DIY Solar - Battery Sizing

For DIY Solar enthusiasts, solar PV systems with batteries or off-grid solar PV systems are one of the most exciting things to build for our own use. Its main advantage over typical on-grid solar PV systems is that it is not required to be connected to our house’s main utility supply, allowing for a safer installation.

To determine the proper size of the battery system that we need to power a certain appliance or appliances, we will need to start with a simple load analysis. It is very simple and straightforward. You will first need to multiply your appliance’s power rating with the number of hours that you will use it in a day. The result is the total energy consumption of that appliance in one day Kindly see the table below for a sample computation:

Getting the sum, we will have the total energy demand that we need to meet every day. This also means that we need to have a battery system with at least 276Wh of energy storage capacity. I said at least because we will still need to consider some things. First is the efficiency of the battery in storing and releasing energy. This is also called round-trip efficiency and is typically around 80%. We will need to divide our total energy demand by 80% or 0.8.

276Wh/0.8 = 345Wh

We will then need to consider the battery’s depth of discharge or DOD. DOD simply means how much energy we have used from the battery in terms of percentage. A 50% DOD means that we have used 50% or half of the battery’s total energy. The higher the DOD when we are using the battery, the lower its lifetime will be. We want our battery to last longer but at the same time, we also want to maximize its energy storage capacity. This is why battery manufacturers recommend a certain value of DOD for their batteries. Their recommended value is the optimum value of DOD that will also result in an optimum lifetime for the battery. It depends on the manufacturer and battery type but is usually around 80%. We will then need to divide the last value that we got by 80% or 0.8.

345Wh/0.8 = 431.25Wh

This value will then be the minimum battery capacity that we will need to supply our total energy demand. We will then need to divide this number with our battery system voltage. This pertains to whether we will have a 12V, 24V or 48V battery system. Take note of what we mean by battery system voltage. If we will be using 12V 3Ah batteries, having a 24V battery system means that we will have 2 strings of batteries in parallel (2*12V=24V). Let’s assume for this example that we will be using 24V.

431.25Wh/24V = 17.97Ah = 18Ah

The 18Ah value is the minimum total Ah capacity of our battery system. Again, if we will be using 12V 3Ah batteries, this means that we will need:

18Ah/3Ah = 6 pieces of batteries

Take note that since we have set that we will be using a 24V battery system, we will have 2 strings of batteries. Strings of batteries must always be the same size, so the total number of batteries that we need must be divisible by 2 (the number of battery strings). In this example:

6 pieces of batteries / 2 strings = 3 pieces of batteries per string.

Even if we have the proper size for our battery system, remember that our solar PV system should still be able to fully charge it everyday for it to be able to supply our total energy demand.

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Engr. Jet Andal has 6 years of experience in the design and installation of residential, commercial and utility-scale solar PV systems. Together, and with the use of solar energy, let us help make the world a better place. You can click here to read all of our other blogs. For aspiring solar PV engineers, you can also check out his Solar PV Engineering Ebook on Amazon on this link.