What is an off-grid solar energy system
An off-grid solar energy system is an power system that is off the grid of an energy company or utility.
An off-grid energy system uses wind or solar energy to generate the energy needed to supply a home.
Benefits of off-grid systems
- Use green and clean renewable energy to reduce carbon emissions.
- Having a completely off-grid solar energy system allows you to live and work off-grid (off-grid life).
- Save money in the long run.
Disadvantages of off-grid solar energy systems
- Large initial investment.
- The efficiency and stability of power generation is greatly affected by weather conditions.
Off-grid solar energy system working principle
During the daytime, the solar panel receives sunlight and converts them into DC output. Part of the DC power is converted into AC power by the inverter to supply household appliances, and the other part charges the battery pack.
Things to consider at the beginning of an off-grid system
As off-grid solar energy system is a huge investment and the payback time is long. Therefore, a combination of cost and quality as well as efficiency should be implemented in the design.
Also as a project that requires long-term maintenance, its scalability should be fully considered.
1, Design your system voltage
This is the most likely to lead to the whole system in the future to be pushed to start over a step.
For example, you may initially think that a 12V system is sufficient, but end up upgrading to a 24V system. The cost of this upgrade will result in a huge waste of cost. This is because you will have to replace the charger/inverter or even reassemble the solar panel array and battery array.
2. Plan with redundancy
We recommend to start with the leanest system and then gradually expand the system according to future life needs. This saves a lot of money in overhead avoiding unnecessary waste.
However, even with the leanest system, you need to consider designing for additional redundancy.
Because there is a loss of conversion efficiency at every step in between, from the solar panels to the inverter.
In addition the average daily energy production capacity of solar panels is greatly affected by the seasons, with very large differences between summer and winter, and between sunny and cloudy days.
All things considered, the daily power demand should be ideally at 80% of the daily power production capacity of the off-grid solar energy system.
Key components of off-grid solar power system
1. Solar panels
It is best to choose monocrystalline panels. Although you will see little difference in price between polycrystalline and monocrystalline panels in the market. However, monocrystalline silicon is better than polycrystalline silicon in the conversion efficiency of sunlight, and this advantage will be gradually presented in the long-term use process.
Please try to choose a high power panel. Larger panels not only have a better price per kWh, but are also more space efficient. It is also possible to design a more streamlined panel array, helping you to reduce much of the wiring work as well as save significantly on future maintenance costs.
Also, it is easy to expand in the future.
Reference price: $0.50/W~0.60/W
2, Solar charger Controller
There are two types of solar chargers: PWM and MPPT. if your budget allows, please use MPPT solar charge controller. Because MPPT charger has higher charging efficiency and more scalability than PWM charger. The value of the extra energy gained by MPPT over PWM in long-term use is much higher than the price difference between them.
Main roles of solar chargers:
- Block reverse current
- Low voltage protection
- Prevent Battery Overcharge
- Configure Control Set Points
- Displays and Metering
- Troubleshooting and Events History
3, Battery Pack
For safety/cycle life/price consideration, please choose lithium iron phosphate(LiFePO4) battery pack.
This is the most expensive part of the system. But you can save a lot of cost by DIY LiFePO4 battery pack.
And by DIY LiFePO4 battery pack, you can build almost any capacity energy storage system.
The advantages of DIY LiFePO4 battery pack.
- Control the quality of the battery, you can use A grade LiFePO4 battery cells to ensure the service life of the battery pack.
- The capacity of the battery pack can be freely customized.
- The capacity of the battery pack can be expanded at any time in the future.
- Easy to make.
- Cost saving.
DIY LiFePO4 battery pack: $0.17/W~$0.24/Wh
4, DC disconnect switch
It is necessary to install DC disconnect switch between solar panel and charger, between charger and battery, between battery and inverter.
It will break the circuit when you need it and make it easy for you to check/repair/upgrade the system.
An inverter converts DC power to AC power.
Future needs should be considered when choosing an inverter, as it has a power limit. A 2kW inverter may be sufficient for the present, but when more power consuming devices are added in the future, a new inverter will have to be purchased.For home applications, it is best to choose an inverter of 3kW or more initially.
In addition to these core components, you will need to prepare DC wires, as well as fuses, connectors, and other accessories.
Steps to designing your solar power system
Building an off-grid solar energy system can be a very expensive project in terms of personal effort, money and long hours of maintenance.
Don’t rush at first, optimize the system design step by step for your specific needs and then complete it in stages.
1. Calculate/track daily power consumption
First find out the electricity bill for the past few months, preferably for the past year, as it covers all seasonal electricity cycles.Calculate the average daily electricity consumption for different seasons/months.
By this point you have a basic understanding of the average electricity consumption of the whole household.
Next make observations about the power consumption and usage time of individual appliances.
Here’s a great power consumption monitor – Kill-A-Watt. it’s easy to use. Have an appliance connected to an outlet through it and it will record all the current passing through and even the total energy consumption over a period of time.
This $20 or so gadget is very helpful for us to grasp the daily power consumption.
When the consumption of all electrical appliances and circuits are mapped out, you should have a very good understanding of the daily habits of power consumption.
Then you can try to calculate the maximum daily power consumption – the kind of situation that uses the most appliances and lighting.
The Calculate Formula of Energy Consumption
Energy consumption = power x usage time
Don’t forget to compare this maximum daily energy consumption with the average daily electricity consumption calculated through your bill to check if there is anything missing.
(Usually we use kWh for solar system energy calculations, while most household appliances are rated in W. The relationship between them is 1kW = 1000W, if you are using Wh to calculate the results, then just divide by 1000 to get the kWh result)
This is a very time consuming work and the most important one, from which all subsequent parameters of the whole system will be designed. So please check and calculate carefully. It is better to take into account the seasonal differences as well.
But don’t stress too much, except for the inverter, solar panels and batteries are scalable, if you leave enough space for them in advance.
Well, you should now come up with a maximum daily consumption figure of, for example, 10kWh/day.
2, Designing the battery bank
2.1 Calculate battery pack capacity
When designing the capacity of the battery bank, in addition to the daily power consumption, you also need to consider the number of days that the home can be kept running in case of low sunlight, such as cloudy or rainy days.
Usually the design should be based on at least two days.
So, if the average daily energy consumption is 10kWh and can support 2 days in the absence of sunlight, so on the AC sideTotal energy required = 10kWh/day x 2days = 20kWh.
Note that it will loose some energy in the process of converting DC power to AC power through the inverter. So to calculate the battery’s total energy should include this lost portion.
The conversion efficiency of different inverter brands ranges from 85% to 95%, with an average value of 90%. Here we use 90% for the calculation.
So, battery pack energy storage = 20kWh/90% = 22.22kWh
2.2 Voltage of the battery pack
The voltage of the battery pack determines the DC-terminal voltage of the entire solar system. Available voltages are 12V, 24V, 48V or even higher. Considering that voltages above 60V are dangerous during operation, it is recommended that voltages below 60V are preferred.
Please note that according to the power formula W=V x I, the lower the voltage, the higher the current for a constant output power.
When selecting a 12V single series battery, attention needs to be paid to whether the current exceeds the output capabilities of the battery pack.
For example, your battery pack is 12V 200Ah with BMS’s max current 100A(you will see many battery packs in the market with a BMS max voltage of 100A).
Then when you run a power of 1200W microwave oven, it is very likely to work improperly.
Because a 1200W microwave oven requires 1333W output from the battery pack at the DC end, which means battery will output 1333W / 12V = 111A, and the BMS’s 100A max current limits the current output.
But if you have 2 12V 100Ah with 100A BMS, then it supports up to 200A current output and you won’t have this problem.
12V is obviously not a good fit in this current 22.22kWh energy storage requirement, because the capacity of battery pack will very large – 22.22kWh / 12V = 1852Ah.
Even charging it with 100A current would take at least 1852Ah / 100A = 18.52 hours. And you’ll have a hard time finding a charger with a higher current.
Let’s look at 24V, 22.22kWh / 24V = 926Ah, it will take more than 9 hours to fully charge it with a 24V 100A charger.
Let’s look at 48V, 22.22kWh/24V = 463Ah, which takes about 5 hours to fully charge with a 100A charger.
Since the conversion of solar panels is at its maximum at noon, we try as much as possible not to let the charger’s current limit the collection of energy. So it is possible that two chargers will be needed here to ensure that all of the energy generated by the solar panels goes through the charger and into the system.
Now that we have determined the system voltage to be 48V, let’s look at how to design the battery pack.
2.3 Battery pack structure
2.3.1 Purchase of finished LiFePO4 battery pack
Total battery capacity = 22.22kWh/48V= 0.463kAh= 463Ah.
Since it is difficult to buy 48V battery pack, however, you can use 2 24V battery packs connected in series to get 48V battery pack.
The most common 24V LiFePO4 battery packs in the market are 24V 100Ah and 24V 200Ah. Can use 10 units 24V 100Ah battery packs to get a 48V 500Ah battery system by 2 series and 5 parallel(2S5P). The total stored energy is 24kWh.
Reference price: $8200
Or 6 24V 200Ah battery packs in 2 strings of 3 to form a 48V 600Ah system. Total energy is 28.8kWh.
Reference price: $12,000
2.3.2 DIY assembly of LiFePO4 battery pack
- LiFePO4 battery cells
- 48V 16S 100A BMS
LiFePO4 battery cell capacity selection
Large capacity LiFePO4 battery cells are 3.2V 230Ah, 3.2V 280Ah, 3.2V 304Ah. 16 cells in series can get 48V battery voltage.
Using 230Ah battery cells, we can get 230Ah x 2 = 460Ah by 16S2P, which is 22.08kWh.
Very close to the daily consumption.
Reference price: $3800
With a 280Ah battery cell, you can get 280Ah x 2 = 560Ah with 16S2P. That is 26.88kWh.
Reference price: $5200
DIY LiFePO4 battery pack main steps
- Connect 16 LiFePO4 cells in series with bus and fix them with tape.
- Unplug the equalization bus from the BMS.
- Connect the BMS in series to the negative terminal of the last cell.
- Connect the 16 balace wires of the BMS to the positive terminal of each battery cell in sequence.
- Plug the balance bus to the BMS.
- Fixing the battery pack with BMS by tape.
According to the characteristics of LiFePO4, it is also necessary to consider making a simple compression device. This will use a wooden board and screw. You can purchase the relevant materials at local stores.
3, Planning of the solar panel
3.1 Total power of the solar panels
One factor that must be mentioned here is the Peak Sun Hour, which is the number of hours of maximum light received throughout the day. This is because the power rating of solar panels is calibrated according to the power generated at midday when the sun is at its strongest.
For example, a solar panel rated at 200W with a local Peak Sun Hour of 3 would produce 200W x 3hour = 600Wh of energy in a day.
Continuing with the above example, assume that the designed battery storage system has a capacity of 560Ah, or 26.88kWh. Since most MPPT solar chargers have a conversion efficiency of 92%-95%. Here we use 92% for the calculation.
So the total energy generated by the solar panel should be 26.88kWh/0.92 = 29.22kWh
3.2 Selection and number of solar panels
For higher conversion rates, choose monocrystalline solar panels.
Choose high power solar panels whenever possible, so that the number of solar panels that you manage will be much less.
Here we take a 320W solar panel as an example.
If the local Peak Sun Hour is 4.
Then the total power of solar panels is 29.22kWh / 4 = 7.3kW
The number of 320W solar panels is 7.3kW / 0.32 = 22.81
To form the solar panel array, we need to use 24 panels.
The total power of 24 320W solar panels is 320W x 24 = 7.68kW
The total energy produced during the day is 7.68kW x 4h = 30.72kWh
Current solar panel prices range from $0.5/W to $0.6/W
Reference price for 7.68kW: $4600
3.3 Installation of Solar Panels
Although most people choose to install solar panels on their roofs, this is not the best option in practice.
First of all, not all roof angles are the best angle to face the sun, and secondly, the angle of sunlight exposure varies from season to season. It also makes maintenance more difficult, and as a long-term project you have to always climb on the roof.
If the site allows, install them on the ground as much as possible. Make an adjustable angle bracket out of wood or metal so you can always adjust the angle to get more sunlight energy.
Once you have determined the type and number of panels, you can figure out the area needed to install the panels.
The 320W solar panel’s size is 39.5 x 4.2 x 65.6 inches, so the area is 2591.2 in² = 18 ft² (1.67 m²). 24 panels will cover 432 ft² (40.2m²).
Once you have the area and number of solar panels, you can plan the location of the installation.
3.4 How to connect the solar panels
Since the input voltage of solar chargers is usually below 150V and the input power is below 5kW, we need to separate the 7.68kW solar panels into 2 groups. 12 pieces in each group.
Each group of solar panels we connect with 4S3P, so the output voltage isSolar Panel Array Output Voltage = 24V x 4 = 96V
4, Select Solar Charger
Since the power generated by the solar panel array at noon is 7.465kW, the charging current after converting this power to 48V is
Charge Current = 7.465kW / 48V = 155A
There are very few solar chargers that can provide 155A, so can use two 100A chargers. They each share the 80A charging current.
Reference price: $700 for 2 units
5. Choosing an inverter
The types of inverters are pure sine wave inverter and modified sine wave inverter. Please choose pure sine wave inverter.
Because modified sine wave inverter is usually used only for resistive loads, such as LCD TVs, cell phone, computers, etc.; while there are many capacitive loads and inductive loads in our life, such as refrigerators, air conditioners, drills, LED lights, and vacuum cleaners, etc.
Choose as much power as possible, at least 3kW or more for home use. For an average daily consumption of 10kWh, a 5kW inverter satisfies the current needs while taking into account that more electrical applications will be added in the future.
Reference price for 5kW pure sine wave: $700
Here, we designed a LiFePO4 energy storage system with a 7.68kW solar panel array and 26.88kWh. Let’s take a look at the main parameters again.
Solar panels: 24 pieces 350W/pcs Total power 7.68kW
Solar charger: 2 units output DC 48V 80A
LiFePO4 battery pack: 48V 560Ah (26.88kWh) – 32 units 3.2V 280Ah LiFePO4 battery cells and 2 16S 100A BMS
Inverter DC 48V input, AC 110V output, 5kW
Let’s look at the total cost of these core components.
- Solar panel $4600
- Solar charger $700
- LiFePO4 battery pack $5200
- Inverter $700
Total price: $11,200
This is not the whole system, you will also need power cables to connect the positive and negative terminals of the battery pack, DC disconnects for maintenance/upgrades, fuses, and other accessories such as compression devices for making the batteries and mounts for the solar energy.
When the whole system design is complete, you can start collecting materials and tools. Of course, this system has a lot of freedom and scalability, you can divide into different stages to gradually expand the number of solar panels and the capacity of the battery pack.
Although the system already covers 2 days of household power consumption, you will still need to consider an additional generator for emergencies.
Any question and inquiries please send email to firstname.lastname@example.org, we answer quickly.
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