for that.

(5 x 5)/0.8 = 31.25 kWh lithium-ion batteries.

You can convert kWh to Ah by dividing the kWh by the voltage of your system. This could be either 12, 24, or 48. If we keep it 24, then it will become 50,000 watts/24=2,083 Ah for lead-acid and 31,250/24=1,302Ah.

If your battery capacity is 200Ah each, you would need 10-11 lead acid batteries or 6-7 lithium-ion batteries. 

Lead-acid batteries are the most common type used in off-grid solar systems. They are comparatively inexpensive and have a long lifespan, but they require regular maintenance and are not as efficient as lithium-ion batteries.

Lithium-ion batteries are a newer technology and have become increasingly popular for off-grid solar systems. They are more efficient than lead-acid batteries, have a longer lifespan, and require less maintenance. However, they are more expensive than lead-acid batteries. Read more on what is the difference between lead-acid and lithium-ion batteries.

Done with all these steps? Congratulations! Your system is now ready to install. You can either install it yourself or consult a professional installer. Since solar is a high-ticket product, and its installation is complex, we recommend doing the process through a certified contractor. Speaking of which, Next Earth Solar is not only professional, but we love everything solar. We not only promise affordable rates but also strive to make the whole solar journey a smooth process!

Off-grid solar maintenance tips

Off-grid solar systems are meant to last long. But that can happen only with proper maintenance. With regular maintenance, you can extend the lifespan of your system and ensure that it continues to provide reliable power.

Here are some maintenance tips for an off-grid solar system:

  • Clean solar panels: Regular cleaning of solar panels is important to ensure that they are able to absorb as much sunlight as possible. Dust, debris, and bird droppings can accumulate on the panels, reducing their efficiency.
  • Check battery levels: Check the battery levels regularly to ensure that they are properly charged and not over-discharged. Over-discharging can reduce the lifespan of the batteries and cause damage.
  • Check wiring: Regularly check the wiring of your off-grid solar system to ensure that all connections are secure and not corroded. Loose or corroded connections can cause problems with the performance of your system.
  • Monitor system performance: Monitor the performance of your off-grid solar system by keeping track of energy production and consumption. This will help you identify any issues that may arise and address them in a timely manner.
  • Replace faulty components, if any, at the earliest: Replace any worn or damaged components of your off-grid solar system as soon as possible. This will ensure that the system continues to give optimal performance.
  • Schedule professional maintenance: Schedule regular professional maintenance for your off-grid solar system. This will ensure that all components are functioning properly and that any issues are addressed on time.

By following these maintenance tips, you can ensure that your off-grid solar system continues to provide reliable power and has a longer lifespan. It’s also important to check the manufacturer’s instructions for any specific maintenance and safety guidelines.

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