When we use sources of energy like sunlight and wind to create power, it's not very efficient if we can't save that power for later. But we can solve this by using special batteries like deep cycle batteries (these are like the ones in your car but designed for more energy storage) such as Lead-Acid and Lithium-Ion batteries. It's important to remember that these batteries can only store a specific kind of power called DC power, not the kind of power that comes out of your home's wall sockets (AC power).

In this article, we'll explain how to figure out the right size for the battery bank - that's the group of batteries - you'll need. We'll measure this in Ampere-hours (Ah), which tells us how much energy the batteries can hold. Also, we'll find out how many batteries you'll need. Just know that batteries are always measured in Ah. And if you're not comfortable doing the math on your own, there's a battery bank size calculator you can use. You'll find it after we walk through an example of how to calculate battery size.

Step 1 - Figuring Out How Much Energy You Need:

The first thing to do is to figure out how much energy you use. You'll need to add up the energy you use from all the devices you have - like lights, appliances, and gadgets - either right away or later when you use the stored power in the batteries. If it's for your home, you can usually find this information on your electricity bill or your energy meter.

If you're using the batteries for something like an RV or a boat, you'll have to add up the energy needs of all the things you'll be powering. (There's an example above  that can help you understand this better.) We've put together a table that shows how much energy some common home devices use. In our example, all the devices together need 900 watts of energy.

Step 2 – How Many Days Without Charging:

This step is about figuring out how many days you can use the energy stored in the batteries without charging them. It's like knowing how many days the batteries can power your stuff when the weather is cloudy and not great for charging. You can find this information from weather forecasts or local weather departments for how much sun you usually get and how many cloudy days there are.

Remember, if you want to use the batteries for more days without charging, you'll need more batteries or even other power sources like portable generators. In our example, we're saying the batteries can power things for 2 days without charging.

Step 3 – Battery System Voltage:

This step is about knowing the kind of battery voltage you have. Most batteries are rated with a DC voltage, like 12V, 24V, 36V, 48V, 72V, and so on. 12V systems are common, but sometimes for solar panels, people use 24V or 48V setups depending on how the system is set up. In our example, we're using 12V batteries.

Step 4 – How Much Battery Energy You Use:

This step is about understanding how much energy you take out of the battery each time you use it. This is called Depth of Discharge (DoD). It's shown as a percentage of the total energy the battery can hold. If you take out more energy, you'll be able to do it fewer times before the battery can't be used anymore. Basically, using too much energy too often will make the battery wear out faster. In our example, we're using up to 50% of the battery's capacity each time.

Step 5 – The Temperature Around:

Unlike solar panels, batteries like moderate temperatures to work well. Temperature changes affect how well the batteries work and how long they last. If it's too hot, the batteries won't last as long, and if it's really cold, the batteries won't be as strong. In our example, the temperature is 60 °F (15.55 °C).

Step 6 – How Many Hours You Need Backup Power:

This step is about knowing how many hours a day you need to use your stuff using the power from the batteries. In our example, it's 3 hours.

Step 7 – Figuring Out Battery Size:

Finally, we can find out how big the batteries need to be. We'll measure this in Ampere-hours (Ah). Here's the formula we use: Battery Capacity in Ah = (Energy Needed in Wh x Days Without Charging x Hours of Use) / DoD % x Battery Voltage

In our example: Battery Capacity in Ah = (900Wh x 2 Days x 3 Hours) / (50% x 12 Volts) Required Battery Capacity = 999 Ah (Almost 1000 Ah) This means you need at least 999 Ampere-hours of battery capacity to run a 900Wh load for 3 hours each day for 2 days without charging.

Step 8 – How Many Batteries You Need:

To find out how many batteries you need, you can divide the total battery capacity by the capacity of each individual battery. In our example, if each battery is 100Ah: Number of Batteries Needed = 999 Ah / 100 Ah = 10 batteries. So, you'd need to connect 10 batteries, each with a capacity of 100Ah, in parallel to run your stuff for 3 hours each day with 2 days of backup.

If you want to use batteries with different capacities like 120Ah, 150Ah, 200Ah, or 250Ah, you just divide the total battery capacity by the capacity of the individual battery you want to use. This tells you how many batteries you need to connect in parallel.