How many solar panels do I need? How big of a solar inverter do I have to buy? How do I calculate battery backup? These are commonly asked questions by people who are installing solar for the first time in India. Let’s address them one by one.
Before we start teaching the simplest way of calculating the size of solar installation in India, we would like you to remember this fundamental formula:
Power (in watts) = Voltage x Current
Keep this formula in your mind because we are going to use it a lot!
How to calculate size of solar system in India?
Most of the solar installations in India are off-grid because our country, India, faces frequent powercuts. Off grid solar installation has 3 key components: solar panels, battery and solar PCU (solar PCU is a solar inverter with built-in solar charge controller). To calculate size of solar system, it is important to follow these steps:
Step 1: Calculate your total load that you want to run
You should know how much power (in watts) your electrical appliances consume. For example, a tube light consumes 40watts, fan consumes 80 watts etc. You should add the electrical load (in watts) that you wish to use. Let’s assume that you added everything and the figure that you get is 1000 watts.
Step 2: Size your solar inverter based on electrical load
After you know the total electrical load, the next thing that you have to do is find a solar inverter that can power the load. In this case where your total electrical load is 1000 watts, you should choose an inverter of 1600 watts. It is advisable to oversize the inverter because unfortunately DC to AC conversion that solar inverters do causes loss of energy. It is also good to know that a 1600 watts inverter comes in 24v (v = voltage). Remember this because we are going to use this fact ahead in our calculations.
Step 3: Calculate the total current of your load
This is easy! Go back to the formula we taught you in the beginning.
Power (in watts) = Voltage x Current
In our example, the power (watts) is 1000 and we already know the voltage to be 24v. Let’s insert these figures into our formula.
1000 (watts) = 24V x current
1000/24 = 41.66 amps
Let’s round it off to 41 amps. Now our solar system needs to generate at least 41 amps of current to power the connected electrical load. Keeping this figure in mind let’s go to our next step.
Step 4: Decide how many hours of battery backup you need – buy battery based on that
The next step in calculating size of solar system in India is to think how many hours of backup you need. Remember, solar PCU/inverter will directly power your electrical load through solar. However, when solar is not available, the solar energy stored in batteries can be used to power load. Let’s say you need backup of 5 hours. Now there is a very simple formula to calculate size of battery based on your total load and backup time required.
Total load (in watts) x hours of backup needed / 24
Why should we divide by 24? Because our inverter is 24v.
Let’s put the figures from our example in this formula:
1000 (watts) x 5 hours / 24 = 208
Let’s round it off to 300AH because it is OK to have extra backup
You can install 2 batteries of 150Ah.
Step 5: Calculate size of solar panels based on battery size and current of electrical load
Yes, sizing of solar panels comes at the last because panels are either going to feed the battery or run electrical load. They need to produce enough voltage and current to charge the battery properly and to run electrical load. So how do we decide the size of solar panels?
Time to remember our fundamental formula again:
Power = voltage x current
And let’s give you another golden formula which will help us here:
Charging Current of Battery = 1/10th of its Total AH.
In this case, we have 300ah so if we divide it by 10 we get 30amps. Our solar panels need to make 30 amps of current to feed our battery bank.
But what about the electrical load? Let’s not forget about that. We already calculated that our electrical load will need 41amps to run. We need to add this to the amps that our battery bank is going to take: 30+41 = 71amps.
Our solar panels should make 71amps.
Fact: On an average, 250 watts solar panels have a voltage of 30v.
Now, let’s put everything together in our fundamental formula:
Power = Volts x Amps
Amps: We calculated in the last step that we need 71amps (30amps to feed the battery bank and 41amps to run the electrical load directly through solar).
Power = 30x71amps = 2130 watts. Let’s round it off to 2500 watts because you can’t have 2130 watts panels.
This is our answer: We need to install panels of 2500 watts to feed our battery bank and run electrical load.
You can go for 10 panels of 250 watts each.
Simple! Wasn’t it?
Note: Please consider this as an honest attempt from our side to simplify complex ideas and calculations. We do not claim that this is 100% accurate. There are other things to consider such as power factor of inverter and other technical things. When you call us for installing solar at your place, our trained team sizes the solar system based on your unique needs.