The idea of going off-grid is incredibly exciting. It’s a lifestyle that allows you to live in a more sustainable way and be free from the limitations of the grid. But there’s one problem. Going off-grid isn’t as simple as plugging in a solar panel and waiting for the sun to shine. Among many other components in the solar energy system, you have to deal with proper and efficient energy storage.
Do note that sizing batteries for solar energy systems depend entirely on energy consumption, budget, the number of days with backup power, and the appliances you want backup power for. As every off-grid solar setup and requirement are different, there is no fast and hard rule when it comes to batteries selection.
Here’s a quick example. If require 10 kWh a day to power your home with 1 day of backup, you will need either a battery bank capacity of 21 kWh for lead-acid batteries or 12 kWh for lithium batteries. Let’s go over them to see why and how you can design this based on your own requirements.
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Battery Storage Design Considerations
This is the part where different people have different preferences in sizing the battery capacity. It might also be subjected to grid availability. Here are 3 ways you can think about, based on different load requirements.
Full off-grid (Standalone)
This is the most ideal solution where you have no access to the grid. You definitely need a large battery bank, large enough to cater to the whole consumption of your home for a few days. If you have not figured out your daily/monthly energy consumption, you can follow this post on how to calculate your power consumption and size your solar array.
A standalone system allows you to power your home with batteries alone during nighttime and cloudy days when charging is not possible.
Cost-Effective
The main idea here is to have an ideal battery capacity to supply enough to your home so that you’ll be able to avoid any peak or demand charges. In this setup, you will require lesser batteries than a full off-grid system. On days when your system is able to generate enough for your home, you can store the access in the grid where net metering applies. With proper management and correct battery capacity, you just might be able to steer clear of buying power from the grid at high prices.
Furthermore, when your battery bank is low, you have a choice to charge it up during off-peak hours.
Resilience
In this case, you rely heavily on the grid but still have a minimal battery capacity to power selected essential appliances when you experience a power failure. It is totally up to the homeowner’s preference on which equipment to have a backup supply for. If you only have a few appliances with small loads, a small battery capacity will suffice.
By now, you should have your home electrical usage ready. So let’s dig deeper into this!
How do I Calculate Battery Size?
Based on your personal energy usage in kWh per day, decide the number of days of backup power you want. Let’s take an example to see how this works for a full off-grid situation:
- Load requirement per day: 10 kWh
- Day(s) of backup: 3
- Total battery storage capacity: 30 kWh
- Rated Wh capacity per battery: 3.8 kWh
- Number of batteries required: 30/3.8 = 7.9 nos.
You can either choose to round it up to 8 to ensure you have more than enough for 3 days or 7 which will still last you slightly more than 2.5 days. But it is best to size it bigger if you’re not on a budget.
If you are planning for the cost-effective method, you probably just need 2-3 numbers of such batteries to last you through peak and demand charges. Of course, that’s assuming you can fully discharge 100% of the batteries. However, this is not the same for all types of batteries.
Types of Battery
There are 2 main types of deep cycle batteries that you can choose from, lithium-ion and lead-acid batteries.
Lithium-Ion batteries are the most common type of batteries you’ll find in residential applications and EVs. These batteries are highly popular for their efficiency, longevity and require no maintenance. These batteries are safe, affordable and most of all, reliable. They can discharge from 80-100% and has a cycle life of 10,000+. This makes them more expensive than lead-acid batteries. If you choose a flooded lead-acid or sealed lead-acid, they have a shorter lifespan and can only discharge up to 50%.
So, taking into consideration the depth of discharge (DoD) and efficiency of the battery for the example above, you’ll need:
For Lithium Battery: Lithium: 30 kWh x 1.2 (account for 80% discharge) x 1.1 (account for 90% efficiency) = 39.6 kWh Number of batteries required: 39.6/3.8 = 10.4 nos.
For Lead-Acid Battery: Lead-Acid: 30 kWh x 1.5 (account for 80% discharge) x 1.2 (account for 80% efficiency) = 54 kWh Number of batteries required: 54/3.8 = 14.2 nos.
Ambient Temperature of Battery Storage
Here is another thing you need to take note of. Your battery performance and capacity are affected by the ambient temperature. Batteries are rated at 77°F and work best around 50-85°F.
When it gets colder, your battery’s capacity decreases. If you live in a prolonged cold climate, you can choose to add additional battery capacity to account for the loss of capacity. It will be a good idea to store your battery in an insulated shed or even mount them near your inverter as those devices put off a little heat to keep them warm.
In summer your battery cap will increase but battery life is shortened. Conversely, in winter times, battery life is extended. So check out the battery operating temperature and size those according to your region’s climate.
What Voltage to Choose for your Batteries?
You might have heard people talking about the different voltages of batteries, 12 volts, 24 volts, and, 48 volts. These are the typical size for your solar energy system. Based on the battery requirement for both types of batteries of 39.6 kWh and 54 kWh:
For Lithium Battery:
12V – 3,300 amp hours
24V – 1,650 amp hours
48V – 825 amp hours
For Lead-Acid Battery:
12V – 4,500 amp hours
24V – 2,250 amp hours
48V – 1,125 amp hours
So which voltage should you choose for your batteries? This usually depends on your rate of energy consumption for an extended period of 3 hours (continuous loads). A good guide will be to keep your continuous AC low.
If you are powering AC loads of less than 1000 watts, 12 volts is a good option. Anything more than that and lesser than 4000 watts, it is best to go for a 24 volts system. 48 volts will be for loads greater than 4000 watts. Keeping the current low means smaller components, thinner wires, and circuit protection parts – eventually greater savings. Higher voltage is preferred in most system components due to its efficiency. So if you’re powering a small off-grid home, 24 volts is ideal.
Sizing it right the first time is crucial as changing the battery voltage will require a re-design of the wiring, inverter, and other related components.
Series or Parallel Connection for Batteries?
Let’s start off on the right foot. You will need identical batteries to rack up to your required battery capacity, whether you are connecting in series or parallel. Connecting your batteries in series adds the voltage together, whereas connecting them in parallel increases the current flow.
Example: 2 12V 150AH in series = 24V 150AH 2 12V 150AH in parallel = 12V 300AH Series-parallel connect of both examples above = 24V 300AH
In a series-parallel connection, you can increase the capacity and voltage of your system to generate the battery bank size you need. However, you should reduce the number of parallel connections as much as possible as it may increase the current draw and becomes less efficient. They also affect each other negatively when the state of charge is different.
If you do connect them in parallel, make sure to arrange them close together with short cables to reduce losses.
Final Note:
With proper sizing, configurations, and energy management, you can optimize your battery system and save a lot of money on your solar energy system. It will undoubtedly provide the energy you need on ‘rainy days’. To conclude, it is essential to know your battery system, its limitations, and its maintenance. The battery system will have its own set of challenges, so it is recommended to get professional help.
More Helpful Solar Energy System Information
- On Grid Vs Off-Grid Solar System – What’s the difference and which system is better for you? In any case, you need to understand the difference before you stay on the grid or off the grid. This post explains it for you.
- How Do I Power My House Off The Grid? – If you are looking for various options to power your off-grid home and don’t know where to look, check out these 5 options in this article and decide for yourself.
- More Lower Or Less Higher Wattage Solar Panels? – Are you confused about choosing the right panel wattage and size? This post compares the cost, efficiency and panels footprint so that you can get the best out of your solar array!
- How To Choose A Solar Charge Controller? – It might be tough to understand if you don’t know how to choose a charge controller, but this post will help you get a clear idea of what to look for and how to choose a charge controller.