In my past 6 years' solar industry experience, we always come across this situation, when a customer has an inquiry for an off-grid solar system, he always has no idea about the solar system capacity, let alone knowing how to wire a solar system.
Based on this situation, today I write this article, helping customers to calculate the solar system capacity, also to choose different components to wire the solar system step by step.
For an off-grid solar system, you need four basic components
Besides the above components, you also need a few more things like DC cables, MC4 Connector, breaker, meter, and fuses, etc.
In the next few steps, I will explain in detail how to choose the above components according to your requirement and how to wire the off-grid solar system.
Step 1: CALCULATE YOUR LOAD
Before you choose the off-grid solar system components, you need to calculate your load (like what home appliance you want to run, basically those machines have its own specification chart to see its rating power), how many hours they need to run every day, and how many rainy days this solar system need to support.
For Example :
Computer = 30Wx 4hr =120
Fan = 60 W x 3hr = 180
TV SET = 100W x 3hr = 300
Total Watt Hour = 120+180+300 = 600
Usually Considering 30% energy lost in the system.
So total Watt Hour per day = 600 x 1.3 = 780 Wh
Now the load calculation is over. The next thing is to choose the right components to match your load requirement.
Step 2: SOLAR PANEL SELECTION
Nowadays solar panel has three common types: monocrystalline, polycrystalline and thin-film one. But the thin-film solar panel is not commonly used in the solar system because of its low efficiency, although it is light and portable, so it is more suitable to use for solar kits.
Thus, for another two types of solar panels, the mono solar panel and poly solar panel, which one is better?
While both of these types of solar panels have cells made from silicon, monocrystalline and polycrystalline panels vary in the composition of the silicon itself. Monocrystalline solar cells are cut from a single, pure crystal of silicon. Alternatively, polycrystalline solar cells are composed of fragments of silicon crystals that are melted together in a mold before being cut into wafers.
Solar panel power and efficiency ratings
Both two types of solar panels vary in the power they generate.
Monocrystalline and polycrystalline solar panels
Comparably, a monocrystalline solar panel typically has more efficiencies and power capacity than a polycrystalline solar panel.
Both two types of solar panels tend to come with 60 silicon cells each, with 72 or 96 cell variants (usually for large-scale installations). But even with the same number of cells, monocrystalline panels are capable of producing more electricity. Usually, the mono solar panels can reach efficiency higher than 20%, while poly solar panels mostly between 17-18%, so even with the same dimension size of the same quantity silicon cell, the monocrystalline solar panel will generate more wattages than polycrystalline one. for example, of the dimension 1960*992*35mm, monocrystalline one now can reach 450W, while polycrystalline one is still lower than 400W.
As a result, if you have a lot of roof space to install the solar system, you can choose the lower efficiency, lower-cost polycrystalline panels. But if you have limited space available and are looking to maximize your electric bill savings, we will suggest you choose the more efficient one- monocrystalline solar panel, and its unit price per watt just a little higher than polycrystalline one.
RATING OF SOLAR PANEL :
The solar panel size should be selected in such a way that it will charge the battery fully during the one day time.
During the 12 hours day time, the sunlight is not uniform, it will differ according to your location around the globe. Mostly we can assume 4-6 hours of effective sunlight which will generate the rated power. And China we most adopt 4 hours as an effective time.
Total Wp of PV panel capacity needed = 780Wh /4 = 195W
By taking some margin you can choose a 200Watt, 18v solar panel.
Some people might ask: the battery is 12V, why you choose an 18V solar panel? It is like the water flow, usually from the higher go into the lower, thus, only when the solar panel voltage is higher than the battery, the power can go into.
Step 3: BATTERY SELECTION
Considerations for choosing a battery include cost, cycle life, installation and maintenance. The most two common types of solar batteries are deep-cycle, lead-acid battery, and lithium-ion battery.
RATING OF BATTERY:
Battery capacity= Load /Voltage = 780Wh/12 = 65Ah, ( as the example system is 12V, but also suitable to calculate the 24V or 48V system)
Practically battery are not ideal, so we have to consider the loss. Let the battery loss is 15%.
So battery capacity required is 65 / 0.85 =76.5Ah
For better battery life, they are not allowed to discharge fully (100% ). For flooded lead-acid battery 60% depth of discharge (DOD) is considered as good practice. So Capacity Required =76.5/0.6 = 127.5Ah
You can select a deep cycle lead-acid battery with a capacity of more than 127.5 Ah.
You can round off to 130 Ah or 150Ah.
Step 4: SOLAR INVERTER AND SOLAR CONTROLLER SELECTION
As we know, the solar panel receives sunshine and convert it into electricity, but that electricity which we called DC can not use to run the home appliances. In that way, we need a solar inverter that can convert DC to AC power. (For some small solar system, which also can just include solar panel solar controller and solar battery, in that condition, then the solar system only can drive DC power machines, like LED light, DC fan, or cellphone charging. )
Square wave inverter is the cheapest one among these three types of solar inverters, but not suitable for all appliances.
Modified Sine Wave output is also not suitable for certain appliances, particularly those with capacitive and electromagnetic devices such as a fridge, microwave oven, and most kinds of motors. which will work at lower efficiency and damage the home appliances.
Thus, for an off-grid solar system, pure sine wave solar inverter is the best option.
RATING OF INVERTER :
The power rating should be equal or more than the total load
For example, computer 30W, fan 60W, TV set 100W, the total is 190W.
By taking some margin we can choose a 200W inverter.
As our system is 12V, so we have to choose a 12V DC to 220V/50Hz or 110V/60Hz AC pure sine wave inverter.
Appliances like Air Conditioner, its starting power consumption is several times greater than their normal working power, usually is 3 times. This should be taken into consideration when we choose a solar inverter capacity.
For example, one air conditioner capacity is 1500W, if you have three sets of this kind air conditioners at home, if you want to turn them on at the same time, then the solar inverter power should be 1500Wx 3x 3=13500W, but if you don't need to turn them on at the same moment, then no need to choose the so big capacity one.
RATING OF CONTROLLER :
For the solar controller selection, we already introduced in the previous R&Q section: https://www.solarhm.net/q3
And nowadays many suppliers produce the hybrid inverter, which includes a built-in controller ( can choose PWM or MPPT), in this way, the customer is easier to wire the solar system and save more space.
Pure Sine Wave Solar Inverter
PWM Solar Charge Controller
Step 5: SERIES AND PARALLEL CONNECTION
1. Series Connection :
No matter solar panel or solar battery, to wire in series means connecting the device positive terminal to the negative terminal.
For example, if you connect 4 solar panels with a rated voltage of 36 volts and a rated amperage of 8 amps in series, the voltage of the series would be 36+36+36+36=144 volts, while the current or amperage is still the same as 8amp.
2. Parallel Connection :
In parallel connection, you need to connect the positive terminal of one device to another's positive terminal, and negative terminal connects to another's negative terminal.
If you wired the same panels from before in parallel, the voltage of the system would remain at 36volts, but the amperage would increase to 32 amps.
Step 6: WIRING
After the 5 steps, we can wire the whole solar system. There are two kinds of wiring diagram, one is without solar inverter, which is mainly for DC system, another is with solar inverter, which can convert DC to AC.
SOLAR DC SYSTEM WIRING DIAGRAM
The DC solar system wiring only includes solar panel, solar inverter and solar battery. At the bottom of the Solar Controller, we can see 3 signs. From left to right, they are the signs for solar panel, solar battery, and DC load. All with both positive (+) and negative (-) .
After solar panel and battery's parallel and series connection, we use their positive side to connect solar controller's positive side, and use negative side to connect the solar controller's negative side.
SOLAR AC SYSTEM WIRING DIAGRAM
Compare to the solar DC system wiring, the AC system is adding one more component-SOLAR INVERTER.
It is same that after solar panel and solar battery's parallel and series connection, we connect solar panel and solar battery's positive (+) side to the solar controller's positive (+) side, and their negative (-) to the solar controller's negative (-). After that, we also connect the battery's positive (+) side to solar inverter's positive (+) side, and battery's negative (-) to solar inverter's negative (-). Then from the solar inverter terminal, we can connect to different AC load.
Noted: from solar panel and battery terminal to solar controller terminal, we need to add a breaker, which can control the voltage and current input.
Step 7: SELECT THE CORRECT DC CABLES
Choosing the correct DC cables is highly essential for solar system's performance and safety. As we know, DC cables are used in between solar panels, solar panel to solar inverter, solar controller to solar battery, and in between solar battery's series or parallel connection. So before running the solar system, you need to be sure the DC cable selected is proportionate to the current and the voltage of the system.
The cross-sectional area of the DC cable in each part of the photovoltaic system is determined according to the following principles:
1. The connecting solar cables in between solar modules or in between solar battery should be 1.25 times of the maximum continuous working current.
2. The solar cables in between solar array or in between solar inverter to solar battery should be 1.5 times of the maximum continuous operating current.
Step 8: MOUNTING THE SOLAR PANELS
As we know, to fix solar panels on the roof, we need the solar mounting bracket, and its material on the market now usually is divided into two types: ALUMINUM ALLOY AND STAINLESS STEEL. From different aspects, we analyze these two types of materials' advantages and disadvantages.
1. For the areas with high wind resistance requirements, stainless steel is better than aluminum alloy because of its strength. But aluminum alloy is lighter, so for the roof top which cannot bear heavy load, aluminum alloy is much better.
2. For the anti-corrosion aspect, aluminum alloy is far superior to steel brackets and easier to maintain.
3. For the price of roof mounting , stainless steel > aluminum alloy> normal steel, so if need to make solar shelter, stainless steel is your best choice, which with high wind resistance and anti corrosion.
And for personal roof top solar PV system installation, most adopts aluminum alloy, which is light and easy to maintain.
And for the solar power plant, which is maximum power generation with lowest cost, normal steel is the best choice.