DIY Solar Inverter | 800W System Installation

How to design an off-grid solar power system( DIY solar inverter ) and how to size solar panel, battery, charge controller and inverter to match the power consumption of that property where the system is going to be installed. In this post, I am going to talk to you about installing a solar Inverter by yourself.

Last year, around this time I had created a Youtube video on how to make a 250 Wh off-grid solar power system. It is still working perfectly powering my office room with clean solar energy. I have decided to upgrade my existing system with another 100 w solar panel and replace the charge controller and inverter with a better one.

When you finish this video you will learn what is an off-grid solar Power system, How does an off-grid solar power system work, What are the components of an off-grid solar power system? How to design a solar inverter, How to Connect components of a solar inverter etc.

How to Design a DIY Solar Inverter

An off-grid solar system has the following components. Solar panel, charge controller, battery, and a power inverter. Finding out the total power consumption of the property where the system is going to be installed and deciding the ratings of each component is known as designing an off-grid system.

The solar panel converts solar energy directly into electricity. This electricity is DC in nature. This electricity can be stored in a rechargeable battery. A charge controller is used to protect the battery from getting overcharged or over-discharged. A power inverter is used to convert the dc output of the battery to the required AC voltage, usually 230v.

1. Calculating Total Power Consumption of the Property

The first step in designing an off-grid solar power system is calculating the total power consumption of your property a day. If You want to know how do we calculate total power consumption just visit my website newphysicist.com, the link is in the description.

Consider I have a laptop which consumes 50w at peak performance which I use daily for four hrs.

that’s 50W*6 = 300Wh

I also have two 10w led bulbs that run for 5rs.

that’s 4*10W*5 = 200Wh

and I have a 50W TV which I use for two hrs

that’s 50W*2 = 100W

So, I have a total power consumption of 600Wh a day. Considering variations in energy use I am adding an extra 200Wh. So in total, The system I am going to build should satisfy 800Wh. This means all the devices on my property consume 800Wh in a day combined. This is how we calculate the total power consumption of the property. Now you can calculate it by yourself. if you have any doubts let me know in the comment section.

Sizing of the Solar Panel for Solar Inverter

Size of a PV System = total load per day/ (PSH*efficiency)

=800/(5*0.85)

= ~200W Solar Panel

I had bought 2 fifty watts solar panels last year for my previous project. I have bought another two 50W panels to upgrade my existing system. So, in total, I have 4 fifty watts solar panels. Each solar panel has a Vmax of 18 volts and an Imax of 2.5A. So that I can connect these panels in several ways depending on the voltage of the solar inverter I am going to build. If I connect all the panels in parallel I can make a 12V system and if all are connected in series I can make a 48V System. But I am going to connect all four panels in parallel and build a 12V system.

These are my previous solar panels manufactured by loom solar. But this time I chose bluebird solar. Both are manufactured by mono-perc technology. But surprisingly bluebird solar panels came with mc4 connectors attached but loom solar didn’t.

Sizing Battery for DIY Solar Inverter

As we discussed earlier we have arranged 4 solar panels in parallel which has an effective output of 22V and a maximum of 8-9A Isc. Since lead-acid titular batteries have a depth of discharge of 50% we are not going to discharge the remaining 50% of the battery. Days of autonomy refers to the total number of days the battery bank should supply power in case there’s any cloudy weather or any unexpected reasons. Here I am going to use just one.

Battery Size = (Total Load*days of autonomy)/maximum DoD

= (800Wh*1)/.5

= 1600Wh

Since we are going to build a 12V system ,

= 1600Wh/12V

= ~ 130Ah Battery

Sizing Charge controller and Inverter

In my previous solar project, I used a 10A Charge controller and a 400VA square inverter. Since I am adding an extra 100W solar panel and I have plans to add more, I thought about going for MPPT Charge controllers. But law cost MPPT charge controllers have a very short life span. A 200W Solar panel array is a very small system for which a PWM charge controller is enough. I bought a 20A PWM charge controller and an 800W sign wave solar inverter.

Check my Violin project: How to Make Violin | DIY Violin Guide

How to Test the Solar Panel Voltage Current Power

On the backside of the solar panel, you can see a label on which its technical specifications are noted. From that, we will get a lot of information about that piece of product. We will get Vmax and Imax, Voc and Isc, and Pmax. The power curve or I-V curve will give you an elaborate understanding of the above measurements. But all the above data is measured under standard testing conditions. So the actual value may be different. However, I am going to show you how to measure the Isc (short circuit current- maximum current a solar panel can supply under given solar radiation), and the Voc ( Open circuit voltage- Maximum voltage a solar panel can supply under given solar radiation.

How to test short circuit current (Isc) of a Solar Panel

An ammeter is used to measure the current in a circuit. I am going to show you how to use a digital multimeter to measure the short circuit current of a solar panel. When we are connecting an ammeter (inside a multimeter) to the positive and negative terminals of the solar panel (in series) we are actually short-circuiting the solar panel. When we short circuit a maximum current flow will happen. At that time the voltage will become zero. First, connect the black test lead to the common input jack and the red test lead to the 10A jack. Then position the function switch to the A range (as shown). Now carefully connect the positive and negative test lead to the positive and negative output of the solar panel respectively. (Don’t hold it for more than 5 seconds since if there’s a large current it will damage the instrument.)

How to meassure the Open Circuit volatege (Voc)

A voltmeter is used to measure the voltage in a circuit. I am going to show you how to use a digital multimeter to measure the open-circuit voltage of a solar panel. When we are connecting a voltmeter (inside a multimeter) to the positive and negative terminals of the solar panel (in parallel) we are not short-circuiting the solar panel. There will be no flow of charge through the circuit. So we will get the maximum voltage the solar panel can supply under given solar radiation. At that time the current will be zero. First, connect the black test lead to the common input jack and the red test lead to the V jack. Then position the function switch to the V range (as shown). Now carefully connect the positive and negative test lead to the positive and negative output of the solar panel respectively.

Fixing and Connecting Solar Panels to for 12V Off-grid System

I have already fixed two of my panels on a stand. I am going to extend that stand to accommodate my new boys along with it. To do that I have to extend my existing stand. I already had a metal cutter and a small welding machine. So it’s not a big deal. After that, I have to connect all four panels in parallel or in series to get the desired voltage. Since I am going to build a 12v system I have to connect all these panels in parallel.

I have a total of 200W solar panels. Since I am living in India there’s a guaranteed 5 hrs of Peak Solar Radiation. It is called PSH or Peak solar hrs. According to the specifications of all these panels are connected in parallel we will get a Voc of 22V and Isc of 12A. Remember this is just the short-circuit current not the actual current and a value of 12A cannot be expected because of wiring and panel loss.

I made the frame this way so that I can later install a single axis solar tracking system that tracks the sun’s daily east-west movements. It’s not worth installing a tracking system these days because modern solar panels are highly efficient. I just wanted to show you how a tracking system works.

Let’s connect all the four 50W panels in parallel and check how much Voc and Isc are we getting. To do that we have MC4 four in one branch connectors. Using it we can easily connect all the panels in parallel as shown in the figure below.

You can Buy mc4 branch connectors from my amazon store. As we discussed earlier we should get 22V Voc and a maximum of 12A (not more than 8A expected). Let’s take the measurements using the multimeter. Then we can decide the AH of the battery we are going to use with this solar inverter.

Redesigning Control Panel

This is the control panel I am using for my existing solar power system. You can see the charge controller, Isolators and control board. This is a simple setup.

10A PWM Charge controller

16A Isolators

Ordinary AC Switchboard

The next step is designing the controls panel. We have to connect the charge controller, a DC MCB for the solar panel, another DC MCB for the battery and an AC isolator to disconnect the inverter from the AC control box. 

PWM Charge Controller

This is a 20A PWM charge controller I am going to use with this system. Charge controllers are used to protecting the battery from getting overcharged or over-discharged. It also prevents the reverse flow of the charge from the battery to the solar panel. 

Our panel size is not large enough to use an MPPT charge controller which is way more efficient than PWM controllers. In the next upgrade, I will replace this PWM with an MPPT charge controller. 

This charge controller has six connectors on the bottom side. Two of them are for solar panels, another two for battery and the last two for connecting DC load.

In an MPPT charge controller, this is where we connect the inverter. Since our PWM controller is not rated enough to handle the possible current flow from the battery, we connect the inverter directly to the battery. 

Most inverters have an over-discharge protection circuit built-in, so it’s not at all a big deal. Otherwise connecting the inverter directly to the battery may lead to frequent discharge which is unhealthy for the battery. 

Inverter

This is the 800W sign-wave inverter I am going to use with this system. As you can see there are two input leads that we have to connect to the positive and negative terminals of the battery.

And there are input, output and neutral connectors. Since we are building an off-grid system we don’t need this Input connector. These are the phase and neutral of the inverter output. 

We can use the input terminal to charge the battery using grid power and inverters inbuilt battery charger during unforeseen circumstances like continuous cloudy days or battery over-discharge etc.

Since we are constructing an off-grid system for a property where there is no electricity grid forget about the input connector on the inverter.

Battery

This 110Ah battery is going to store the solar energy and power the inverter. This is a lad acid tubular battery, which usually has a depth of discharge around 50%.

Let’s connect the inverter and the charge controller’s battery leads to the positive and negative terminals of the battery. The first and second DC MCBs are used to isolate and protect the charge controller from the solar panel and the battery respectively.

Here we can see an AC isolator which is used to isolate the AC control panel from the inverter. Make sure the inverter is off and connect the output phase and neutral to the AC isolator. 

Lets Connect

Now we can connect the solar array to the first MCB. Make sure you are connecting positive and negative leads from the panel array to the same on the charge controller. You should pay attention when connecting the battery too.

Interchanging the polarity will damage the charge controller. I suggest you use different coloured wires for positive and negative polarity so that you will not get confused. You can use this Wire gauge calculator to determine the DC wire gauge.

Great. Sun is shining bright on the solar panel, which has already started producing power but the MCB blocks the power to reach the charge controller and then the battery. 

Testing

Just turn on the first MCB and check how much voltage the solar panel array is generating. Great, 21.6 V. It is the open-circuit voltage of the solar panel array under current solar radiation.

Now let’s turn the second MCB between the battery and the charge controller. Keep this in mind. When turning on an offered system always turn on the battery first. It will help the charge controller to identify the battery and its voltage.

Turning on the MCB to the battery automatically turn on the charge controller output. There’s a switch on the charge controller to turn it off. The battery is a new one that’s why it already read 12.2V.

Then you can turn on the solar panel. If you turned on the battery while the solar panel is connected, the charge controller will malfunction. You can see that the battery voltage is rising which means the solar array has started charging the battery.

Now we can turn on the inverter and, AC isolator and test the AC loads using this control box. Perfect. Our 800W off-grid solar power system is ready to harness unlimited solar energy and power your office, workshop or outhouse. Enjoy.