How to choose the right solar inverter and battery at home?

What is a Solar Inverter?

Solar Inverter

Inverter is a device used to convert direct current (DC) into alternating current (AC). The home inverter converts the DC stored in battery into AC. Solar inverter consists of solar panels, charge controller, batteries and inverters.In a solar inverter, the photovoltaic solar panel produces variable direct current (DC). Solar inverter converts the DC into alternating current. This alternating current can be fed into commercial electrical grid or off grid electrical network. The solar panel can be placed at home, office, etc. The converted AC power can be used by power appliances. Solar panel cannot produce alternating current by themselves. So an inverter is used for this purpose.  

Solar Inverter contains large capacitors which can be used to store power and also it can be used to improve the output waveform. The solar inverter consists of some small components such as wiring, hardware etc. Central inverter is used in most homes. It is simply a box placed in some location and it converts DC into AC. Another type of inverter is microinverters. It uses single photovoltaic module for the operation. Micro inverter converts DC into AC from each panel.  These inverters directly and independently work under solar panel. It needs more installation time and it has high cost. Figure below shows the connection of photovoltaic solar panel to grid.

Solar inverter versus normal inverters

The inverter of solar inverter and normal inverter are same. The difference of solar inverter from normal inverter is that the solar inverter consists of solar charge controller and some switching circuits. Solar inverter has a terminal for connecting battery and solar panels of correct rating. The battery is charged from the output of solar panels when it is under sufficient sunlight. It allows reducing the electricity bill.

The normal inverter can be converted into solar inverter by connecting charge controller along with a automatic transfer switch. If the solar panel gives sufficient supply to charge the battery, then the automatic transfer switch will be routed to the battery. At that time it does not take supply from main supply. If the solar panel does not give sufficient supply, then the battery charges from the main supply by automatically routing the automatic transfer switch.

Solar inverter can be classified into 3.They are given as follows:-

• Stand alone inverter –It is used in isolated systems. The DC energy is drawn from battery. The battery is charged from photovoltaic arrays. These are not interfaced with the utility grid.
• Grid-tie inverter – Its phase matches with utility supply. It consists of a special circuit for matching the voltage and frequency of the grid. It is connected to the utility grid and gets disconnect ed when it does not detect the presence of grid. That is  it will automatically shut down during utility outage.
• Battery backup inverters – It gets energy from battery. It uses an onboard charger to manage the battery charge. The excess energy will be given back to the utility grid. If utility outage occurs, then it will give AC supply for selective loads.

It is usually seen that due to the transients from the power grid or photovoltaic panel and aging of components; solar inverter may fail.

There are 3 major types of solar inverter based on the output it produce:-

• Sine wave – Local utility and generator produces sine wave. The rotating AC machinery produces sine wave output. Majority equipments are work on sine wave. So these devices work with full specifications. Motors, microwave ovens produced full output only when it gets sine wave power. It is more expensive than others.
• Modified sine wave – It is like a square wave but is not exactly a square wave. All equipments will work fine but will have low efficiency. So these devices will use more power due to its low efficiency.
• Square wave – It is the cheapest inverter. But they are seldom in use. It is used to operate universal motors without any problem.

Majority of the AC appliances operate in modified sine wave. Some appliances such as computer, television etc are work based on the DC current. For that purpose AC is rectified into DC for its operation.  For inductive load, the current and voltage are out of phase. It uses more power which causs the motor or transformer to run hotter. The solar inverter is very useful during utility outages occur. If the solar inverter is not maintained properly then it will drain huge amount of electricity. 

How  to Choose the Right Solar Inverter at Home?

There are different types of sizes, brands, models etc. Choosing right solar inverter among this requires some basic knowledge about the terms. Some of the basic factors are given below.

• Safety certification :– It is certified by any independent testing laboratory
• Watts: - It is the power used by a device when turn on. The Watt is defined as joules per day. If a device uses 20amps at 12 hours then it is a 120 watts device. It is the product of volts and amps.
• Watt-hours:- It is the product of how many watts and how many hours the device used. If a 100 watt bulb is used for 8 hours, then it uses 900 watt-hours.
• Amps: - It is the measure of electrical current at moment. It is defined as 1 coulomb per second.
• Amp-hours: - It is the product of amps and times. It is the main measure of battery capacity. Most inverters operate using its battery. The Amp-hour capacity determines the battery capacity and it gives an idea about how long it will work.
• For home; Total Harmonic Distortion (THD) less than 6 percent is sufficient. RMS voltage regulation must be less than or equal to plus or minus 5. Peak voltage regulation must be less than or equal to 10 percent. 
• Efficiency: - The ratio of output power to the input power is termed as efficiency. It is difficult to achieve 100 percent efficiency. Because some power is lost in the form of heat. The efficiency of solar inverter depends on load. The specifications of the inverter contain an efficiency graph. It is the graph of efficiency versus load.
• Internal protection: - The solar inverter components must be protected from overload, lightning surges, etc. It must contain sensing circuits to sense the problems and automatically shut down. If the supply voltage is less, it must shut off. Otherwise the battery will over discharge and even damaged.

Points to be remembered when choosing an inverter:-

• Sine wave inverter is better, because it reduces heating and will increase the longevity of inverter. The square wave inverter produces a humming sound. This humming sound can be reduced by using sine wave inverter. The sine wave inverter supply is better than the supply from grid.
• Choose the inverter with capacity according to the load.
• The idle power consumption of inverter must be low.
• It must consume low electricity from grid.
• The battery for inverter must be branded.

To choose the right inverter for user requirements, they must know some basic calculations. Using this simple calculation the user can easily buy inverters according to their need. Inverter must be bought according to the power requirement. For example, If the user wants 2 tube lights, 2 fans, 2 CFL and 1 television to run at the time of utility outage; then the power consumed by these appliances is supposed to be

1 tubelight – 60 watts, 1 CFL -25 watts, 1 fan- 70 watts, 1 television- 120 watts.

Then the power requirement =2* 60 +2*20+2*25+1*120

                                              = 120+40+50+120

                                               =330watts

So the total power requirement is 330 watts.

Then the power of the inverter is the ratio of power requirement in watts to the power factor (efficiency).

Power of inverter = power requirement / power factor

Most of the inverters have efficiency 60% or 70 %

Assume power factor =0.7

Then power of inverter = 330/ .7 = 471 VA, the inverter of specific rating can be choose.

How To Choose the Battery for Solar Inverter Wisely?

The life and performance of solar inverter depends upon the battery. The battery capacity means the maximum hours that the battery can run all the equipments. The battery capacity is termed as Amps-hour (Ah). The battery capacity can be defined as the product of power requirements in watts and backup hours in hours divided by battery voltage in volts.

Battery capacity= power requirement * backup hours / battery voltage.

Using this formula, the user can easily choose the battey for their requirement.

If battery backup is considered to be 3 hrs and battery voltage is 12V then the battery capacity is calculated using above expression.

Battery capacity = (330*3)/12 = 82Ah.

Battery with particular capacity can be used.