How to Properly Size an Off-Grid Home Solar System

With the increased need for sustainable and eco-friendly practices, it is no surprise that one of the biggest proponents of solar energy and other renewable forms of energy is the government. The government may offer benefits and incentives to encourage and motivate people to use off-grid home solar system power.

Homeowners and other property owners have two options when it comes to solar power; off-grid systems and on-grid solar power systems.

The off-grid home solar systems, also known as stand-alone solar power systems are not connected in any way to the grid. The solar system is responsible for catering to the electricity needs of the home/building. To buy one of these systems you need to make sure you get the proper size system and according to, you should calculate carefully.

On-grid solar power systems are connected to the grid and are simply used to complement and reduce dependence on the grid. Some people prefer on-grid solar systems because they are less costly and lead to savings in energy costs.

Benefits of Off-Grid Solar Systems

Although off-grid solar systems are typically more expensive than on-grid solar systems, some advantages justify this high initial investment;

1. It Is Eco-Friendly

Perhaps one of the biggest benefits of off-grid solar systems is the fact that they are eco-friendly as they rely on renewable energy. It is a way of giving back to Mother Earth and ultimately securing future generations.

The high initial investment is, therefore, a little cost to pay considering for individuals who are focused on going green. Many experts also agree that using off-grid solar power can significantly help to reduce a home’s carbon footprint.

2. Independence From Power Utility Companies

One of the defining features of off-grid solar systems is the fact that they do not use any electricity from the grid. This gives the homeowner total autonomy from power utility companies. Subsequently, this means that homeowners should not expect to receive any electricity.

Independence from power utility companies culminates in an even more important benefit for homeowners that are of energy security and reliability. In fact, it is fair to say that off-grid solar powered homes have relatively more energy security than traditional grid-powered homes particularly in areas that are prone to blackouts and brown-outs.

3. Suitability In Remote Areas

In cases where the home or property in question is not connected to the local grid or now power is available in an area, installing off-grid solar systems is one of the best and one of the most affordable options.  There are several initiatives going on in Africa to help these energy poverty areas.

Further, the installation of the off-grid system is significantly simpler than that of the traditional electrical grid. This is especially so in remote areas where homes are often located far away from electricity poles

(Source: Commsprepper)

How Off-Grid Solar System Works?

Stand-alone solar power systems are comprised of several critical components. Firstly, the system includes an array of solar panels on the roof or mounted next to the house.

When the sun shines on these panels, they generate electricity which is then used to cater to the power needs of the home and also to charge the batteries. The second major component of the solar power system is the inverter which is used in converting DC electricity into AC electricity which is then used around the home.

The third major component is the battery which is used to store DC power for use at night or during times when the sun is not shining e.g. on cloudy days.

According to experts, for a solar power system to work effectively, it is important to ensure that it is properly sized for the home’s daily energy needs in mind. Further, one must make sure that the system is adequately sized to replace any energy used from the battery bank effectively. Sizing, therefore, emerges as a critical process in the installing off-grid systems.

What to Consider Before Sizing an Off Grid Solar System?

There are two main things to keep in mind when sizing off-grid systems. While most of them may seem straightforward, many people often ignore them, resulting in a small and ineffective off-grid solar system.

1. Consumption Habits

Opting for an off-grid solar power system means that all the electrical needs of the home are powered using solar energy. For homeowners who are currently on the grid but what to change to solar power completely, this means tracking daily electricity usage.

In some cases, homeowners may even have to reduce their electricity consumption to manageable and feasible levels before installing off-grid systems. An appropriate size is one that is capable of powering the home and sufficiently charging the battery.

2. Budget

Although off-grid solar systems are expensive, there are those that are more affordable. In most cases, smaller sized solar power systems are less costly than larger systems. For example, a solar panel with a 140-watt capacity may cost anywhere from $1000 up while one top-end system of between 3,000-watts and 6,000 watts may cost between $30,000 and $70,000.

How to Size Off-Grid Solar Systems?

Although sizing an off-grid solar power system is not a complicated process, many people often get it wrong. It is for this reason that many experts suggest that individuals must first get acquainted with solar power systems. A good way to do this is through seeking the advice and insights of experts and professionals within the field.

 The following steps are important in sizing a system.

1. Determine Required Electricity Need

This is by far one of the most important steps in designing and sizing an independent solar power system. Surprisingly, it is also one of the steps many homeowners skip. This explains why there is so much emphasis on determining one’s daily consumption needs.

For those who are using the national grid but want to switch to solar power altogether, one can determine use by just analyzing the electricity bill.

When designing off-grid solar systems, it is always important to consider oversizing the system to a certain degree. This is necessary for purposes of charging the batteries which are then used for emergencies and also for night use.

Load analysis includes calculating what the loads are, how many they are, how many watts each load is and how long each load is used every single day. Luckily, for those not accustomed to electrical math, online load calculators are there to assist in calculating these loads in a faster manner.

2. Determine The Number Of Solar Panels Needed

Solar Panels

The second significant step in sizing an off-grid solar system starts with a determination of the amperage of the solar panel one intends to use. The amperage of a single solar panel then gets multiplied by the number of sun hours available in the home owner’s locality.

Experts also recommend factoring in factoring in the cold months and other “bad” months regarding sun hours. However, the inclusion of these “bad sun days” largely depends on factors such as location and time of year. Therefore, if it is a holiday home only used in summer, then one may not have to factor in the months of winter.

The result of this calculation will then help in determining how many solar panels are required to run the off-grid solar system reliably and efficiently. For example, consider a situation where the daily consumption of an individual is 11 Amps at 12 V DC. An individual opts on using a 75 Watt solar panel producing 4 Amps every hour in an area that receives 3.5 hours of sunlight.

A single solar panel, therefore, produces (4 Amps x 3.5 hours) 14 amps per day. Since the required is 11 amps a day, a single solar panel is enough. However, if the daily usage was higher, say 15 amps a day, then the individual would require two solar panels to cater to the everyday usage. According to experts, it is critical for homeowners to factor in at least a 20% average in the calculation to factor in the “not so sunny” days.

3. Determine The Amount Of Battery Storage Required

Battery back-up is a vital component of any off-grid solar system. Homeowners must ensure that they have enough back-up to help them through the nights and those cloudy days/stormy/snowy days.

Some homeowners may want to make sure they have enough storage to last them at least a day or two while others may feel a need to have enough storage to last them at least 3 or 4 days of power.

Further, it is important to consider factors such as the presence of other energy sources such as a generator or turbine. In such a case, battery storage would only serve a supporting role.

Another factor to consider is the temperature under which the batteries are stored. The rule is that batteries stored in cold rooms consume more energy and homeowners would, therefore, require larger battery storage.

Consider a situation where the individual using 11 Amps on a daily basis decides to have adequate storage to last five days. To obtain the required storage, one would like multiple the daily load by the desired number of storage days (11 x 5). This would mean that the individual needs a battery capable of producing at least 55 Amps to have enough back-up for five days. The more power one requires in the home, the larger the battery back-up required.

4. Select An Appropriate Solar Charge Controller

Solar charge controllers have two primary functions within an off-grid solar power system. Firstly, it facilitates and optimizes the process of charging the deep cycle batteries using the energy generated by the solar panel. Secondly, they prevent electricity from flowing out the batteries and through the solar panels when there is no sun.

Solar charge controllers, therefore, protect the battered bank by ensuring it does not get overcharged. Subsequently, this enhances the longevity of the batteries.

One must ensure that the solar charge controller chosen can take in the voltage of the solar panel array and output this power to the battery bank’s energy reserves. It is also equally as important to ensure that the solar charge controller has the required capacity to handle the current from the solar panel array.

To get an estimate of how many amps a controller needs, one needs only to divide the wattage of the solar panel arrays by the voltage of the battery bank. Alternatively, one may opt to many the short-circuit current of the solar panel array by 1.56.

There are two general types of solar charge controllers; Maximum Power Point Tracking (MPPT) controllers and PWM (Pulse Width Modulated) controllers. MPPT solar charge controllers are preferred in systems where the voltage produced in the solar panel array is different from the voltage of the battery bank. This is because these controllers can detect the required optimum operating voltage and amperage of the solar panels and ultimately match them with those of the battery bank.

MPPT Controllers

According to experts, using MPPT controllers can result in between 15 and 30% more power produced from the solar array as compared to a PWM solar controller. However, MPPT controllers are also more expensive than PWM solar charge controllers.

5. Select An Appropriate Inverter For The Off-Grid Solar System

It is not enough to have a well sized solar panel array and efficiently charged batteries. It is necessary to convert it into AC to make the power produced in the solar power system usable if the energy generated is used to power electrical in the home. Inverters are responsible for converting DC into AC which is utilized by the electric grid.

Experts note that an efficient inverter will ultimately lead to a more efficient off-grid solar power system. To calculate the most appropriate inverter size for one’s needs, one must calculate the maximum wattage of the home. This is very much like the first step of sizing the off-grid solar power system in which the wattage of the different appliances and devices in the house are added up.

The total wattage of the home is equal to the required inverter size. This means that for a home with a total wattage of 300 Watts would require an inverter that is also 300 Watts.

In some situations, the total capacity of solar panels is often greater than that of the inverter. In such a case, the panels are described as “oversized.”

Many solar power enthusiasts posit that over-sizing the panels, also known as overclocking the inverters can have a positive effect on the average efficiency of the inverter. On the other hand, homeowners who plan on expanding their off-grid solar systems may opt to oversize the inverter such that it has a capacity higher than the ability of the solar panel array.

However, unlike under-sized inverters (over-sized panels), over-sizing the inverters is not recommended as it often results in less efficient energy production from the off-grid solar system.

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2 thoughts on “How to Properly Size an Off-Grid Home Solar System

  • July 4, 2018 at 7:12 pm

    Way too complicated for me as an end user. Give me a formula to plug into rather than an in-depth analysis of components.

    • July 5, 2018 at 7:19 pm

      Dear Les,
      If you read the beginning of the article you will see a link that points to a solar guide, inside that article you will find helpful charts and an excel spreadsheet you can download to help you calculate what you need. Every household is different, due size, number of appliances, usage requirements. There is a learning curve when introducing solar energy into your household, whether it is for camping with an RV or boating or for your house. Good luck!


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