Which solar solution is right for you?

Solar power has now been widely exploited and used, making a great contribution to saving costs and protecting the environment. To install and use an effective solar power system, it is necessary to calculate and select a solar power system and installed capacity suitable for each family.

Solar power system is a system that absorbs light and generates electricity for equipment to use. The system consists of solar panels that absorb light to generate DC electricity and then pass it to the Inverter to convert the DC current into AC for devices to use. There are  3 popular solar power systems for households:

1. On-grid solar power system

2. Off-grid solar power system

3. Grid interactive solar power system (Hybrid)

Currently, thanks to the mechanism to encourage the development of solar power and the most economic efficiency, the On-grid solar power system  is popular and prioritized to be installed for households.

Working principle of grid-tied solar power system:

Calculate suitable power system:

Calculating Watt-hour of solar panels

Due to the losses in the system, as well as considering the safety when the sunny days are not good, the Watt-hour of the solar panel must be higher than the total Watt-hour of the full load, according to the following formula:

Watt-hour of solar panels = (1.2-1.3) x Total Watt-hour of full load used (where 1.2 to 1.3 is a factor of safety)

Calculating the required solar panel capacity

To calculate the size of  solar panels to use, we must calculate the required Watt-peak (Wp) of  solar panels. The amount of Wp that a solar cell produces depends on the climate of each region of the world. The same solar panel but placed in this place, the level of energy absorption will be different from when placing it elsewhere. To design correctly, one must survey each area and come up with a factor called “panel generation factor”, roughly translated as the power generation factor of a solar cell. This “panel generation factor” is the product of the collection efficiency and solar radiation in the region’s less sunny months, its unit is   (kWh/m2)/day).

The absorption of solar energy in Vietnam is about 4.58 kWh/m2/day, so taking the total Watt-hour of solar panels to provide divided by 4.58 we will have the total Wp of solar panels. There are larger regions of solar absorption and there are also smaller regions. In the calculation, an average of 4 kWh/m2/day can be calculated.

Each PV that we use has its Wp parameter, taking the total required Wp of the solar panel divided by its Wp parameter we will get the number of solar panels needed.

The above results only tell us the minimum number of solar panels needed. The more solar cells there are, the better the system will work, the longer the battery life will be. If there are few solar cells, the system will run out of power on shady days, draining the battery more and thus reducing the battery life. If designing many solar cells, the cost of the system will be high, exceeding the allowable budget, sometimes unnecessary. How many solar cells are designed depends on the redundancy of the system. For example, a solar system with a 4-day backup, (called autonomy day, is the days when there is no sunshine for solar cells to generate electricity), it is imperative that the battery capacity increases and the number of solar cells must increase God. In addition, SolarV has a smart grid compensation or smart grid switching system that will solve the problem of power outages or lack of electricity for shady days for areas where solar power system is connected to the grid.

Calculating the inverter

Currently, there are two types of standard sine inverters that we can use to calculate: high frequency standard sine inverters and low frequency standard sine inverters (also known as inverters using high frequency).

If the design chooses a high frequency standard sine inverter, the inverter must be large enough to be able to respond when all loads are turned on, so it must have a capacity of at least 150% of the load capacity, it is best to choose 200% of the load capacity because there are times when it is necessary to start the devices. If the load is a motor (or a refrigerator, an air conditioner, etc.), it is necessary to calculate more capacity to meet the starting time of the motor. Usually, the starting current of equipment with a large motor is about 5-6 times higher than the current when running stably, but it is possible to use a soft start method to avoid choosing too large a capacity inverter.

If you choose a standard sine inverter to use a boost, you can choose a capacity from 125 to 150% to be able to use it, but the disadvantage of this type of inverter is the large consumption.

Choose an inverter whose rated input voltage matches the rated voltage of the battery. For the solar system connected to the grid, we do not need a battery, the nominal input voltage of the inverter must match the nominal voltage of the solar cell system.

Calculation of battery

Battery used for solar system is deep-cycle type. This type allows for very low discharge to the tank and allows for quick refilling. This type has the ability to charge and discharge many times (with many cycles) without internal damage, so it is quite durable and has a long life. There are 2 methods of calculating the battery:

Method 1: Based on the amount of electricity produced.

Battery capacity must be able to hold =  1.5 to 2 times the amount of electricity produced per day. The discharge efficiency of the battery is only about 70 – 80%, so divide the Wh produced by the solar cell by 0.7 – 0.8 and then multiply it by 1.5 to 2 times, we have the Wh of the battery. In case the demand for use is mainly during the day, it is only necessary to design the amount of battery stored equal to the amount of electricity produced from the solar cell.

In an independent solar system for daily use, in order to increase the battery life (2, 3 times normal), the battery should not be discharged deeply, should protect the battery at a voltage above 11V (for batteries). 12V) and switch to mains or grid offset.

Method 2: Based on the usage load, specifically as follows:

The number of batteries needed for the solar system is the number of batteries that are enough to provide electricity for the autonomy days when the  solar panels  cannot produce electricity. We calculate the battery capacity as follows:
– The discharge efficiency of the battery is only about 80%, so dividing the Wh of the load consumed by 0.8 we have the Wh of the battery

– With a deep of discharge DOD (deep discharge) of 0.6 (or less than 0.8), we divide the Wh of the battery by 0.6 to get the battery capacity

The above results tell us the minimum battery capacity for a solar system without backup. When the solar system has the number of backup days (autonomy days), we must multiply the battery capacity by the autonomy-day number to get the number of batteries needed for the system.

Solar charge controller design

The solar charge controller has an input voltage that matches the voltage of the solar cell and the output voltage corresponds to the voltage of the battery. Because there are many types of solar charge controllers, you need to choose which type of solar charge controller is suitable for your solar system. For large solar cell systems, it is designed in many parallel arrays and each array will be in charge of a solar charge controller. The capacity of the solar charge controller  must be large enough to receive electricity from the PV and enough capacity to charge the battery.

Normally, we choose Solar charge controller with current Imax = 1.3 x short circuit current of PV

SolarV has designed Solar charge controllers that use MPP pulse and peak voltage charging technology, so the charging efficiency is higher and the battery is more durable, the charging performance is equivalent to MPPT chargers but the price is cheaper. Pulse charging technology makes the battery more durable, including MPPT charging. You can see more here:

Specific examples:

Calculating the solar system for a household in a remote area has the following usage requirements:
– 1 18 Watt light bulb used from 6-10 pm.
– 1 60 Watt  fan per day uses about 2 hours.
– 1 75 Watt refrigerator running continuously

Determine the total amount of electricity consumed

Determine total power consumption per day = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours) = 1,092 Wh/day
(Refrigerator automatically shuts off when cold enough, so it’s considered running for 12 hours, rest for 12 hours)

Calculating solar cells (PV panel)

✅PV panel =1,092 x 1.3 = 1,419.6 Wh/day.
✅Total Wp of PV panel =1,419.6 / 4.58 = 310Wp
✅Choose the type of PV with 110Wp, the number of PV needed is 310 / 110 ~ 3 panels

Calculate inverter

✅Maximum total capacity used at a time =18 + 60 + 75 = 153 W
✅Inverter power =153 x 125% # 190W
✅However, there is a refrigerator in the system with a starting current of about 5-6 times (6 x 75 = 450w)
✅So choose inverter power must be greater than 450W.
✅We can choose the type of inverter 500W or more. Note that you must choose a standard sine inverter to be safe for the refrigerator.

Calculate Battery

With 2 days of backup, tank capacity = 178 x 2 = 356 Ah
So choose 12V/400Ah deep-cycle battery for 2 days backup.

If only used during the day, there is no need to calculate backup, choosing a 12V-200Ah battery is enough.

Calculate solar charge controller charge

Parameters of each PV module: Pm = 110 Wp, Vm = 16.7 Vdc, Im = 6.6 A, Voc = 20.7 A, Isc = 7.5A
So solar charge controller = (3 PV panels x 7.5 A) x 1.3 = 29.25 A
Choose a solar charge controller with a current of 30A/12 V or greater.


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