How To Calculate Solar Panel Battery And Inverter Size

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In the quest to go off grid, sizing and installation of just solar panels is considered important. But right selection of solar panel batteries, charge controller, and inverter is equally important. If any one of these components is not compatible with others, your system will work inefficiently; in worst case, it might not work at all. Here’s my step by step guide on how to calculate solar panel battery and inverter:

How To Calculate Solar Panel Battery And Inverter

Calculate Solar Panel Battery Inverter and Charge Controller

So to get started, you will just need to use solar angle calculator and inverter size calculator.

how to calculate solar panel battery and inverter

Required Tools:

– Inverter size calculator

– Solar angle calculator

Things Needed?

– Solar panel

– Battery

– Charge controller

– Inverter

Step by step Guide:

Load Estimation

Load installed at the facility leads to sizing of all components as correct maximum wattage assessment, and energy calculation is start off point. Suppose that you have three tube lights of 100W each, and three laptops of 70W each. Maximum Watt rating would be (3 * 100W) + (3 * 70W) = 510W.

Now let’s suppose that on average tube lights are powered for 4 hours a day and laptops for 8 hours a day. Energy requirement of whole load for one day is (3 * 100W * 4h) + (3 * 70W * 8h) = 2880Wh. Now is the time to account for energy losses. Voltage drops, temperature variations etc. will cause actual available energy to be less than supplied one. Let’s suppose that 20% energy is lost before reaching load. So energy demand can be said to be 2880Wh * 1.2 = 3456Wh.

Sizing Solar Panel Batteries

After knowing watt-hour requirements, we directly know the capacity of our system batteries. But before making decision, there are two more factors to consider.

First, for how many days you want your battery to back up your system. Suppose, we need 2 days back up. Second, how deeply you can discharge your battery. For deep cycle discharge batteries, it is advisable that you get it discharged up to 50% of its total capacity.

So the battery you need, must be of 3456Wh * 2 * 2 = 13824Wh or above. Keeping it simple, you need batteries with total 14kWh capacity. To convert it to Ah capacity, we have to be aware of volt rating of our system. Let’s say our solar panel is that of 12V and we are designing the whole system at 12V. So Ah rating would be 14000kWh / 12V = 1166Ah. So you need one or more batteries with total energy storage capacity of 1166Ah minimum.

Charge Controller

Charge controller is another crucial component in solar panel systems. It keeps your batteries from over charging so they don’t get damaged. Moreover, controller ensures that current flow is unidirectional. Or that current flows from solar panel to battery only and not from battery to solar panel.

As this could happen at night time when solar panels are not producing any charge that could go to batteries. Choosing right charge controller refers to selecting a controller with some specific amperage rating.

To calculate ampere rating for your charge controller, Watt rating of your solar panels and system voltage must be know. For Example, you have 120W solar panels installed and system operates at 12V. Then ampere rating of controller would be 120W / 12V = 10A. While keeping safety margin, you can select a controller with 12A rating. But margin should not be too large as this might lead to overcharging of your batteries.

Similarly, if you design your system at 24V with same 120W panels. Then controller with ampere rating of little above than 120W / 24V = 5A can be selected.

Inverter Selection for Solar Array

Once your DC power is stored in batteries in the form of charge, inverters are there to make this DC power available for your AC load. While sizing inverter there are three important characteristic factors to be considered.

Maximum load that could be run on the inverter at some moment.
Input DC voltage that would be typically same as that of battery voltage
Output AC voltage specifications e.g. 120 or 220V at 50Hz or 60Hz (It depends on utility grid. Different countries choose standards of their own)

While estimating load at step 1, we assumed total load of 510W. We also assume that your utility grid is rated at 120V at 60Hz. In such case inverter must be rated above 510W / 120 = 4.25A. Inverters have some inherent losses so after accounting for 20% efficiency, you shall buy an inverter rated at almost 4.25A * 1.2 = 5.1A. In this case we assumed that batteries were operating at 12V. So inverter should be such that they accept 12V DC, provide 120V/60Hz AC, and have minimum ampere rating of 5.1 amperes.

How To Calculate Solar Panel Battery And Inverter FAQ

What happens if your charger controller is too small in amps compared to the solar panels?

While having a charge controller with large amperage capacity can overcharge and, ultimately, damage batteries. Controllers with less than needed ampere rating have their own disadvantages. Simply, if you have enough energy available at your solar panels, it will not be captured and stored fully in your batteries. So you have wasted some capacity, some energy of your system. As batteries will not store expected charge, you will not be able to run all your load for needed time because your batteries will run out of charge before expected time. Please note that capacity loss will be higher in sunny days. Here’s my guide to best solar charge controller.

How to choose a suitable battery and inverter?

Inverter must be chosen according to load. Maximum load that could be connected to inverter, must be estimated and then 20 to 25% efficiency margin must be adjusted. Moreover, protection systems of inverter, usability, input and output voltage ratings, size, technology etc. must be considered while choosing an inverter.

Battery, being the backbone of solar system, must also be chosen carefully. First of all, its amp-hour capacity should be good enough that it could power all the load for required time. Moreover, energy density, charging efficiency, discharge profile, and shelf life are among many other factors to be considered while choosing a battery. Here’s my guide to best 3000 watt inverter.

In a Nutshell

To make your solar power system work efficiently and safely, each component in the system must be compatible with the other and thus chosen vigilantly. For this, you must how to calculate solar panel battery and inverter before buying any component. Load at your home or office determines specifications for each individual component in the system.

Read my guide on solar panels cost here.