How many solar panels, how big of a solar inverter to buy? How to calculate battery backup? These are the common questions to size up for solar off grid system. Off grid solar installation has 4 key components: solar panel, battery, charge controller and power inverter. To calculate size of solar system, take a look of following 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 40W, fan consumes 80W 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 1000W.

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 1000W, you should choose an inverter of 1600W. 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 1600W inverter comes in 24V nominal voltage.

Step 3: Calculate the total current of your load

In our example, the power (watts) is 1000 and we already know the voltage to be 24V, where Power (W) = Voltage(V) x Current(A).

1000 (watts) = 24V x current
Current =1000/24 = 41.66A

Let’s round it off to 41A. Now our solar system needs to generate at least 41A of current to power the connected electrical load.

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 is to think how many hours of backup you need. Remember, power 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 (W) x hours of backup needed / 24V

Why the 24? Because our inverter is 24V. Let’s put the figures to the formula:

1000 (W)x5Hours/ 24V = 208AH

Let’s round it off to 300AH because it is advisable 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

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? Let’s consider another useful formula 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 30A. Our solar panels need to make 30A of current to feed our battery bank. Let’s not forget about the loads. We already calculated that our electrical load will need 41A to run. We need to add this to the amps that our battery bank is going to take: 30+41 = 71A. Our solar panels should make 71A. On an average, 250W solar panels have a voltage of 30V. Use the power formula: Power = Volts x Amps

Amps: We calculated in the last step that we need 71A (30amps to feed the battery bank and 41amps to run the electrical load directly through solar).

Power: 30x71A = 2130W. Let’s round it off to 2500W because you can’t have 2130W panels.

This is the answer:-
We need to install panels of 2500W to feed our battery bank and run electrical load. You can go for 10 panels of 250W each.

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