How to calculate the optimal battery size for your solar setup

 

Choosing the right battery bank is crucial when installing a solar system. The battery bank stores the energy from your solar panels and supplies power when needed.

The ideal battery size depends on several things:

  • How much energy you use daily
  • How long do you want your battery to last
  • The type of battery you choose
  • Your budget

This post is a guide on battery sizing for solar energy systems. It will help you find the right size battery for your needs.

 
Option A- Buy what you can afford and make do with how long the battery lasts

This is the most straightforward and least recommended approach. Don't buy the most giant battery you can afford. Think about how much power you need and how long you want your system to last. This approach can lead to several problems, such as:

  •     Running out of power frequently and having to rely on backup generators or grid power
  •     Damaging the batteries by over-discharging them or not charging them properly
  •     Wasting money on batteries that are either too small or too large for your system

 
Option B- Determine a target runtime and buy just enough battery to cover that

This approach is better than the first, but it still needs improvement. Think about how long you need backup power. This is for times when the sun isn't shining, or the power goes out. Choose a battery that can store enough energy to power your needs for that length of time. For instance, if you need 12 hours of backup power, your battery should have enough capacity to last 12 hours.

This way is better. You'll have enough power for as long as you need. However, it still has some drawbacks, such as:

  •     Not accounting for variations in power consumption or solar generation.
  •     Not leaving any margin for safety or future expansion.
  •     Not optimising the battery life or performance.

 
Option C- Calculate what you need to run the system for one day and have enough battery power to cover the typically day after including the power from the panels for the typical day plus some margin of error

 
This is the ideal approach for battery sizing. Figure out how much power you use in a day. Then, buy a battery that can store enough power to cover the you after considering the energy from the sun during the daytime. This way, you'll have enough power when the sun is out. This approach has several advantages over the previous ones, such as:

  •     Ensuring you have enough power for the day.
  •     Leaving some extra capacity for unexpected events or future expansion.
  • Optimizing the battery life and performance by avoiding deep discharges or overcharges.

 
How to Calculate Your Battery Size

 Choosing the right battery size is crucial for going off-grid with solar power. Your battery should be able to power your home or business to meet your requirement, even if it's too cloudy to charge it efficiently with solar power. Using some basic math, this blog post will teach you how to find the ideal battery bank size.

Before we go further, please have the following facts at hand:

  •   The type of battery affects how much power you can safely use. The type of battery you use affects how much power you can draw from it without harming it. Don't use lead acid, tubular, or AGM batteries past 50% of their charge. This can damage them. Going below this level can damage the battery. If you go below 50% regularly, you will damage the battery and shorten its lifespan. Following this rule will help your battery last longer.
  • LiFePO4 and LiPo are two common types of lithium batteries. It can be hard to tell them apart. These batteries can use up to 90% of their capacity without damage. However, it's best to use only 80% for safety. Better still you can rely on the battery management system of the manufacturer. Lithium batteries allow you to use most of their power (for example, 160-180Ah out of 200Ah). Other batteries, like wet, lead, or tubular, only let you use half their power. So, a 200Ah battery gives you 100Ah of usable power.

Here's the formula to figure out how many batteries you need. Battery life (hours) = Battery capacity (Ah) x Depth of discharge (DoD) x Battery voltage (V) / Daily energy consumption (Wh)

Before using the formula, find out your daily energy use. This is the total watt-hours (Wh) used by everything you plug in. You can estimate this number using a power meter or an online calculator. Our sample house uses 500 Wh of energy per day. Our sample battery voltage is 51.2V. So if you have a 100 Ah lithium battery and only use 90% of its charge, you get 100 x 0.9 = 90 Ah 90 x 51.2 = 4608 Wh 4608 / 500 = 9.22. Our example house uses 500 Wh of energy per day. A 100Ah lithium battery can power it for about 9 hours.

Of course, like most things in life, it could be more complex. Other things can affect how long the battery lasts. Using more power or having bad wiring can drain the battery faster. Using less power can make it last longer.

But at least you have a good ballpark figure to work with.. Now, it's becoming clear why you need to optimise your consumption. It would be best if you did so to maximise your system and funds. Wasteful, inefficient power consumption is your enemy.

Conclusion

Battery sizing is an essential aspect of designing a solar system. Knowing your battery size helps you know how long it will last before needing a recharge. It also affects how well your batteries perform over time. To pick the correct battery, figure out how much power you use daily, then add a bit extra for safety. Remember that different types of batteries have different capacities. However, this post should give you the guide on getting the optimal battery size for your needs.

 



Back to Solar 101